Host plant pubescence: Effect on silverleaf whitefly Bemisia argentifolii fourth instar and pharate adult dimensions and ecdysteroid titer fluctuations

摘要

The ability to generate physiologically synchronous groups of insects is vital to the performance of investigations designed to test insect responses to intrinsic and extrinsic stimuli. During a given instar, the silverleaf whitefly, Bemisia argentifolii, increase in depth but not in length or width. A staging system to identify physiologically synchronous 4^th instar and pharate adult silverleaf whiteflies based on increasing body depth and the development of the adult eye has been described previously. This study determined the effect of host plant identity on ecdysteroid fluctuations during the 4^th instar and pharate adult stages, and on the depth, length and width dimensions of 4^th instar/pharate adult whiteflies. When grown on the pubescent-leafed green bean, tomato and poinsettia plants, these stages were significantly shorter and narrower, but attained greater depth than when grown on the glabrous-leafed cotton, collard and sweet potato plants. Thus, leaf pubescence is associated with reduced length and width dimensions, but increased depth dimensions in 4^th instars and pharate adults. For all host plants, nymphal ecdysteroid titers peaked just prior to the initiation of adult development. However, when reared on pubescent-leafed plants, the initiation of adult development typically occurred in nymphs that had attained a depth of 0.2 to 0.25 mm (Stage 3 – 4). When reared on glabrous-leafed plants, the initiation of adult development typically occurred earlier, in nymphs that had attained a depth of only 0.15–0.18 mm (Stage 2 Old - early 3). Therefore, based on ecdysteroid concentration, it appears that Stage-2, -3 and -4/5 nymphs reared on pubescent-leafed plants are physiologically equivalent to Stage-1, -2 Young and -2 Old/3, respectively, nymphs reared on glabrous-leafed plants. The host plant affected the width but not the height of the nymphal-adult premolt ecdysteroid peak. However, leaf pubescence was not the determining factor. Thus, host plant identity affects physiological events as well as structural characteristics during whitefly nymphal and adult development.

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EIA

enzyme immunoassay

class="kwd-title">Keywords: whitefly-plant interactions, 20-hydroxyecdysone class="head no_bottom_margin" id="s1title" style="text-transform: uppercase;">IntroductionThe silverleaf whitefly, Bemisia argentifolii, is polyphagous, attacking more than 500 different species of plants including food, fiber and ornamental species. This homopteran causes hundreds of millions of dollars of damage in crop losses each year by feeding on plant phloem, transmitting plant pathogenic viruses and producing honeydew that causes stickiness in cotton and other produce and supports the growth of sooty mold (; ). Recently, precise staging systems for identifying silverleaf whitefly instars and for tracking developmental progress of 3^rd instar and of 4^th instar/pharate adult whiteflies grown on green bean plants has been described (). During any given instar, length and width measurements remain the same while depth increases (; ,). Thus, the staging systems for 3^rd instars and for 4^th instar/pharate adults, respectively, were based wholly or partly on increasing body depth. The development of a precise staging system for silverleaf whitefly 3^rd and 4^th instars and pharate adults made it possible to monitor ecdysteroid (the molting hormone, its precursors and metabolites) titer fluctuations during these stages and to determine premolt peak chronology and size. The external appearance of the silverleaf whitefly as well as its developmental time have been reported to vary depending upon the identity of the host plant (; ; ; ; ). Thus, it became important to determine if the nature of the host plant affects developmental progress and ecdysteroid titer fluctuations in 4^th instar and pharate adult silverleaf whiteflies. Here we report that host plant identity influences not only mean length and width of silverleaf whitefly nymphs and pharate adults, but also depth attained. We also describe the effect of the host plant on the timing, magnitude and shape of the ecdysteroid peak in last instar and pharate adult silverleaf whiteflies.