1995 Movement & Dispersal Research

Managing Insect Resistance to Transgenic Corn:

Implications for Migrating Insects.

Richard L. Hellmich and Leslie C. Lewis.

Iowa Agriculture and Home Economics Experiment Station, and Department of Entomology, Iowa State University; USDA-ARS Corn Insects Research Unit, Ames, Iowa

Transgenic corn that expresses a toxin gene from Bacillus thuringiensis (Bt) will be commercially available for the 1996 field season. Bt-corn could replace synthetic insecticides as a control measure for insect pests, particularly the European corn borer, Ostrinia nubilalis (Hübner). Dramatic control of insects via transgenic plants, however, has many scientists concerned about high selection pressure from these toxins and the subsequent adaptation by pest insects to these toxins.

Our lab is actively pursuing research related to resistance management of the European corn borer. Although the corn borer is the primary pest that will be affected by Bt-corn, there are implications for insect migrants, particularly black cutworm, Agrotis ipsilon (Hufnagel), corn earworm, Heliocoverpa zea (Boddie) and true armyworm, Pseudaletia unipuncta (Haworth). The following paragraphs summarize proposed resistant management strategies and consider issues related to corn-insect migrants.

Several strategies have been proposed for managing insect adaptation to plants that express the Bt endotoxin (Gould 1988a & b, 1989; Raffa 1989). These strategies include using multiple endotoxins, mixing Bt corn seeds with non-Bt corn seeds, expressing endotoxin in specific plant tissues, and increasing refugia (i.e., alternate hosts or non-Bt corn).

Pyramiding two or more toxins could be very effective, but some pests are resistant to two or more toxins (i.e., cross resistance) (Gould et al. 1992). Further, developing and maintaining multiple-endotoxin hybrids would be expensive for seed companies.

Mixing Bt and non-Bt corn seeds would require hybrid-seed companies to maintain Bt and non-Bt versions of each hybrid (Eric Sachs, Monsanto Co., personal communication). Maintaining a twofold increase in the number of hybrids would be a logistic nightmare for seed companies. This strategy also is undesirable because corn borer larvae frequently move among corn plants (Paula Davis, Cornell University; William B. Showers, Iowa State University, personal communications). A larva could ingest sublethal quantities of Bt from a Bt plant then move to non-Bt plant and survive. Consequently, a mixed-seed strategy actually could hasten insect resistance (Mallet & Porter 1992; Tabashnik 1994).

For similar reasons, expressing endotoxin in specific tissues also could promote European corn borer resistance. A larva could potentially move from toxin-containing to toxin-free tissues on the same plant (Mallet & Porter 1992).

A growing consensus from the scientific community is that refugia will play a critical role in any resistance management program (Gould 1986; Mallet & Porter 1992; Tabashnik 1994). Refugia is simply another word for alternative hosts that will support the growth of susceptible insects. Presumably, susceptible insects from refugia, if present in sufficient numbers, will mate with resistant insects and dilute out resistance genes.

Resistance probably will not be a problem with black cutworm or true armyworm. Both of these insects have a large number of alternate hosts. Resistance could be a problem with corn earworm. Some scientists are particularly concerned about corn earworm resistance in areas where both Bt-cotton and Bt-corn are planted (Rick Roush, personal communication). The issue is whether or not there is sufficient refuge in these areas to reduce selection pressure. If corn earworm populations do become resistant how quickly will resistance spread to other regions of North America? Certainly migratory routes and non-Bt refuge in these other regions are important issues that the committee might want to discuss.

References

Gould, F. 1986. Simulation models for predicting durability of insect-resistance germ plasm: A deterministic diploid, two-locus model. Environ. Ent. 15:1-10.

Gould, F. 1988a. Evolution biology and genetically engineered crops. BioScience 38:26-33.

Gould, F. 1988b. Genetic engineering, integrated pest management and the evolution of pests. Trends in Ecology and Evolution 3/TIBTECH 6:S15-19.

Gould, F. 1989. Ecological-genetic approaches for the design of genetically engineered crops, pp. 146-151. In D. W. Roberts and R. Granados (eds.), Proceedings of a symposium: biotechnology, biological pesticides and novel plant-pest resistance for insect pest management. Boyce Thompson Institute Conference, July 1988, Boyce Thompson Institute Publications, Ithaca, NY.

Gould, F., Martinez-Ramirez, A., Anderson, A., Ferre, J., Silva, F., and Moar, W. J. 1992. Broad-spectrum resistance to Bacillus thuringiensis toxins in Heliothis virescens. Proc. Natl. Sci. USA 89: 7986-7990.

Mallet, J., and Porter, P. 1992. Preventing insect adaptation to insect-resistant crops: Are seed mixtures or refugia the best strategy? Proc. Soc. Lond. B 250: 165-269.

Raffa, K. F. 1989. Genetic engineering of trees to enhance resistance to insects. BioScience 39:524-534.

