1996 Movement & Dispersal Research

Dispersal Behavior by Bemisia tabaci, the Sweet Potato Whitefly

David N. Byrne (state representative), Rufus Isaacs, and Klaas H. Veenstra

Department of Entomology, University of Arizona, Tucson, AZ 85721

L-R: Rufus Isaacs, Klaas Veenstra, Ryan White, David Byrne, Andres Amaya

The focus in this report is to discuss the progress made this past year concerning our understanding of short (< 1.0 km) and long-range (>5.0 km) dispersal by Bemesia tabaci. As a part of this effort our laboratory has been conducting research on the host-location and selection phase of flight using a horizontal wind tunnel. We have found there is an exponential reduction in take-off as windspeed increases at air flows up to 40 cm/s airflow.

Other experiments concerning potential for egg load were conducted. We found no relationship between distance captured from a marked source field and number of mature eggs per female. Another experiment confirmed that females lay significantly fewer eggs on the older leaves. Egg laying was also positively correlated with weight of the female. We expected that vitellogenin/vitellin levels would decrease as a function of the amount of time spent away from suitable hosts, as females would eventually need amino acids for other life processes. The reverse was true. The amount of vitellogenin/vitellin per female increased significantly with time removed from plants. We are also now investigating the vertical components of whitefly dispersal in the field. Preliminary results show the intense concentration of the flying population near to the ground. Also, marked whiteflies were routinely caught in the highest traps (24 ft) on a series of trestles. This was true even in the trestles placed at the field edge. This indicates that a portion of the population dispersing from a field immediately rises to reasonably high altitudes. More recent work with traps suspended from balloons has shown that whiteflies can be captured at heights of at least 150 ft during early morning hours.

Introduction

On a worldwide basis Bemisia tabaci (Gennadius), the sweet potato whitefly, continues to be one of the principle pests of crops grown for food and fiber (Berlinger 1986, Byrne et al. 1990). Because there are no viable alternatives to chemical control available (e.g., resistant varieties or profitable alternative crops), growers are left with one principle control strategy; the application of pesticides. It has not been uncommon in the past for growers in the southwestern US to make biweekly applications of insecticides to cotton and vegetable fields throughout the late summer/early fall seasons. This is not environmentally sound nor is it cost effective. We feel that a better understanding of whitefly dispersal behavior will lead to management strategies that are less pesticide dependent.

During the last nine years, various members of our laboratory have examined what we term mid-range dispersal (by our definition on the scale of a few kilometers) (Blackmer and Byrne 1993a and 1993b, Blackmer et al. 1995a and 1995b, Byrne et al. 1996a and 1996b). The focus in this report is to discuss the progress made this last year concerning our understanding of short (< 1.0 km) and long-range (>5.0 km) dispersal by B. tabaci.

Laboratory Experiments in 1995-6

Host Finding. In the past year, our laboratory research on whitefly flight has shifted focus from solely migratory behavior to include the host-location and selection phase of flight. In December 1995, a seed grant was awarded to enable us to build a horizontal wind tunnel for the study of whitefly flight behavior, especially as it relates to host plant location. The tunnel has now been calibrated and operates at between 0 and 35 cm/s providing conditions suitable for flight by these small insects. The preliminary results presented here are extremely encouraging and indicate that tunnel conditions are suitable for examining whitefly flight. We started by testing the impact of windspeed on take-off behavior, because our field observations indicated little take-off as windspeed increased after sunrise. There is an exponential reduction in take-off as windspeed increases. The tunnel is equipped to allow the introduction of visual and volatile chemical stimuli and we are currently investigating the impact of stimulus combinations on take-off. This has already been used to demonstrate that in the presence of a visually attractive stimulus, whiteflies that weigh only 50 mg are capable of flight upwind against a 40 cm/s airflow (0.9 mph).

A more detailed analysis of flight behavior is also being conducted, using a motion-analysis system in the Department of Neurobiology. With this, we can digitize insect flight tracks and then calculate aspects of their flight such as flight speed, ground speed, rate of turn and turn angle.

