EFFECT OF MANUAL DEFOLIATION ON POTATO YTELD AT MAXIMUM ABUNDANCE OF DIFFERENT STAGES OF COLORADO POTATO BEETLE, LEPTINOTARSA DECEMLINEATA (SAY), IN THE FIELD'·'

Defoliation of 'Kennebec' and 'Superior' potaioes by the Colorado potato hectle (CPR), Leptillotarsa decemlineato (Say), was simulated by hand defoliation in a 5 year field study. Defoliations corresponded to peak population levels of larval and adult CPB. Defoliation was done by removing O. 12.5, 25. 50, 75, 100% of the foliage. lind cutting the stem at ground level, 8S well as removing the top third of the plant. At the time of peak spring and summer adult emergence of CPS, 'Kennebec' and ;Superior' potato plants were able to sustain total defoliation without. significant decrease in yield, When the larvae of this pest. had reached their population peak, the t\...o culti\'ars sllstained approximately 25 and 12.5% defoliation, respectively, without any decrease in yield. !(ey Words: Potatoes, artilicial defoliation, yield, J. Agric. Entomol. 6(4): 217-226 (October 1989) Potatoes are one of the most important agricultural crops in Quebec. and are vulnerable to complete defoliation by the Colorado POll:lto beetle (CPS), Leplinotarsa decemlineala (SAY). In southern Quebec, the CPB has two complete generations pel' year. The adults emerge from the soil in late spring and lay eggs on potato leaves (Mailloux et al. 1988). The eggs hatch and the Imvae develop into pupae in the soil to emerge as adults by the end of July. These summer adults feed intensively for about one week, then some of them enter reproductive diapause, whereas others give rise to a second generation. Defoliation is of concern to potato growers because CPB populations can defoliate and kill the potato plants before tuber initiation. When developing a management program fol' potatoes, it is important to understand plant responses to different levels of defoliation as related to CPB seasonal abundance. Insect-damage simulation studies are often used to establish yield·damage relationships without. considering pest abundance or its economic injury level. A decision to treat based on the level of defoliation would be easier, more direct and rapid, than a decision based on pest abundance. Decreases in potato yield caused by hail have been reported by Tokatori et a1. (1952), Sparks et "'liniSlcre de l'Agricullure, des Pccheries et de l'Alimentation dll Quebec, Conlribution ;# :174. , Agricult.ure Canada. St-Jcnn·sur·RichcliclJ, Contribut.ion ;# 335-8!)'()..1.H. , Research Station, A,::riculture Canllda. St-Jcan-sur-Richclicu Quehec. Canada J3B 6Z8