Tabashnik, B. E. 1994. Delaying insect adaptation to transgenic plants: seed mixtures and refugia reconsidered. Proc. Soc. Lond. B 255:7-12.

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Effects of Bt transgenic corn on non-target lepidopteran species.

Clinton D. Pilcher, Marlin E. Rice, Leslie C. Lewis, and John J. Obrycki

Iowa Agriculture and Home Economics Experiment Station, and Department of Entomology, Iowa State University; USDA-ARS Corn Insects Research Unit, Ames, Iowa

Two years of research has been completed (1994-1995) testing effects of Bt transgenic corn on secondary lepidopteran corn pests including black cutworm (Agrotis ipsilon), true armyworm (Pseudaletia unipuncta), and corn earworm (Heliocoverpa zea). Field studies were used to analyze injury caused by each of these pests to Bt corn. Corn plants were infested with early stage larvae of each species. Later plants were rated for injury. Bioassays were run in the laboratory to determine what impact Bt transgenic corn would have on developmental times and survival of these corn pests.

Results show that the Bt corn has no effect on the black cutworm. However, there is an impact on armyworm and corn earworm. If the corn earworm feeds on the leaf tissue, then there is a toxic effect, however, typically the corn earworm lays its eggs in the corn silks. The toxin is only expressed in the leaf and pollen tissues of the tested hybrid. Therefore, there is no effect on the corn earworm to ear-tip feeding. The results here would be different given variability in expression of different tissues within the plant. True armyworm are susceptible to mortality as well, but are mainly impacted by a delay in development. Under armyworm infestations of early instar larvae, the Bt corn could reduce the amount of leaf defoliation.

Publications

Keaster, A. J.,. J. A. Grundler, M. A. Jackson, M. D. McCorcle, W. B. Showers, M. O. Way, R. D. Parker, J. B. Giezentanner, K. Schwindt, and J. R. Raulston. 1995. Occurrence and winter activity of black cutworm moths along the Texas Gulf Coast, 1987-1991. Southwestern Entomologist, Suppl. No. 18, Sept. 1995: 135-154.

Sappington, T. W., H. W. Fescemyer, and W. B. Showers. 1995. Lipid and carbohydrate utilization during flight of the migratory moth, Agrotis ipsilon (Lepidoptera: Noctuidae). Arch. Ins. Biochem. Physiol. 29: 397-414.

Showers, W. B., A. J. Keaster, J. R. Raulston, J. L. Goodenough, W. H. Hendrix, III, M. O. Way, and J. F. Robinson. 1995. Seasonal migration of the black cutworm. Southwestern Entomologist, Suppl. No. 18, Sept. 1995: 119-134.

Showers, W. B., M. J. Weiss, M. E. Derrick, and W. H. Hendrix, III. 1995. Potential movement on surface airflow of a bivoltine population of European corn borer (Pyralidae: Lepidoptera) into a historically univoltine habitat. Environ. Entomol. 24: 835-840.

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Field Examination of Short-Range Whitefly Migration

The sweet potato whitefly, Bemisia tabaci (Gennadius) (also known as B. argentifolii ), is an increasingly important agricultural pest worldwide. The application of pesticides is the principal control option available to growers. This strategy has become increasingly less effective in recent years as a result of heightened levels of pesticide. Optional strategies such as cultural control techniques are needed. One possible set of alternatives is to alter crop placement and to manipulate irrigation schedules based upon models that predict when, where and how far whiteflies are going to migrate. The development of such techniques requires a better understanding of whitefly flight behavior, more specifically short-range migration. Much of this information has already been generated in the laboratory), but these results needed to be corroborated with field studies.

Laboratory populations of the sweet potato whitefly, Bemisia tabaci, have been shown to consist of both migratory and trivial flying morphs. The behavior of these forms as part of the process of short-range migration was examined under field conditions. Insects were marked in a field of melons using fluorescent dust during two consecutive growing seasons. During the first growing season, passive traps used to collect living whiteflies were placed along 16 equally spaced transects radiating from the field to a distance of up to 1.0 km. Wind out of the northeast consistently carried migrating whiteflies to traps placed along transects in the southwestern quadrant because cold air drainages dictate wind direction during early morning hours in the desert Southwest. For this reason, during the second season traps were laid out over fallow ground in a rectangular grid extending 2.7 km to the southwest of the marked field. If dispersal was entirely passive, patterns could be described using a diffusion model. Statistical examination of the data, however, demonstrated that the distribution on all days was patchy. Geostatistical techniques were used to describe the observed patchiness. Traps in the immediate vicinity of the marked field caught more whiteflies than the daily median. Large numbers were also collected from near the periphery of the grid. Whiteflies were far less prevalent in the grid's center. As a result, the distribution of captured whiteflies can be described as bimodal (the second peak 2.2 km from the source probably constitutes the distance for short-range migration for sweet potato whiteflies). These patterns confirm behavior observed in the laboratory, i.e., a portion of the population are trivial fliers that do not engage in migration and are consequently captured in traps near the field and a portion initially to cues associated with skylight, ignoring cues provided by the ground, and fly for a period of time before landing in distant traps. During both years movement out of the field had an exaggerated directional component on 13 of 14 days.