Videotaping of flights will also be done in concert with a wingbeat frequency monitor that will enable us to start understanding how whiteflies generate the power to fly upwind against relatively fast airflows. This work has just started, but we are finding that these exceptionally small insects are capable of quite sophisticated responses to stimuli from the environment during flight.

Flight Physiology. It is often assumed that migration and reproduction involve a large trade-off. For example, once the winged forms of aphids start to reproduce, they are no longer capable of migratory flight. However, in B. tabaci reproductive capability does not seem to inhibit migratory behavior or capability. For example, we found no relationship between distance captured from a source field and number of mature eggs per female (Veenstra & Byrne in prep.). In a previous study Blacker et al. (1995) showed an association between migratory behavior and high quantities of vitellogenin/vitellin. Individuals collected from the ceiling of a greenhouse had higher vitellogenin/vitellin levels than those collected from plants. They also found a positive correlation between flight duration and vitellogenin/vitellin levels. A possible explanation for this could be that a lack of suitable oviposition sites leads to migratory behavior. We examined this possibility by looking at the change in vitellogenin/vitellin levels when individuals were restricted to non-preferred oviposition sites (= old leaves). We also examined what happened to vitellogenin/vitellin levels when whiteflies were removed from plants for varying amounts of time. We confined individual female whiteflies to either old leaves of mature plants, young leaves of mature plants, or to young leaves of young plants. It is well known that whiteflies have a strong preference for oviposit on younger leaves, especially on cucurbits. In addition we froze a sub sample of the transferred whiteflies. This experiment confirmed that females lay significantly fewer eggs on the older leave, even in a no-choice situation. Egg laying was also positively correlated with weight of the female.

When females were confined to old leaves they did have higher vitellogenin/vitellin levels than at the beginning of the experiment, or than in females that were confined to young leaves. This indicates that the lower oviposition is not merely the effect of less feeding on older leaves or of poorer phloem quality in older leaves. More importantly, these results indicate that elevated levels of vitellogenin/vitellin can result from exposure to non-preferred oviposition sites.

In order to examine what happens to the vitellogenin/vitellin levels in individual whiteflies when they have no longer access to phloem for nutrients, we conducted the following experiment. At four different times during the day, we collected whiteflies from a melon plant. A third was frozen immediately, another third was kept alive off the plants for 4 h, and the remaining third was kept alive off plants for another 24 h. We expected that vitellogenin/vitellin levels would decrease over time, as females would eventually need amino acids for other life processes. The reverse happened. The amount of vitellogenin/vitellin per female increased significantly with time removed from plants. Apparently vitellogenin synthesis continues, even when whiteflies are no longer ingesting phloem.

Although we found before that migrating whiteflies were reproductively active we hypothesized that migrating individuals would, at least temporarily, reduce the amount of resources diverted to reproduction. Thus, we expected a much lower level of vitellogenin synthesis in migrating individuals. To test this hypothesis we collected whiteflies outside a cotton field (2-5 m) with fan traps and either froze the whiteflies right away, or froze them after they had been in an environator for 8 h . In addition we collected a field sample for comparison and subjected it to the same treatment. Whiteflies increased significantly in vitellogenin/vitellin content, whether they were collected from the field or were collected in muffin fan traps (Fig. 3b). In addition, the number of mature eggs per female (= egg-load) increased significantly over time. The fact that the "source by time" interaction is not significant, indicates that the increase in vitellogenin/vitellin and egg-load over time does not differ between whiteflies from the two collection methods.

Field Experiments 1995-6

We are also now investigating the vertical components of whitefly dispersal in the field. In the summer of 1995 and 1996 at the Yuma Agricultural Center we placed sticky traps on six wooden trestles downwind transect from a field of vegetables dusted with DayGlo® dust. These frames were placed at the field edge and at 20 m intervals along the transect. They supported traps at ground level, 2.4, 4.8 and 7.3 m. At each height there were four sticky traps spaced 30 cm apart (each trap representing a replicate). Preliminary results support the vertical distribution patterns reported by Byrne and von Bretzel (1987), showing the intense concentration of the flying population near to the ground. It is also important to note that marked whiteflies were routinely caught in the highest trap (24 ft) on each trestles. This was true even in the trestles placed at the field edge. This may indicate that a portion of the population dispersing from a field immediately rises to reasonably high altitudes. We also found early indications of gender associated differences in flight behavior.

