Developmental rates of the native milfoil weevil, Euhrychiopsis lecontei, and damage to Eurasian watermilfoil at constant temperatures

The native aquatic weevil Euhrychiopsis lecontei (Dietz) is a potential bio logical control agent of Eurasian watermilfoil (Myriophyllum spicatum L.). The weevil reduces the viability of milfoil by mining plant stems. We deter mined the influence of temperature on the developmental rates of the weevil and damage to Eurasian watermilfoil stems. Single E. lecontei eggs were la id on rooted plants in individual tubes filled with water and 16 such tubes were randomly assigned to each of eight environmental chambers set at cons tant temperatures of 15, 19, 21, 23, 25, 27, 29, and 31 degrees C with a 16 -h daylength. Weevils and plants were monitored daily and development times were recorded for the egg, larval, and pupal stages. Length of watermilfoi l stem damaged (cm) was estimated at 21, 25, and 29 degrees C. Developmenta l rate was linearly related to temperature, up to 29 degrees C; the develop mental maximum appeared to be between 29 and 31 degrees C. Average egg hatc h occurred in 12.0 days at 15 degrees C and in 4.2 days at 31 degrees C. Av erage larval development time took 20 days at 15 degrees C and 6.1 days at 31 degrees C. Complete egg to adult development ranged from 16.6 days at 29 degrees C to 61.7 days at 15 degrees C. The lower developmental threshold was between 8.2 and 10.5 degrees C, and egg to adult development required 3 09 +/- 27.6 (2 SE) degree-days above 9.8 degrees C. Daily stem damage incre ased with temperature but total damage (by larvae) was equal across tempera tures and averaged 15.1 +/- 1.9 cm. Field temperature data indicated that u p to five generations could be completed in a typical summer in Minnesota l akes. (C) 1999 Academic Press.