Effects of a potential biocontrol agent, Euhrychiopsis lecontei, on Eurasian watermilfoil in experimental tanks

Eurasian watermilfoil (Myriophyllum spicatum L.) is a submersed macrophyte, exotic to North America where it is a major nuisance. One potential biological control agent is the native weevil Euhrychiopsis lecontei (Dietz) (Curculionidae). To determine the effects of known densities of the weevil on Eurasian watermilfoil in controlled conditions, we stocked weevils into outdoor 0.38 m³ tanks containing watermilfoil. Watermilfoil stems (20 cm long; 150 m^-2) were planted in each tank and given 3 weeks to root and grow. Plants developed extensive roots and grew 10-15 cm prior to stocking. Weevils were then applied at four stocking densities (0, 6, 12 or 24 adults) to 16 tanks arrayed in a Latin square design. Plant length was measured and weevils were counted weekly in the tanks. Sediment nutrients, plant mass and nutrient content, and weevil densities were determined from a sample of plants prior to stocking and at 3 and 4 weeks after stocking. After 4 weeks, all plant material was removed, biomass was determined and weevils were counted. Weevils survived in the stocked tanks and eggs appeared soon after introduction. There was an average of over 200 weevils (25 adults) in each of the stocked tanks at 4 weeks. Weevil stocking density resulted in a significant decline in watermilfoil biomass (P < 0.005) with biomass in the tanks stocked with 24 weevils reaching only 40% of the control. Root biomass also declined with weevil density (P < 0.005) and biomass in the high density tanks reached only 55% of the control. However, detached sunken and floating watermilfoil biomass increased with density of weevils and no change in total above ground biomass (sum of standing, floating and sunken watermilfoil) was found, indicating that effects were caused by plant damage rather than direct consumption. Periodic estimates of plant height, mass and root mass showed that effects on plants resulted from reductions in stems and roots from peak levels at Weeks 2 or 3 and were thus not simply a suppression of growth. Percent sugars and total nonstructural carbohydrates declined with stocking density in both roots and shoots. The total stock (g per plant) of sugars, starch and total nonstructural carbohydrates was reduced in the roots. Weevil densities of ≈ 300 m^-2 can have rapid and substantial effects on Eurasian watermilfoil both above and below ground. Herbivory by weevils may have long term effects via disruption of plant carbohydrate stores that are essential for overwinter survival and subsequent regrowth.