More recent work with traps suspended from balloons have shown that whiteflies can be captured at heights of at least 150 ft during early morning hours. These preliminary studies represent our attempts to examine the long range component (i.e.> 5.0 km) of whitefly dispersal.

Future Objectives

During the last nine years, various members of our laboratories have examined what we term mid-range dispersal (by our definition on the scale of a few kilometers) (Byrne et al. 1996). The focus in some future work will be to enhance our understanding of the extensive movement of whiteflies between adjacent crops and the behavior that is associated with this movement. Briefly, our specific near goals are as follows.

Determine which plant factors result in whitefly dispersal away from summer hosts.
Examine the behavioral basis for location of fall hosts, particularly as it relates to plant age.
Determine if there are differences in host plant acceptance as a function of plant age.
Determine the physical and chemical basis for host plant acceptance.
Determine the distribution patterns of whiteflies following short-range dispersal between adjacent crops of varying ages.

References

Berlinger, M. J. 1986. Pests. in: J. G. Atherton and J. Rudich (eds.) THE TOMATO CROP - A Scientific Basis for Improvement. pp. 391-441. Chapman and Hall, London.

Blackmer, J. L. and D. N. Byrne. 1993 a. Environmental and physiological factors influencing phototactic flight of Bemisia tabaci. Physiological Entomology 19: 336-342.

Blackmer, J. L. and D. N. Byrne. 1993 b. Flight behaviour of Bemisia tabaci in a vertical flight chamber: Effect of time of day, sex, age and host quality. Physiological Entomology 18: 223-232.

Blackmer, J. L., D. N. Byrne & Z. Tu. 1995a. Behavioral, morphological and physiological traits associated with migratory Bemisia tabaci (Homoptera: Aleyrodidae). Journal of Insect Behavior 8: 251-267.

Blackmer, J. L., V. A. Lindley and D. N. Byrne . 1995b. Histological examination of flight muscle development and breakdown in Bemisia tabaci (Homoptera: Aleyrodidae): relationship to age and flight behavior. Journal of Morphology 226: 213-221.

Byrne, D. N. and P. K. von Bretzel. 1987. Similarity in flight activity rhythms in coexisting species of Aleyrodidae, Bemisia tabaci (Gennadius) and Trialeurodes abutilonea (Haldeman). Entomologia Experimentalis et Applicata 43:215-219.

Byrne, D. N., T. S. Bellows and M. P. Parrella. 1990. Whiteflies in agricultural systems. Pp. 227-261 In D. Gerling (ed.)Whiteflies: Their Bionomics, Pest Status and Management , Intercept Limited, Wimborne, UK.

Byrne, D. N. and J. L. Blackmer. 1996a. Examination of short-range migration by Bemisia. In D. Gerling and R. T. Mayer, (eds.) Bemisia :1995 Taxonomy, Biology, Damage, Control and Management. pp. 17-28 Intercept Publications. Wimborne, UK.

Byrne, D. N. Rathman, R. J., T. V. Orum, and J. C. Palumbo. 1996b. Migration and dispersal by Bemisia tabaci. Oecologia 105: 320-328.

Cohen, S. and M. J. Berlinger. 1986. Transmission and cultural control of whitefly-borne viruses. Agriculture, Ecosystems and Environment 17: 89-97.

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Joint Project on the Dispersal of Stable Flies: Phenology of Dispersing Flies

Carl J. Jones (UI), Gerald L. Green (KSU), Scott A. Isard (UI, co-rep from IL), Jerome Hogsette (USDA-ARS, MAVERL), Alberto Broce (KSU), Yu-Jie Guo (KSU), Michael E. Irwin (UI, co-rep, IL), Douglas Keen (UI), Mark Belding (UI), Geoffrey Hutchinson (UI), with important assistance from Elson Shields (Cornell), and Steve Rutherford (pilot from Garden City, KS).

