Use of the Occurrence of Peak Colorado Potato Beetle (Coleoptera: Chrysomelidae) Egg Hatch for Timing of Bacillus thuringiensis Spray Applications in Potatoes

Abstract

Two field experiments were done to evaluate spray timing relative to peak egg hatch, of Bacillus thuringiensis var. san diego (M-One Insecticide, Mycogen Corp., San Diego, Calif.) for control of Colorado potato beetle, Leptinotarsa decemlineata (Say) in potatoes. In the first experiment, 186.1 ± 4.5 (x ± SEM) cumulative DD ( n = 3 yr, 10°C base threshold) were recorded from first oviposition by overwintered beetles until peak (33-64%) egg mass hatch. In 1989, when no rainfall occurred during the period when spray treatments were made, effective potato beetle control was achieved with B. thuringiensis if the first spray was applied within 9 d of peak egg hatch and subsequent sprays were applied at either 5- or 10-d intervals. In 1990, when heavy rainfall occurred after the first spray, spray interval (5 or 10 d) had a greater effect on potato beetle control than did timing of the first spray application, and only the 5-d schedule treatments resulted in effective control. Results of a second field experiment in 1990 demonstrated that a 7- or 10-d interval between sprays of B. thuringiensis (following the initial spray at peak egg hatch) was too long to result in acceptable control of Colorado potato beetle if rainfall occurs after the initial spray. However, a 10-d spray schedule using an encapsulated formulation of B. thuringiensis var. san diego (MYX-1806, Mycogen Corp., San Diego, Calif.) resulted in effective control of Colorado potato beetle. Methods for estimation of peak potato beetle egg hatch in potatoes for use in timing of B. thuringiensis spray applications are presented.