Degree-day model for timing insecticide applications to control Dioryctria amatella (Lepidoptera: Pyralidae) in loblolly pine seed orchards
Because Dioryctria amatella (Hulst) is a key pest in loblolly pine, Pinus taeda L. (Pinaceac), seed orchards in the southeastern United States, improved timing of insecticide applications would be valuable for its control. To time two fenvalerate (Pydrin® 2.4 EC) applications we tested four variations of a degree day model that was developed to predicted when various proportions of D. amatella eggs would hatch during the spring generation. WC compared reductions in Dioryctria spp. cone damage to unsprayed checks and a standard operational spray regime of four monthly applications of fenvalerate. In addition, we examined seeds from healthy cones to determine if sprays to control D. amatella also reduced seed damage caused by Leptoglossus corculus Say (Heteroptera: Coreidae) and Tetyra bipuctata (Herrich-Schäffer) (Heteroptera: Scutelleridae). Trials were conducted from 1984 to 1986 in two orchards in South Carolina and one in Alabama. Degree day accumulations (threshold = 11°C) were begun on the day when the cumulative number of male D. amatella equaled or exceeded five captured in IS Pherocon 1C® traps baited with 100 mg of Z-11-hexadecenyl acetate. One application per year was insufficient to control D. amatella or reduced seed-bug damage. Two sprays based on D. amatella phenology significantly reduced coneworm and seed bug damage, and were as effective as four sprays applied monthly. None of the treatments reduce spring cone losses, which are primarily caused by Dioryctria merkeli Mutuura and Monroe. Several variations of the model performed well, but we suggest that the best, based on efficacy and ease of use, was when sprays were applied immediately alter five males were caught (degree-day = 0) and again when the model predicted 50% of the spring generation eggs had hatched.