For further information contact David N. Byrne or look for,

Byrne, D. N., R. J. Rathman, T. V. Orum and J.C. Palumbo. Localized migration and dispersal by Bemisia tabaci. Oecologia in press.

Byrne, D. N. and J. L. Blackmer. 1995. Examination of short-range migration by Bemisia. In (D. Gerling and R. T. Mayer, Eds.) Bemisia '95: Taxonomy, Biology, Damage, Control and Management. pp Intercept Publications. Wimborne, Eng. accepted.

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True Armyworm and the Reproduction-Flight Syndrome Revisited

Seth J. Johnson & Abner M. Hammond

Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, LA 70803

Lizhi Luo, Visiting Scientist

Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, China

J.D. Lopez, K. R. Beerwinkle, J. K. Westbrook, and J. F. Esquivel

USDA-ARS, Areawide Pest Management Research Unit, College Station, TX

There is evidence that the true armyworm like a number of other noctuid pests in North America, is an annual poleward migrant in the spring. Migration enables it to colonize new habitats and thus track its resources in a northward progression as climate allows for their availability. Migration appears to occur in the spring on strong southerly synoptic scale winds that many other insect migrants in the Mississippi River Drainage Basin (MRDB) also use in poleward migrations (Johnson 1995). Circumstantial evidence for migration by true armyworm includes: no evident diapause (Guppy 1969, Fields & McNeil 1984); inability to survive the winter in areas where temperatures of < 0°C last more than 80 days, which would include much of its summer range in the northern latitudes of the U.S. and most of Canada; and moths often occur synchronously over large geographic areas in the spring at least three weeks prior to emergence of the local population in areas where it can overwinter (Fields & McNeil 1984, McNeil 1986, 1987). Additional evidence for migration was presented in a recent study that found exotic legume pollen on adults in Missouri and Iowa indicating they could have migrated at least 1300 km northward in the spring from southern Texas and northern Mexico where plants producing these pollens are distributed (Hendrix & Showers 1992). There is also some evidence that the true armyworm is a true seasonal migrant with a southward return migration in the autumn. In the northern latitudes of the MRDB and southern Canada the moths are sexually inactive, the females delay calling and males are not responsive to sex pheromone in late summer (Turgeon & McNeil 1983, McNeil 1987). These physiological and behavioral changes indicate the insects may be premigrants and possibly in a physiological syndrome in which there is an accumulation of energy resources, development of a flight apparatus and depression of reproductive systems (Johnson 1969). The most compelling evidence of a reproduction-flight syndrome in the TAW was presented by Luo, Johnson, Showers, Hammond and Sorenson (unpublished data) at last years NCR-148 meeting. Simultaneous measurement of behavior and physiological conditions during late Spring 1994 in a source population (New Madrid, MO) and a downwind immigrant population (Ames, Iowa) strongly suggested that the TAW fits the oogenesis-flight syndrome model. The source population was unmated, males were unresponsive to sex pheromone, female reproductive tract development was much reduced and the female's flight muscle was larger than in the immigrant population. In the immigrant population, however, 78% of females were mated, 228 males were captured per night in a pheromone trap, the female reproductive tract was larger than the source populations and was mature, and the female's flight muscle was smaller. The general specialized behaviors involved in the state are persistent, straightened out movement, and depression of vegetative responses (Kennedy 1961). This physiological and behavioral premigratory condition has been called the "oogenesis-flight syndrome" (Johnson 1969) and/or "reproduction-flight syndrome" (Dingle 1974).

Our results in 1994 strongly suggested the true armyworm is a seasonal migrant with a well defined "reproduction-flight syndrome." We wanted to validate our 1994 results and also examine flight fuel and flight potential in a southern premigrant source population and a northern population composed of recent immigrants. The variables we measured included: 1) adults collected in sex pheromone and black-light traps, 2) female mating status and ovarian development, 3) weight of dorsal-longitudinal muscle, 4) lipid (flight fuel) levels, and 5) flight potential.

Materials and Methods

Populations. The lower Rio Grande Valley (LRGV) of Texas was selected as the site of the source population based on previous research (Lingren et al., unpublished) that indicated blooming citrus in the LRGV served as a sink for large numbers of sexually unresponsive and unmated TAW that appeared to disperse downwind on southerly winds in late winter-early spring. The College Station area was selected as the downwind immigration site based on 1990-1991 pheromone trap data (Beerwinkle et al., unpublished) that indicated peak catches of males in early and late March in 1991 and 90, respectively, which was considered to be compatible with anticipated migration events in 1995.

Atmospheric Analysis.

