Bacillus thuringiensis subspecies kurstaki reduces competition by Parapoynx diminutalis (Lepidoptera: Crambidae) in colonies of the hydrilla biological control agent Cricotopus lebetis (Diptera: Chironomidae)

The hydrilla tip mining midge, Cricotopus lebetis Sublette (Diptera: Chironomidae), is a biological control agent of the invasive aquatic weed hydrilla, Hydrilla verticillata (L.f.) Royle (Hydrocharitaceae). Mass rearing of the midge for augmentative releases can be compromised by competition from the adventive hydrilla leafcutter moth, Parapoynx diminutalis Snellen (Lepidoptera: Crambidae). The objective of this study was to evaluate the biorational pesticide Bacillus thuringiensis (subspecies kurstaki) (Btk) for reducing competition by the moth in laboratory colonies of the hydrilla tip mining midge. Dose response bioassays with Btk were completed with both species to determine the minimum concentration that kills the moth without affecting midge production. Adult moth emergence was significantly reduced at concentrations of 2.0 mL Btk per 3.8 L of well water. A concentration of 0.2 mL Btk per 3.8 L of well water also reduced the number of emerging moths by 50%. However, midge development declined, although not significantly at the higher 2.0 mL Btk concentration compared with the control. Therefore, concentrations of 0.2 or 2.0 mL Btk per 3.8 L of well water would be appropriate for controlling competition by P. diminutalis in C. lebetis colonies. The lower concentration (0.2 mL Btk) would be appropriate for maintenance control due to its minimal impact on the midge. However, a severe infestation of P. diminutalis might justify the higher concentration of 2.0 mL Btk for maximum impact on the moth. Implementing selective pest management strategies in a biological control agent colony can help increase production and improve the quality of the insects.