University of Illinois at Urbana-Champaign (=UI); Kansas State University (=KSU); Cornell University

Stable flies are a major constraint to cattle production in the Midwest; they are also a significant nuisance pest of humans and other domestic animals. Overwhelming numbers of adults have frequently been encountered hundreds of kilometers from the closest site where larvae could have developed. Our research is intended to create fundamental understanding of the mechanisms of stable fly dispersal.

During June 1996, we sampled stable flies from resting sites, in the surface layer of the atmosphere, and high in the planetary boundary layer. Thousands of flies were collected. We have determined the sex and mated status for a large subsample and the flies are currently being graded to determine their chronological and physiological ages.

Flies were collect at locations proximal and distal to feedlots on the Kansas State University Agricultural Extension and Research Station in Garden City, KS. We made approximately 75 truck collections with large nets (in a variety of configurations that included 1, 2, and/or 3 m heights) over a variety of routes that included circling the feedlots immediately outside the fence, loops around feedlots at a distance of 1 to 2 km, and collection runs over roads that were more than 5 km away from the nearest feedlot. We also deployed a large number of Alsynite (attractant) traps in a grid pattern around a large feedlot and at sites along country roads among wheat fields between 3 to 10 km from the nearest feedlot.

Finally, we deployed a net from a remotely controlled aircraft that flew at a variety of altitudes between 5 and 50 m above the feedlots. Air temperature and humidity, wind speed and direction, air pressure, soil temperature, solar radiation, and precipitation were measured and averaged or totaled over half hour intervals at the Agricultural Station throughout the two-week study period

Although we have not completed the physiological and chronological age grading analyses, a number of important findings have emerged from our field project. During the daylight hours, flies often moved from the feedlots to nearby resting sites on fences and other structures, on plants in corn and wheat fields, and on the lower branches of trees and bushes. During these trips the flies flew within their flight boundary layer at altitudes less than 3 m. Flies were caught in the truck trap nets at the 1, 2, and 3 m levels at a ratio of approximately 25:4:1. Despite numerous airplane sorties, we were unable to catch a single fly in the planetary boundary layer above their flight boundary layer. Many aerial samples were obtained above the feedlots during early morning and evening hours while flies were moving within their flight boundary layer from the feedlots to resting sites in nearby fields. Even when winds were light and the airplane was able to sample the atmosphere 3 m above the backs of the cattle (4-5 m altitude), we were unable to capture flies with the aircraft mounted net.

We collected flies from the distal traps (3 - 10 km away from the nearest feedlots) each day. We have not been able to identify spatial or temporal patterns in the number of flies collected at these site. On some days there would be very few or no flies in the distal traps while on other days there would be as many as 200 flies in a single trap and very few in the others. We did not have a day when flies were abundant in more than one or two of the distal traps.

Thunderstorms with updrafts that might carry flies long distances and deposit them in high concentrations far from feedlots were not present in the Garden City area during some of the 24 hr periods when flies were found in great abundance in the distal traps. For this reason, we suspect that stable flies are traveling long distances within their flight boundary layer. We have yet to determine a wind speed above which the flights are primarily downwind.

Preliminary laboratory analysis has revealed physiological differences between populations of flies caught in the stationary traps near the feedlot and those in more distal locations. In many cases, the physiological ages of the flies caught at the distal locations were younger than the ages of the flies caught near the lots. It appears that at least some stable flies are moving long distances within their flight boundary layer before they are maturing eggs.

This year, for the first time ever, there have been numerous reports from ranchers in the Garden City area that flies are appearing in high numbers on cattle grazing several kilometers from known larval development sites. This new development is consistent with our findings and leads to the speculation that stable flies are altering their behavior to disperse long distances within their flight boundary layer in search of their initial blood meal.

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