Bioassay and Efficacy of Bacillus thuringiensis and an Organosilicone Surfactant Against the Citrus Leafminer (Lepidoptera: Phyllocnistidae)

Abstract

Two laboratory bioassays were developed for testing insect pathogens for control of larval citrus leafminer, Phyllocnistis citrella Stainton, in isolated citrus leaves. An organosilicone surfactant and several commercial preparations of Bacillus thuringiensis (Berliner) (Bt) were tested in these bioassays. One bioassay tested the effect of direct contact of a test mixture on second or third instars by injecting the mixture into a leaf mine surrounding the head of a larva. The second bioassay tested the effect of a test mixture when topically applied to the surface of a leaf mine above a larva. Effects in this second bioassay were only observed if the mixture or its components first penetrated through the mine epidermis, and then contacted and affected the insect within the mine. In both bioassays, blue food coloring was included as a tracer to observe movement of liquid into mines and through the gut of the larva. An organosilicone surfactant, Silwet L-77, enhanced penetration of solutions into mines and killed larvae when applied topically at adequate concentrations (LC50 = 0.026% in water) or when injected into mines (LC50 = 0.011%). Three commercial Bt preparations were active when applied topically at high concentrations or when injected at lower concentrations. When L-77 was added at its LC10 (0.01%) to Bt in the topical bioassay, activity of one of the three Bt preparations, or activity of all three preparations considered together, was significantly higher than without the L-77. Up to 90% mortality resulted at 0.01% L-77 and 20% Bt. Mortality positively correlated with dose of all of the Bts when topically applied with 0.01% L-77. These results suggested that an increased concentration of surfactant and decreased concentration of Bt at commercial rates might be effective against citrus leafminers in citrus groves or nurseries. This was tested on old nursery trees (3 yr after planting) and young grove trees (2 yr after planting). Application of any of the three Bts plus L-77 in the nursery resulted in reduced populations of live larvae and pupae in new foliage, to approximately 50% of the population in water-treated controls. Bts without L-77 yielded counts that were intermediate to, but not significantly different from, control and combined Bt-L-77 treatments. Bt applied with L-77 in the grove did not significantly reduce percentage damage to leaves and number of larvae per leaf when assessed at 14 days. However, when assessed at 21 days, damage and number of larvae were significantly reduced by treatment with Bt plus L-77 or L-77 alone. Bt and L-77 reduced leaf damage and larval numbers to a level not significantly different from treatment with Agri-Mek and spray oil. Assessed at 14 or 21 days, the L-77 treatment resulted in significant reduction of larvae.