Atmospheric transport analyses of wind trajectories from Moore Air Base in the LRGV revealed SE wind trajectories into LRGV from March 14-18 but strong SE flow and northward trajectories from March 19-26. The SE flow from March 19-26 was due to prefrontal flow as weather fronts encroached on central Texas during this period. Estimated trajectories show that TAW could have flown 497 km to Navasota (within 32 km of College Station) on March 22. Estimated northward trajectories on March 19-21 were between 322 and 404 km. Winds during this period could have supported multiple-night flights from LRGV to College Station on March 19-24. Minimum air temperatures were 11.7 - 20.0°C from March 18-24 in College Station which is above the 10°C flight and trap capture threshold.

Male attraction to sex pheromone traps.

TAW adults were monitored in the LRGV with black-light and pheromone traps and hand captured in citrus groves for behavioral and physiological analyses. The pheromone trap line consisted of 5 traps in an east-west transect from Laguna Vista to Citrus City and operated from Jan. 22-April 15. The two black-light traps were operated at Moore AFB and Weslaco, respectively. TAW adults were not as abundant in the citrus groves in 1995 because of lower populations and cold induced delayed and partial citrus bloom in February and March. However, we did collect approximately 100 TAW over 6 nights from March 14-19th, collecting from 7:00 pm to 12:00 midnight near Mission, TX. TAW were monitored with pheromone and black-light traps at the immigration site. A SW-NE pheromone trap line (3 sites) from Bastrop to North Zulch and an intersecting (at Caldwell) SE to NW trap line (3 additional sites) was operated daily from February 22-March 32. Three additional pheromone traps were operated at College Station from February 4-May 31. Two black-light traps were operated near College Station from February 4-May 31. The pheromone and black-light trap captures both indicated that peak TAW capture at the immigration site occurred between March 20-24th.

Female mating status and ovarian development.

Moths used in this part of the study were collected by hand, at night, either feeding on citrus blossom nectar or resting on citrus trees or else in black-light traps. Mating status was determined by dissection and counting the number of spermatophores in the bursa. Ovarian development was determined by dissection and measuring the length of the right ovary from the base of the ovariole pedicel to the distal end of the terminal filament. The width of the ovariole was taken at the widest point of the follicle at 5OX with a dissecting microscope.

Weight of dorsal-longitudinal muscle.

The entire dorsal-longitudinal muscle was removed from the thorax after oven drying at 50°C for 48 hours and weighed.

Lipid Analysis.

Whole body lipids were analyzed using a modified Folch et al. (1957) procedure as described by Fescemyer & Hammond (1988).

Flight Potential.

Single-night flight performances of both sexes from the two experiments were measured with a 32-channel, computer monitored flight-mill system (Beerwinkle et al. 1995). The following flight variables were measured: longest single flight duration (min), total flight duration (min), total flight distance (km), and flight speed (km/hr).

Experimental Subjects.

The longest run of strong S.E. wind flow was March 19-24 and resulted in the highest trap catches of TAW between February & April in the College Station area. Therefore, since there was strong evidence migration was taking place, collections of TAW in LRGV immediately prior to this event and during the event in the College Station area were used in the lipid and flight potential studies. Moths collected during earlier but less spectacular migration events at both locations were used to determine mating, reproductive tract development, flight muscle and body weights.

Results and Conclusions.

In the emigrant population, an average of only 0.92 males and 0.175 moths were collected per trap per night in pheromone and black-light traps, respectively. However, TAW moths were present in citrus groves feeding on blooms at night and 20-40 could be collected by two people per night. No mated females were found in a sample of 99 examined. Mean basal ovariole width, length and wet weight were 0.266 mm, 4.169 cm and 9.1 mg (n=99), respectively. Dry weight of the dorsal longitudinal muscle in females was 6.328 mg. This population had a fresh body weight of 184.7 mg and 53.2% whole body lipids. The longest single flight duration, total flight duration, distance flown and flight speed were 125.8 min, 340.9 min, 21.0 km, and 3.646 km/hr (n=17), respectively.

In the immigrant population, many more adults were collected in pheromone and black-light traps. Two separate pheromone trap networks captured means of 5.8 and 4.23 male moths/trap/night. The two black-light traps captured a mean of 6.57 males and 9.44 females/trap/night. Fifty seven percent of the 138 immigrant females examined were mated with a mean of 0.891 spermatophores per female. Mean basal ovariole width, length, and wet weight were 0.397 mm, 4.576 cm, and 27.0 mg (n=138), respectively, which were significantly higher (P< 0.001) than those in the emigrant population. The ovariole length was significant at the P< 0.05 level. Dry weight of the flight muscle in the female was 5.175 mg and was significantly lower than the value in the emigrant population (P< 0.05). This population had a fresh body weight of 161.6 mg and 21.4% whole body lipids, which were both significantly less than the emigrant population (P< 0.001). The longest single flight duration, total flight duration, distance flown and flight speed were 24.0 min, 133.3 min, 6.8 km, and 2.966 km/hr (n=42), respectively. All of the flight potential variables except flight speed were significantly less than those in emigrant population (p< 0.05). Comparison of all of these variables between the two populations suggests that the incidence of migration in P. unipuncta is in the post-teneral stage of adults. The migrants are young and mate after they arrive at new habitats. Pre-migrants have high whole body lipid levels and flight potential. Migrants also undergo obvious changes in behavior and reproduction status after migration: their mating activities are depressed before migration and initiated after migration, a condition which fits the "oogenesis-flight syndrome" hypothesis well.

