Submitted to: International Conference on Arthropods Chemical Physiological ...
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2001
Publication Date: N/A
Interpretive Summary: Whitefly activity results in crop losses that total hundreds of millions of dollars. Environmental considerations concerned with pesticide use and reports of the development of pesticide resistance in whiteflies have resulted in increased emphasis on the use of cost-effective biological control strategies in IPM programs. These include the enhanced use of natural enemies such as parasitic wasps and the development of environ mentally-safe biopesticides. However, little information is available concerning the regulation of whitefly development, information that is essential for identifying hormonal systems to target and times to apply treatments, and for improving upon/developing new biological control technology. Here, we review the development of a system of markers for identifying physiologically synchronous 3rd and 4th stage whitefly nymphs, and present timetables for the final two whitefly molts. The effect of whitefly stage parasitized on the growth, development, longevity and vitality of the parasite, Encarsia formosa, is also described. Parasite developmental time is the shortest and parasites are more robust when 3rd and 4th stage nymphs are parasitized. In order for the parasite to undergo its last larval molt, it must receive a signal from its whitefly host, a signal associated with the whitefly's molt to its adult stage. Information generated concerning the control of whitefly molting should be useful to other scientists whose focus is the discovery of new biopesticides and the mode of action of these control agents. In addition, for the artificial production of large numbers of natural enemies (such as insect parasites) to control whitefly damage, researchers must have information concerning the interactions that occur between the whitefly host and its parasite.
Precise staging systems for identifying whitefly instar and for obtaining physiologically synchronous third instar and fourth instar/pharate adult whiteflies have been developed. Results from histological studies and/or whole body ecdysteroid determinations indicate that the larval molt from the third to the fourth instar occurs just prior to Stage 5 of the third instar and that the larval-adult molt occurs during Stages 4/5 or six (depending on whitefly species) of the last instar. Metamorphosis to the adult is characterized by an anterior to posterior gradient with the adult eye beginning to form before the wing and the anterior portion of the wing beginning to invaginate before the posterior portion of the wing. The aphelinid wasp Encarsia formosa exhibits more synchronous development when third or young fourth instar whiteflies are selected for parasitization than when younger host instars are used for oviposition. When older instars sare parasitized, adult eclosion is also more synchronous and an adult wasp longevity is greater. Parasitoid egg hatch and the molt from the first to the second instar do not appear to be triggered by host cues. However, in the greenhouse whitefly, the parasitoid's molt from the second to the third instar is never observed until the host has reached Stage 5, the stage associated with early pharate adult development. Therefore, a cue(s) that occurs at the onset of host metamorphosis appears to trigger the final larval molt of E. formosa.