Effects of fungicides on a mycophagous coccinellid may represent integration failure in disease management

Abstract The adults and larvae of halyziine coccinellids (Coleoptera: Coccinellidae: Halyziini) are obligate mycophages on hyphae and conidia of powdery mildew (PM) (Erysiphales) fungi, that are plant pathogens warranting chemical control in many managed systems. These insects have been observed to reduce PM severity through consumption. Fungicide applications, however, may interfere with this ecological service. Five commercial fungicides were topically applied to the mycophagous coccinellid, Psyllobora vigintimaculata , in the laboratory to gauge contact toxicity. In order to detect interference in the field, population density of naturally occurring P. vigintimaculata was assessed weekly in a northern California vineyard ( Vitis vinifera , cultivar “Chardonnay”) over 3 years in relation to PM ( Erysiphe necator ) severity and in the presence of various fungicides. Wettable sulfur was toxic to adults in the laboratory, resulting in complete cohort mortality 24 h after application. Topical applications of a strobilurin fungicide (trifloxystrobin) and a demethylation inhibitor fungicide (myclobutanil) also resulted in significant adult mortality. Rapid and complete larval mortality was observed in the laboratory after contact with wettable sulfur and myclobutanil. There was no effect on survival after contact with the PM-antagonistic bacterium, Bacillus subtilis . Vineyard density of P. vigintimaculata was reduced in vines receiving applications of sulfur and myclobutanil, even when considering the covariate PM severity. The microbial antagonist, Streptomyces lydicus , did not significantly affect insect density. Our study questions the integration of chemical disease management with naturally occurring mycophagous agents in some agricultural systems.