Literature cited

Beerwinkle, K. R., J. D. Lopez, Jr., D. Cheng, P. D. Lingren, & R. W. Meola. 1995. Flight potential of feral Helicoverpa zea (Lepidoptera: Noctuidae) males measured with a 32-channel, computer-monitored, flight-mill system. Environ. Entomol. 24:1122-1130.

Dingle, H. 1972. Migration strategies of insects. Science 175: 1327-1334.

Fields, P. G. and J. N. McNeil. 1984. The overwintering potential of the true armyworm, Pseudaletia unipuncta (Lepidoptera: Noctuidae), populations in Quebec. Can. Ent. 116: 1647-1652.

Guppy, J. C. 1969. Some effects of temperature on the immature stages of the armyworm, Pseudaletia unipuncta (Lepidoptera: Noctuidae), under controlled conditions. Can. Ent. 101: 1320-1327.

Fescemyer, H. W. & A. M. Hammond. 1988. Effect of larval density and plant age on size and biochemical composition of adult migrant moths. Anticarsia gemmatalis Hubner (Lepidoptera: Noctuidae). Environ. Entomol. 17: 213-219.

Folch, J., M. Lees & G. H. Sloane Stanley. 1957. A simple method for the isolation and purification of total lipids from animal tissue. J. Biol. Chem. 226: 497-509.

Hartstack, A. W., J. A. Witz and D. R. Buck. 1979. Moth traps for the tobacco budworm. J. Econ. Entomol. 72: 519-522.

Hendrix, W. H., III and W. B. Showers. 1992. Tracing black cutworm, and army worm (Lepidoptera: Noctuidae) northward migration using Pithecellobium and Calliandra pollen. Environ. Entomol. 21: 1092-1096.

Johnson, C. G. 1969. Migration and Dispersal of Insects by Flight. Methuen, London.

Kennedy, J. S. 1961. A turning point in the study of insect migration. Nature 189: 785-791.

McNeil, J. N. 1986. The true armyworm, Pseudaletia unipuncta (Haw.): A possible migrant species. In The Movement and Dispersal of Agriculturally Important Biotic Agents. (Edited by D. R. MacKenzie, C. S. Barfield, G. G. Kennedy and D. J. Taranto). pp. 435-441. Claitor's Publishing Division, Baton Rouge.

McNeil, J. N. 1987. The true armyworm, Pseudaletia unipuncta: a victim of the pied piper or a seasonal migrant. In Recent Advances in Research on Tropical Entomology, ed M.F.B. Chaudhury, pp. 591-597. Insect Science and its Applications. 8, Special Issue. Nairobi' ICIPE Science Press.

Turgeon, J. J. and J. N. McNeil. 1983. Modification in the calling behavior of Pseudaletia unipuncta (Haw.) (Lepidoptera: Noctuidae), induced by temperature conditions during pupal and adult development. Can. Ent. 115: 1015-1022.

Publications:

Johnson, S. J. 1995. Insect migration in North America: Synoptic scale transport in a highly seasonal environment. In Insect Migration: Tracking Resources Through Space and Time. (eds. V. A. Drake & A. G. Gatehouse) Cambridge University Press, Cambridge. pp. 31-66.

Wei, X. & S. J. Johnson. 1995. Velvetbean caterpillar: Surviving freezing weather in Louisiana. Florida Entomol. 78:1-3.

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Gene Flow and Resistance Management in European Corn Borer

David A. Andow & Don N. Alstad

Departments of Entomology and Ecology & Evolutionary Biology
219 Hodson Hall, University of Minnesota, St. Paul, MN 55108

Gene flow and migration are critical processes for evolution of resistance because migrants can homogenize variations in allelic frequencies that result from locally varying selection environments, and these processes determine the rates at which local adaptations spread and become regional and national concerns. We are estimating genetically significant long distance migration to document the subcontinental structure of Ostrinia populations and enable us to predict the rates at which highly resistant lines can be expected to spread.

Direct empirical measurement of dispersal over long distances is problematical. The number of dispersers required to effect significant long distance gene flow is very small. Moreover, even if one succeeds in counting the number of individuals that move between demic subunits of a natural population, one must still assess their reproductive success to determine whether the movement has any genetic consequence. Slatkin (1980, 1981, 1985) has proposed indirect gene flow measures that circumvent this difficulty, and Slatkin and Barton (1989) have tested these methods by simulation. The most robust method is based on an inverse relation that Wright (1951) demonstrated between Nem (effective population size multiplied by the proportion of new migrants into the deme, the number of migrants per generation) and his Fst statistic, a standardized measure of interdemic variance in allelic frequency. Gene exchange between demes reduces this local differentiation.

