Because of the high growth potential of Eurasian watermilfoil (up to 10cm per day), and ability to grow at cool water temperatures (10 degrees C, at the threshold for successful weevil development (Mazzei et al. unpublished)), it is unlikely that any invertebrate herbivores will be able to control healthy Eurasian watermilfoil populations early in one growing season. The plants will be able to outgrow herbivore damage until later in the summer, unless other stressors are operating. Thus effects from one growing season to the next must be important for effective long term control. Examination of these effects is difficult, but is essential to being able to predict control and understand long term control mechanisms.
Plant response to herbivores and competition from other plants can be as important to successful biological control of weeds as control agent densities (Newman et al. 1998). Observations at our five study sites suggest that plant competition and a positive response by native plants to habitat left open by damaged milfoil may be important for successful suppression of Eurasian watermilfoil. Sites that have retained or increased native plant diversity have been less likely to have continued increases or rebounds in Eurasian watermilfoil populations. Stem mining does reduce both carbohydrate concentrations and stocks (quantity per plant) in both shoots and roots. These reductions may influence overwinter survival and spring regrowth (Madsen 1993). Reductions in carbohydrate stores from weevil damage may be important, however, if native plants do not colonize these areas the Eurasian watermilfoil can regrow without competitive stress. Investigations on the role of competition, sediment nutrients, carbohydrate status and plant community response is ongoing.