Interaction of Insect Size and Bacillus thuringiensis on Larval Mortality of Palpita quadristigmalis and Endoparasitoid Emergence

Abstract

Pest management in ornamentals and landscaped areas relies largely on synthetic pesticides, which usually are non-selective and may negatively impact human health, natural enemies and pollinators. It is important to develop integrated, biorational management plans for ornamentals. Palpita quadristigmalis Guenee (Lepidoptera: Crambidae) was observed in northeastern Mexico in 2017 causing severe defoliation and branch dieback of privet, Ligustrum sp. (extensively grown there as hedge and shade trees). We assessed the effect of Bacillus thuringiensis (Bt) (toxin crystals and spores) on privet leaves upon mortality and foliage consumption by P. quadristigmalis larvae, as well as the effect of Bt on larval parasite emergence. Larvae of three different instars (4th, 5th and 6th) were exposed to three concentrations of B. thuringiensis (DiPel®, Bayer) (1.0, 2.5 and 5.0 g/L) in the laboratory. Mortality increased while foliage consumed (as total area and percent) decreased in response to increasing Bt concentrations. At 48h, mortality values of 65.0-97.5% were observed across Bt-exposed larvae; also, there was a maximum of 2.5% foliar area consumed by all Bt-exposed larvae, compared to foliage consumed by unexposed larvae. Overall, 1.7 % of larvae used in these tests (Bt-exposed and control) were parasitized (Braconidae 25%, Ichneumonidae 12.5% and Tachinidae 62.5%). Bt apparently did not have negative effects on insects that naturally parasitize larvae of P. quadristigmalis. Bacillus thuringiensis-based products can have significant potential for the management of P. quadristigmalis larvae.