Estimates of Fst require the characterization of allelic frequencies from multiple sampling localities. We are using allozyme electrophoresis on cellulose acetate (stained with agar overlays) to obtain these data, because it is an expedient and cost-effective technique (Hebert 1989, Richardson et al. 1986). This electrophoretic technique produces diploid nuclear genotypes in addition to allelic frequency data, an advantage (along with cost, speed, and simplicity) over alternative approaches that characterize the DNA. We have identified at least 7 polymorphic electrophoretic loci and have optimized expression of 5 of these loci on cellulose acetate gels, confirming previous work by Bernie May and Chuck Mason. We are now processing samples from the field.

Although it is hard to guess a priori what levels of gene flow we will find over long distances in O. nubilalis, it is clear that O. nubilalis are quite vagile. During their initial invasion of Minnesota after 1943, Chiang (1961, 1972) documented dispersal of 400 km westerly and 650 km northwesterly in 4 and 7 years, respectively. Observations of adult moths at light traps, before locally maturing insects eclose suggest significant annual migration from distances of a few hundred km (Chiang et al. 1965). Because of these observations, the geographical scale of our sampling effort is large. We are estimating long distance gene flow analyses at the subcontinental scale, sampling from at least 16 localities.

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Aphid populations and PVY spread in the Red River Valley

Chris D. DiFonzo

Department of Entomology, 219 Hodson Hall, University of Minnesota, St. Paul, MN 55108

Potato virus Y (PVY) potyvirus spread to indicator plants and captures of alate aphids in tile traps were monitored weekly in 1992 - 1994 in the Red River Valley of Minnesota and North Dakota. Yellow tile traps caught significantly more aphids overall than green tile traps, and were significantly preferred by Aphis helianthi, Capitophorus elaeagni, and Rhopalosiphum maidis. Thirty-four species were identified from green and yellow traps in 1992, 25 in 1993, and 26 in 1994. Green traps alone were used to obtain unbiased estimates of comparative landing rates of all aphid species. Intervals of greatest aphid capture at all sites were between 16 July and 13 August in 1992, and 11 July and 15 August in 1993. Exceptions were southern sites, where aphid captures increased beginning 2 July (Ada, MN, 1992) and 13 June (Prosper, ND, 1994). In 1993, the interval of greatest aphid capture tended to be wider, but ended 23 August. Aphid captures at all sites were 3 to 25 times greater in 1992 and 1994 than in 1993.

PVY infection of indicator plants exposed at the trapping sites was also greater in 1992 (25 plants) and 1994 (18 plants) as compared to 1993 (2 plants). Approximately 89% of PVY spread to indicator plants occurred between 8 July and 19 August. Eight species comprised 91% of the aphids collected in green traps during intervals of PVY transmission to indicator plants: Acyrthosiphon pisum, Aphis helianthi, Capitophorus elaeagni, Lipaphis erysimi, Rhopalosiphum maidis, Rhopalosiphum padi, Schizaphis graminum, and Sitobion avenae. Seven of these species were previously reported PVY vectors. We found that A. helianthi was also capable of PVY transmission under laboratory conditions. These eight species are all associated with crops and weeds common in the Red River Valley. Our data suggests that while the relative importance of individual PVY-vector species will vary from year to year and location by location, total aphid captures may be the best indicator of the risk of PVY spread. Furthermore, the epidemiological implication of this information is that Red River Valley seed potato growers should concentrate not on reducing numbers of aphids in the region, but on eliminating sources of PVY within the crop and on isolating seed fields as much as possible from potential PVY and aphid sources.

In order to share our research results with growers, an "Aphid Alert" newsletter was distributed weekly during the 1994 field season to all Minnesota and North Dakota seed potato growers. The newsletter reported trap catch and grain aphid survey data from various sites in the Red River Valley. The newsletter also featured short articles on use of crop borders, aphid identification, aphid life cycles, and other topics related to potato viruses and aphids.

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The ascent phase of aphid migration and dispersal.

Scott A. Isard, Mark Belding, Michael E. Irwin, Gail E. Kampmeier.

University of Illinois at Urbana-Champaign, Dept. of Geography, Urbana, IL 61801; Illinois State Water Survey, Champaign, IL 61820; University of Illinois, Dept. of Natural Resources & Environmental Sciences; Illinois Natural History Survey, Champaign, IL.

We have completed the third year of a research program to study the ascent phase of flight. Our research objective is to determine the biological and environmental factors that govern the ascent phase of aphid flight. Once take-off occurs, we postulate that physical forces within the surface layer (SL) and the aphids' flight aptitude (high vs. low) combine to govern whether aphids climb into the planetary boundary layer (PBL) and consequently are in a position to move long distance or remain within the SL and are restricted to local dispersal. We thus hypothesize that: the relative strength of buoyant and mechanical forces in the SL and the aphids' flight aptitude combine to govern the trajectories of alatae during the ascent phase of flight.

