EFFECTS OF A POTENTIAL BIOCONTROL AGENT, EUHRYCHIOPSIS-LECONTEI, ON EURASIAN WATERMILFOIL IN EXPERIMENTAL TANKS

Eurasian watermilfoil (Myriophyllum spicatum L.) is a submersed macrop hyte, exotic to North America where it is a major nuisance. One potent ial biological control agent is the native weevil Euhrychiopsis lecont ei (Dietz) (Curculionidae). To determine the effects of known densitie s of the weevil on Eurasian watermilfoil in controlled conditions, we stocked weevils into outdoor 0.38 m(3) tanks containing watermilfoil, Watermilfoil stems (20 cm long; 150 m(-2)) were planted in each tank a nd given 3 weeks to root and grow. Plants developed extensive roots an d grew 10-15 cm prior to stocking, Weevils were then applied at four s tocking densities (0, 6, 12 or 24 adults) to 16 tanks arrayed in a Lat in square design. Plant length was measured and weevils were counted w eekly in the tanks. Sediment nutrients, plant mass and nutrient conten t, 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 pl ant material was removed, biomass was determined and weevils were coun ted. Weevils survived in the stocked tanks and eggs appeared soon afte r introduction. There was an average of over 200 weevils (25 adults) i n each of the stocked tanks at 4 weeks. Weevil stocking density result ed in a significant decline in watermilfoil biomass (P <0.005) with bi omass in the tanks stocked with 24 weevils reaching only 40% of the co ntrol. Root biomass also declined with weevil density (P < 0.005) and biomass in the high density tanks reached only 55% of the control. How ever, detached sunken and floating watermilfoil biomass increased with density of weevils and no change in total above ground biomass (sum o f standing, floating and sunken watermilfoil) was found, indicating th at effects were caused by plant damage rather than direct consumption. Periodic estimates of plane height, mass and root mass showed that ef fects 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 growt h. Percent sugars and total nonstructural carbohydrates declined with stocking density in both roots and shoots. The total stock (g per plan t) of-sugars, starch and total nonstructural carbohydrates was reduced in the roots. Weevil densities of approximate to 300 m(-2) can have r apid and substantial effects on Eurasian watermilfoil both above and b elow ground. Herbivory by weevils may have long term effects via disru ption of plant carbohydrate stores that are essential for overwinter s urvival and subsequent regrowth.