Host plant effects on activity of the mitosporic fungi Beauveria bassiana and Paecilomyces fumosoroseus against two populations of Bemisia whiteflies(Homoptera : Aleyrodidae)

Laboratory bioassays were conducted to determine the effect of host plant o n mycosis in two geographically distinct populations of early 2nd-instar ny mphs of Bemisia argentifolii Bellows & Perring from the entomopathogenic fu ngi Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces fumosoroseus (W ize) Brown & Smith. Mycosis in B. argentifolii nymphs varied according to t he host plant on which the nymphs were reared but not according to the popu lation. Both populations of whiteflies reared on cotton were consistently s ignificantly less susceptible to infection by either fungus than when reare d on melon. We hypothesized that the cotton plant produced a fungal inhibit or that may confer protection on whiteflies feeding (and possibly sequester ing) upon it. Germination of conidia of both fungi was strongly inhibited ( below 12% germination) on the cuticle of nymphs reared on cotton but was ov er 95% on the cuticle of nymphs reared on melon. We further hypothesized th at the terpenoid gossypol, produced by many cultivars of cotton, might have been involved in antibiosis. Gossypol mixed with Noble agar at five concen trations was tested for its effects on germination of conidia of both fungi . P. fumosoroseus was highly tolerant of gossypol, even at the relatively h igh concentration of 1000 ppm, while B. bassiana tolerated gossypol at conc entrations up to 500 ppm and strong inhibition only occurred in presence of gossypol at 1000 ppm. Our in vivo findings on cotton and on the insect's c uticle pointed at a potential host plant-mediated antibiosis. The in vitro tolerance of P. fumosoroseus and partial tolerance of B. bassiana to gossyp ol disagreed with our in vivo data. Gossypol concentrations higher than 100 0 ppm might have increased the sensitivity of the fungi in our in vitro tes ts. Sequestered gossypol (and/or other cotton plant allelochemicals) by B. argentifolii nymphs would explain, at least partially, the insect's defense against the pathogens.