The influence of environmental factors on the ascent phase of aphid movement is dependent on the flight aptitude of alatae. Our laboratory experiments suggest that if aphids do not fly during the relative narrow temporal window when their flight aptitude is high ("migration flight mode") they enter a long period during which their flight aptitude is relatively low and consequently, they are in a "local dispersal flight mode". We postulate that once aphids pass into a "local dispersal flight mode," the opportunity for long-distance movement is all but lost. Aphids can enter this "local dispersal flight mode" because of delays in take-off due to conditions such as darkness, precipitation, low temperature, and strong winds.

Eight post flight initiation scenarios follow from our hypothesis. For aphids that take off when their flight aptitude is high ("migration flight mode"): A1) flight trajectories have a slight upward component when the buoyant forces do not exist and the mechanical forces are very small within the SL (atmospheric stability = non-turbulent); A2) flight trajectories are primarily horizontal when the buoyant forces are small relative to the mechanical forces within the SL (atmospheric stability = stable); A3) flight trajectories have an upward component when the buoyant forces are positive and approximately equal to the mechanical forces within the SL (atmospheric stability = neutral); and A4) flight trajectories have a steep upward component when the buoyant forces (directed upward) are large relative to the mechanical forces within the SL (atmospheric stability = unstable).

For aphids that take off when their flight aptitude is low ("local dispersal flight mode"): B1) flight trajectories have a downward component when the buoyant forces do not exist and the mechanical forces are very small within the SL (atmospheric stability = non-turbulent); B2) flight trajectories have a downward component when the buoyant forces are small relative to the mechanical forces within the SL (atmospheric stability = stable); B3) flight trajectories are primarily horizontal and short when the buoyant forces are positive and approximately equal to the mechanical forces within the SL (atmospheric stability = neutral); B4) flight trajectories have an upward component when the buoyant forces (directed upward) are large relative to the mechanical forces within the SL (atmospheric stability = unstable).

On clear, calm days, temporal oscillations of aphid flight activity and atmospheric stability within the SL have similar diurnal periods, but their patterns are dissimilar. For example, the first peak in aphid flight activity generally occurs between 0600 and 0800 when the SL is stable. Thus it is postulated that flight trajectories of aphids in the SL are primarily horizontal and ascent by aphids through the SL and into the PBL above during the morning hours is unlikely (scenarios A2 and B2). Such ascent, therefore, may be impossible even for alatae in the "migration flight mode". In contrast, the second peak in aphid flight activity generally occurs between 1600 and 2000 when the SL is unstable. Consequently, it is proposed that any upward movement of aphids out of the SL during the afternoon and early evening hours is enhanced by atmospheric forces; it, therefore, requires relatively little energy expenditure (scenarios A4 and B4). In fact, at that time, once take-off has occurred, aphid movement out of the SL may be unavoidable, and thus aphids in the "local dispersal flight mode" may be carried long distances.

Approximately 200 experiments have been run on 50 nights. Initially, many of the studies were focused on creating the airflow regimes needed to evaluate our hypothesis. During the 1994 growing season, we concentrated on substantiating our scenarios using R. padi.

The results of two experimental runs evaluate the effect of the vertical wind speed gradient on R. padi flight trajectories. An isothermal (26°C) vertical profile was maintained in the wind tunnel. For the first experiment, wind speed at canopy level (0.3 m) was 0.35 m/s and increased logarithmically to 1.1 m/s at the 2.1 m measurement level, while for the second experiment, the vertical wind speed gradient was approximately one-half as steep, increasing logarithmically from 0.25 to 0.5 m/s between the same levels. Grid square coordinates have been converted to an angular measurement of the aphids' mean flight trajectories and displayed as vectors; the length of each vectors represents the percent of the aphids initiating flight that passed through the corresponding grid square. For example, 59% of the 22 aphids that initiated flight in the first run had mean flight trajectories between 11 and 22° above the canopy while the flight trajectories of the remaining aphids were less than 11°. In contrast, 63% and 8% of the aphids that flew during the second run, when the vertical wind speed gradient was small, had mean flight angles between 31 and 45°, and 45 and 55°, respectively, while only 29% had mean flight trajectories less than 22°. Over the course of these and numerous other experiments, we have learned that aphids seldom take-off from the vial when wind speeds in the lowest air layer exceed 0.4 m/s. Regardless of the wind speed, the trajectory of aphid flight is almost always downwind, and when the speed of the air layer in which aphids are flying exceeds 0.75 m/s, their flight trajectories are primarily horizontal and downwind.

