Dispersal Capabilities of the

Whitefly Parasitoid Eretmocerus eremicus

& Other Insects

David Byrne and David E. Bellamy
Department of Entomology
University of Arizona
Tucson, AZ 85721

We are interested in movement by Eretmocerus eremicus Rose & Zolnerowich, a parasitoid of the sweet potato whitefly, Bemisia tabaci (Gennadius). This aphelinid has shown enough promise as a biological control agent that it has been incorporated into several IPM programs backed by the public and private sectors. A better understanding of dispersal by E. eremicus will assist in designing release systems for this and other parasitoids.

Using a vertical flight chamber we learned that unmated female E. eremicus fly for significantly longer periods of time than unmated males (34 versus 7 min) and that mated females fly for significantly longer periods than male counterparts (10 versus <1 min).

	The dispersal behavior of E. eremicus is also being investigated in the field using fluorescent dust as a marker. In 1999 field experiments traps were placed along annuli placed 3, 5, 7 and 10 m away from release points.
	Eighty-seven percent of the 4,153 parasitoids captured in replicated trials were males. Sex ratios were near parity upon release. Females, as predicted from flight chamber studies, were leaving these relatively small plots.

We speculate that these differences in patterns of dispersal can be explained in terms of resources. The foremost requirement for dispersing males is the need to mate. They may do so in release containers, or in close proximity. Dispersing females may mate in containers or they may postpone mating. We are still in the process of examining spermathecae. They must also find hosts, which were in short supply in these experiments. To do so, they seem to have left plots in 1999 and flew beyond the 10-m annuli.

	In 2000 we expanded our design to 25 m. Even so, the proportion of captured females increased significantly out to the farthest annuli. Since the number captured was diminishing, we allowed for dilution effects, we believe that E. eremicus has relatively limited capability to disperse.

We seem to be on the verge of finding out why inoculative releases of E. eremicus to aid in the control of the sweet potato whitefly are not effective in the Southwest. This may be explained, in a large part, by dispersal capabilities. The sweet potato whitefly has the capacity to fly for hours in laboratory and to travel miles in the field (Blackmer & Byrne 1993, Byrne et al. 1996). By contrast, in laboratory studies involving E. eremicus have shown that this insects ability to disperse are comparatively limited (Table 1) (Bellamy and Byrne 2000). It is our contention that in a discontinuous cropping system such as we find in Arizona (i.e., agricultural areas are often isolated from others by expanses of desert), whitefly parasitoid dispersal can not keep pace with their migrating whitefly hosts. This is in contrast to situations such as those found in Florida and California where herbaceous weeds and crop hosts often form extensive continuums.

Table 1. Flight duration, in minutes, for the whitefly parasitoid Eretmocerus eremicus,
as determined in a vertical flight chamber (total flown = 92).

Mating Status	Unmated	Mated
Females	34.4 a1	10.2 b
Males	6.7 b	0.7 c

In an attempt to explain why sweet potato whiteflies and E. eremicus have different flight characteristics, and by extension different abilities to disperse, we are in the process of comparing wing beat frequencies and wing loading (wing surface area/body mass) values (Byrne et al. 1988). We have already found that wingbeat frequencies are not fixed for a species, rather they are effected by such factors as gender and temperature. If we can explain why whiteflies and these parasitoids are not the best candidates for inoculative releases will lead us to examine other, more effective, methods of deploying biological control agents.

References cited

Bellamy, D. E. and D. N. Byrne. 2000. Effects of gender and mating status on self-directed dispersal by Eretmocerus eremicus (Hymenoptera: Aphelinidae), a whitefly parasitoid. Ecological Entomology submitted.

Blackmer, J. L. and D. N. Byrne. 1993. 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.

Byrne, D. N., S. L. Buchmann and H. G. Spangler. 1988. Relationship between wing loading, wingbeat frequency and body mass in homopterous insects. Journal of Experimental Biology 135: 9-23.

Byrne, D. N., R. J. Rathman, T. V. Orum and J. C. Palumbo. 1996. Localized migration and dispersal by the sweet potato whitefly, Bemisia tabaci. Oecologia 105: 320-328.

Recent publications

Blackmer, J. L. and D. N. Byrne. 1999. Changes in amino acids in Cucumis melo in relation to life-history traits and flight propensity of Bemisia tabaci. Entomologia Experimentalis et Applicata 93: 29-40.

Blackmer, J. L. and D. N. Byrne. 1999. The effect of Bemisia tabaci on amino acid balance in Cucumis melo. Experimentalis et Applicata 93: 315-319.

Byrne, D. N. 1999. Migration and dispersal by the sweet potato whitefly, Bemisia tabaci. Agricultural and Forest Meteorology 97: 309-316.

Isaacs, R., M. A. Willis and D. N. Byrne. 1999. Modulation of whitefly take-off and flight orientation by wind speed and visual cues. Physiological Entomology 24 311-318.

Isaacs, R., M. Cahill and D, N. Byrne. 1999. Host plant evaluation behaviour by Bemisia tabaci and its modification by external or internal uptake of imidacloprid. Physiological Entomology 24: 101-108.

Veenstra, K. H. and D. N. Byrne. 1999. Does dispersal affect the reproductive physiology of the sweet potato whitefly, Bemisia tabaci? Physiological Entomology 24: 72-75.

Bellamy, D. E. and D. N. Byrne. 2000. Effects of gender and mating status on self-directed dispersal by Eretmocerus eremicus (Hymenoptera: Aphelinidae), a whitefly parasitoid. Ecological Entomology submitted.

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