Eurasian watermilfoil (Myriophyllum spicatum L.) is a non-native, aquatic, invasive species common throughout North America. The native aquatic milfoil weevil (Euhrychiopsis lecontei Dietz), whose natural host is the native northern watermilfoil (Myriophyllum sibiricum Komarov), has expanded its range to include M. spicatum. Previous studies show that it prefers the non-native Eurasian watermilfoil over native watermilfoils for feeding and oviposition. Previous studies also suggest that milfoil weevils that develop on M. spicatum have shorter development times and a greater mass. Eurasian watermilfoil and northern watermilfoil have hybridized, and hybrids can be more invasive than their parent species. One hypothesized mechanism for increased invasiveness in the hybrid watermilfoil is resistance to the milfoil weevil. To test for resistance, we compared development times, mass, and survival of the milfoil weevil reared on Eurasian, northern, and the Eurasian × northern hybrid watermilfoils. We followed weevil development from egg to adult on individual rooted plants (n = 17–20 for each taxon) in laboratory experiments at 26°C. Neither development times (total and individual stages) nor adult eclosion mass were significantly different among hosts. Mean development time from egg to adult ranged from 19.7 to 20.3 d, and mean mass ranged from 1.3 to 1.5 mg. Weevil survival rates differed significantly among the taxa and were lowest on northern watermilfoil (45%), intermediate on the hybrid (61%), and highest on Eurasian watermilfoil (88%), but stem diameter may account for some of these differences. This study suggests that hybrid watermilfoil is not exceptionally resistant to milfoil weevil herbivory but rather possesses resistance intermediate between the native and exotic hosts.
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