Observations of free flight of R. padi in our recent wind tunnel studies confirm that maximum flight aptitude occurs during the first day of adult life. The mean angle of ascent for 0.5 to 1-day-old, 1 to 2-day-old, and 2 to 3-day-old alates that flew were 34°, 24°, and 17° respectively under a neutral atmospheric stability regime, while the proportion of aphids that initiated flight (64%, 48%, and 15% respectively) decreased equally dramatically with age. Three to 4-day-old R. padi alates were incapable of flight. These results on differential flight aptitude of R. maidis and R. padi alate age classes concurs with previous finding that the flight muscle in many species of aphids starts to autolyse 2-3 days after adult molt (Johnson 1954).

Another set of experiments examined the combined effects of variations in atmospheric stability and environmental preconditioning on 0.5 to 1-day-old R. padi flight trajectories. The data are from a set of studies first run on 25 April 1994 (a1 and a3) and repeated on 26 October 1994 (d1 and d3). Numerous similar experiments were run during the intervening months. The aphids flown in these experiments were raised on barley plants in the growth chambers at 23°C and 14 hrs of light each day. Alates for experiments a1 and a3 were produced as a result of crowding, when the R. padi populations were allowed to become large. In contrast, alates for experiments d1 and d3 were produced as a result of deteriorating host conditions, after barley plants were subjected to water stress. In each of the experimental sequences, the important meteorological control over the ascent phase of aphid flight is demonstrated; the vertical component of the aphids' flight trajectories is more pronounced under unstable atmospheric conditions (a3 and d3) than during the stable airflow regimes (a1 and d1) while the trajectories during neutral conditions (not shown) were intermediate. During the course of this experimental sequence we observed differences in flight trajectories that likely resulted from changes in the condition of the maturing host plants over the intervening two months (not shown). Stressing the host plants appears to have altered the flight capacity of R. padi as well, resulting in relatively weakly flying aphids (compare experiments a1 with d1 and a3 with d3). The effect of host plant stress appears most pronounced during unstable atmospheric conditions. For example, the mean ascent flight angles for all alates in experiment a3 was 56° while the mean ascent flight angles in experiment d3 was only 28°.

See more about flight chamber and future plans

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Mark/Release/Recapture Studies of Male Tobacco Budworm Moths in Mississippi

John C. Schneider

Entomology and Plant Pathology, Mississippi State University, MSU, MS 39762 email: JSchn@Ra.MSState.edu

Movement of male Heliothis virescens (Lepidoptera: Noctuidae), tobacco budworm, was studied by mark/release/recapture during emergence of the over wintering generation in the Delta (a major cotton and soybean production area) near Greenville, MS in 1982, 1983, 1992, and 1993. Persons involved in the conception and performance of the work were M. Laster and W. Kitten and those in the administration/funding were E. King and D. Hardee. My involvement has been in advisement and analysis.

Larvae were reared on a diet containing Calco Red dye which colors the adult's fat body. Adults eclosed after placement in the field at 25 sites in a grid with a 3-4 km spacing. Periodic placement of pupae over a 4-6 wk period resulted in ca. 4,446, 7,452, 57,624, and 63,648 males (plus a like number of females) eclosing from each site. Males were recaptured using a similar grid of 120 pheromone traps centered on the release area and extending a distance of about 35 km from the center of the release area in 1982 and 1983 and a grid of 48 pheromone traps similarly arranged plus a satellite grid of 28 traps about 50 km from the release area in 1992 and 1993. The release area and satellite trap area locations were reversed between 1992 and 1993.

The cumulative number of marked males (adjusted for variation in trap efficiency using the number of unmarked males caught in the trap) were analyzed using a model based on the following assumptions: 1) movement follows the Fickian diffusion equation with constant coefficient of diffusion, D, 2) males die off at a constant rate, u, and 3) trap efficiency, la, is constant over time. An estimate of u was available from the rate of reduction in numbers of marked males caught following cessation of release in 1982 and 1983. The model has been published by Turchin and Thoeny (1993. Ecological Applications 3: 187-198). Nonlinear regression was used to obtain parameter estimates.

The results are given in the following table:

Year    r0.9    u/D      la/D       u      D      la
        (km) (x10-5/ha) (x10-4)   (d-1)  (ha/d) (ha/d)
1982    29.6     11.0    24.9     0.13    1137   2.8
1983    23.5     18.0    19.9     0.08     458   0.9
1992    61.9      2.6     5.5    <0.1>   (3910) (2.2)
1993    42.6      5.4     6.4    <0.1>   (1860) (1.2)

Estimates of ratios of parameters are obtained from the fitted model, so estimates of individual parameters are available for 1982 and 1983 only. If the average, observed value for u is used for 1992 and 1993, the estimates shown in parentheses are obtained. The estimates of radius of the circle centered on a release site that would contain 90% of trap captures are shown under r0.9.

Movement was greater in 1992 and 1993 probably due to a greater tendency to move as reflected in increased values for D. Absolute trap efficiency la, which is the slope of number of males caught per trap night vs. number of males per hectare, had a value of ca. 2 ha/d.

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