Differences in pathogenesis and virulence against insects among isolates of the fungus Paecilomyces fumosoroseus (Plutella xylostella)

Diamondback moth, Plutella xylostella, larvae damage various crucifer crops worldwide. Alternative management tools are important because of insecticide resistance. The fungus Paecilomyces fumosoroseus is registered for use against diamondback moth in the U.S.; Diamondback moth second instars were sprayed with 8 Paecilomyces fumosoroseus isolates at ≈ 4,000 conidia/cm2. The most virulent isolates (≤47% larval mortality), including ARSEF 4461, came from Homoptera. The least virulent isolate (2% larval mortality) was ARSEF 1576. At ≈ 6 x 105 conidia/cm2, isolate 1576 infected 9% of second instars. Infection by isolate 1576 was not enhanced by pregermination or passage through diamondback moth by injection.; The original host may have been stressed or wounded before being colonized by isolate 1576. Mortality of diamondback moth larvae dipped in isolate 1576 was higher when starved (40%) than fed (10%). Injected conidia of isolate 1576 infected 44% of larvae.; Isolate virulence was correlated with conidial length and germination speed in broth. Blastospores of isolate 1576 are shorter than those of isolate 4461, and they germinate and proliferate more slowly in broth. However, isolate 1576 grew faster and produced more conidia than isolate 4461 on agar.; Fewer conidia of isolate 1576 attached to larval cuticle, compared to more infective isolates (P < 0.05). After 36 h on larval cuticle, the percentages of spores germinated for isolates 1576 and 3682 were 3% and 95%, respectively. Unlike more virulent isolates, isolate 1576 did not elicit melanized lesions on cuticle. Hyphae of virulent isolate 4461, but not isolate 1576, were visible in cuticle cross-sections of diamondback moth and fall armyworm, Spodoptera frugiperda, ≤30 h after inoculation. Isolate 4461 proliferated faster than isolate 1576 in hemolymph of fall armyworm. The reasons for poor attachment, germination, penetration, and proliferation of isolate 1576 in vivo remain to be determined.; The sawfly Acantholyda erythrocephala was similar to fall armyworm in susceptibility to injected blastospores of isolates 1576 and 4461. In small (<0.3 g) fall armyworm larvae injected with blastospores, isolate 1576 caused less mortality and took longer to infect than isolate 4461. Large fall armyworm larvae were equally susceptible to both isolates when injected.