Laboratory and Field Evaluation of

Flight by the Whitefly Parasitoid Eretmocerus

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

The sweet potato whitefly (crawler stage, left) continues to concern Arizona cotton and vegetable growers. This is in spite of the success imidacloprid, Admire®, on vegetables and buprofezin, Applaud®, on cotton. Because of signs of developing resistance to both compounds (Denholm et al. 1998, Dennehy et al. 1999), we need to continually search for methods other than chemical control. Alternative strategies now have added significance because more and more enterprises are interested in organic farming.

The use of biological control agents, such as the whitefly parasitoid Eretmocerus spp., (right) has demonstrated promise in the past (Simmons and Minkenberg 1994). In order for this strategy to be completely successful, however, much more has to be learned about proper ways to deploy these insects under field conditions. Our laboratory has already developed some expertise in insect movement.

Important questions that are currently unanswered concern the method of parasitoid release and the behavior of commercially produced natural enemies. In attempts to address these, we have learned a number of things. In a vertical flight chamber (left) we determined that unmated parasitoid females fly for an average of 34.35 min, mated females for 10.15 min, unmated males for 6.68 min and mated males for only 0.68 min (Table 1)

 

Table 1. Flight duration, in minutes, for the whitefly parasitoid Eretmocerus eremicus, as determined in a vertical flight chamber.

SEX/MATING STATUS	Unmated	Mated
Males	6.68 a*	0.68 b
Females	34.35 c	10.15 a

*Numbers followed by the same letter are not significantly different according to a Tukey-Kramer multiple comparisons test.

This would indicate the flight behavior of females is most responsible for avoiding mating between closely related individuals. More importantly, it provides an explanation for field data collected last summer at the Marana Agricultural Center. In replicated field trials employing the design shown in Fig. 1, we captured almost no females.

Figure 1. Field plot design showing layout of traps (filled circles) on annuli of 3-m, 5-m, 7-m, and 10-m. The filled circle in the center at the intersection of the sector lines is the release point. Each field was divided into 6 sectors for statistical analysis.

Actual field  Release point

Fan traps were placed in plots during the course of the summer. We captured 4,101 wasps in our four 20-m plots. Of these, 3,555 were male and only 546 were female. In addition, male trap catches fell dramatically with distance from the release site (Fig. 2). All most none were captured in the 10 ­m annuli. We had carefully considered previous data (Byrne et al. 1996) when deciding on the scale of our experiment. The scale chosen provided a great deal of information concerning male dispersal, but clearly plot size needs to be expanded to draw conclusions about female movement. We are searching for reasons why there are such dramatic differences in flight behavior (both in the field and laboratory) that are apparently tied to gender and mating status.

Figure 2. Histogram showing mean trap count of male and female E. eremicus as a function of distance from the release point.

We have also been examining how post harvest handling procedures affect parasitoid behavior. Wasps are said to have been harvested when parasitized whitefly pupae are removed from plants. Commercial operations such as Novartis BMC routinely chill their wasps to 12º C for 48 hr. before shipping. We have found that this has a profound effect on emergence patterns and mortality (Fig. 3). Wasp emergence is delayed for at least two days after field deployment and mortality (particularly among females) increases significantly. This information should be critical when deciding on field release strategies.

Figure 3. Mean 72 hour emergence patterns for five replicates of 1,650 chilled (shaded) and unchilled (unshaded) E. ethiopia. A two day delay in emergence is apparent in the chilled specimens.

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.

Denholm, I., M. Cahill, T. J. Dennehy and A. R. Horowitz, 1998. Challenges with managing insecticide resistance in agricultural pests, exemplified by the whitefly Bemisia tabaci. Philosophical Transactions of the Royal Society of London B 353: 1757-1767.

Dennehy, T. J. M. Wigert. X. Li and L. Williams III. 1999. Arizona whitefly susceptibility to insect growth regulators and chloronicotinyl insecticides: 1998 season. University of Arizona Cotton Report 15 pp.

Simmons, G. S. and O. P. J. M. Minkenberg. 1994. Field-cage evaluation of augmentative biological control of Bemisia argentifolii (Homoptera, Aleyrodidae) in southern California cotton with the parasitoid Eretmocerus-nr-californicus (Hymenoptera, Aphelinidae). Environmental Entomology 23: 1552-1557.

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