|Blackburn, Michael - Mike|
Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/26/2003
Publication Date: 8/29/2003
Citation: GELMAN, D.B., BLACKBURN, M.B. AGE-SPECIFIC INTERACTION BETWEEN ENCARSIA FORMOSA (HYMENOPTERA: APHELINIDAE) AND ITS HOST, THE SILVERLEAF WHITEFLY, BEMISIA AGRETIFOLII (HOMOPTERA: ALEYRODIDAE). JOURNAL OF INSECT SCIENCE. 2003. Vol. # 3 Pg. 28 Interpretive Summary: Whiteflies are tiny insects that attack more than 500 different species of food, fiber and ornamental plants causing crop losses that total hundreds of millions of dollars each year. Concern about the development of pesticide resistance in whiteflies as well as the need to reduce pesticide usage because of environmental concerns has resulted in increased emphasis on the use of whitefly natural enemies such as the parasitic wasp, Encarsia formosa. This wasp has been found to successfully control the greenhouse whitefly (GHWF) and to a lesser extent, the silverleaf whitefly (SLWF) in greenhouses. Because E. formosa is such a good biological control agent, there has been considerable interest in artificially rearing this wasp. Therefore, it became important to track wasp development in the SLWF and to determine why the GHWF is a more suitable host than the SLWF. Here we report that as was true for the GHWF, parasitization of the SLWF 3rd or early 4th stage is the best for promoting synchronous development and adult emergence of the parasitic wasp, and that the initiation of adult development in the SLWF is a prerequisite for the parasite's molt from the 2nd to the 3rd larval instar. Our results suggest that the GHWF may be the preferred host for E. formosa because of the more leisurely developmental rate of the parasite in the GHWF, differences in the concentration of molting hormone and/or differences in body size, body shape and cuticle thickness in the two species of whiteflies. Information generated will contribute to the design of artificial rearing systems for this useful parasitic wasp.
Technical Abstract: The effect of host age (instar parasitized) on the growth and development of Encarsia formosa was studied. E. formosa was able to parasitize and complete its life cycle no matter which instar of Bemisia argentifolii was provided for oviposition, but parasitoid development was significantly slower when 1st or 2nd instar rather than 3rd or 4th instar nymphs were parasitized. The parasitoid egg was typically deposited in the vicinity of the ventral ganglion. Host age had no significant effect on the day in which E. formosa nymphal hatch was first observed, i.e., on the 3rd day post-oviposition. However, mean durations for embryonic development and for the development of the 1st instar parasitoid and the pupa were significantly longer when 1st rather than 3rd or 4th instar nymphs were parasitized. Interestingly, no matter which instar was parasitized, the parasitoid did not molt to the 3rd instar until the 4th instar host had reached a depth ?0.23 mm (Stage 4-5) and had initiated adult development. Histological studies revealed that eye and wing structures were either in the process of disintegrating or were adult in nature whenever a 3rd instar parasitoid was present. It appears, then, that the parasitoid's molt to its last instar is associated with the host whitefly's nymphal-adult molt. However, the initiation of the host's final molt, while a perquisite for the parasitoid's 2nd-3rd instar molt, did not necessarily trigger this molt. Although host instar parasitized had a significant effect on various aspects of parasitoid development, it did not significantly influence the mean size of the parasitoid larva, pupa, or adult nor did it affect percent emergence rate or adult longevity. However, adult parasitoid emergence was more synchronous when 2nd, 3rd and 4th instar nymphs were parasitized than when 1st instars were parasitized. Results are compared with those reported for the Trialeurodes vaporariorum/E. formosa system and provide possible explanations for why the GHWF is a more suitable host than the SLWF for E. formosa.