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 biological control agent of Eurasian watermilfoil (Myriophyllum spicatum L.). The weevil reduces the viability of milfoil by mining plant stems. We determined the influence of temperature on the developmental rates of the weevil and damage to Eurasian watermilfoil stems. Single E. lecontei eggs were laid on rooted plants in individual tubes filled with water and 16 such tubes were randomly assigned to each of eight environmental chambers set at constant temperatures of 15, 19, 21, 23, 25, 27, 29, and 31°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 watermilfoil stem damaged (cm) was estimated at 21, 25, and 29°C. Developmental rate was linearly related to temperature, up to 29°C; the developmental maximum appeared to be between 29 and 31°C. Average egg hatch occurred in 12.0 days at 15°C and in 4.2 days at 31°C. Average larval development time took 20 days at 15°C and 6.1 days at 31°C. Complete egg to adult development ranged from 16.6 days at 29°C to 61.7 days at 15°C. The lower developmental threshold was between 8.2 and 10.5°C, and egg to adult development required 309 ± 27.6 (2 SE) degree-days above 9.8°C. Daily stem damage increased with temperature but total damage (by larvae) was equal across temperatures and averaged 15.1 ± 1.9 cm. Field temperature data indicated that up to five generations could be completed in a typical summer in Minnesota lakes.