Chemically Mediated Host-Plant Selection by the Milfoil Weevil: A Freshwater Insect–Plant Interaction

Abstract

The milfoil weevil Euhrychiopsis lecontei is a specialist aquatic herbivore that feeds, oviposits, and mates on the invasive freshwater macrophyte Myriophyllum spicatum. We characterized the weevil's preference for M. spicatum, and through bioassay-driven fractionation, isolated and identified two chemicals released by M. spicatum that attract E. lecontei. Mass spectrometry and nuclear magnetic resonance spectroscopy were used to identify the attractive compounds as glycerol and uracil. Dose-response curves for glycerol and uracil indicated that weevil preference increased as sample concentration increased. Weevils were attracted to a crude sample of M. spicatum-released chemicals from 0.17 to 17 mg/l, to glycerol from 18 to 1800 microM (0.0017-0.17 mg/l), and to uracil from 0.015 to 15 microM (0.00014-1.4 mg/l). Although glycerol and uracil are ubiquitous, weevils are likely responding to high concentrations that are released as a result of the rapid growth of M. spicatum. Uracil concentration was greater in the exudates of M. spicatum than other Myriophyllum spp. E. lecontei was attracted to glycerol at a concentration similar to that at which terrestrial insects are attracted to sugar alcohols. This is the first example of a freshwater specialist insect being attracted to chemicals released by its host plant. Analysis of the water milfoil-weevil interaction provides further understanding as to how insects locate their host plants in aquatic systems.

Ubiquitous Water-Soluble Molecules in Aquatic Plant Exudates Determine Specific Insect Attraction

“…sea urchins [19], [20] and estuarine snails [21]). Only one study concerning a plant-insect interaction in freshwater has been described [22]. In this study the authors demonstrated that the milfoil weevil ( Euhrychiopsis lecontei Dietz, 1896; Coleoptera: Curculionidae) was attracted by chemicals released by an invasive host-plant ( Myriophyllum spicatum Linnaeus, 1753; Haloragaceae) for feeding, ovipositing and mating.…”

“…Host plant attractions often involve a mixture of VOCs-like, and the effective concentration of one attractant can be modified, when diluted in a specific mixture [23], [24]. However, this phenomenon was not observed by Marko et al [22], who detected no synergism between attractants in the E. lecontei-M. spicatum relationship.…”

Chemical profile of the North American native Myriophyllum sibiricum compared to the invasive M. spicatum

“…These differences suggest that M. spicatum and M. sibiricum exhibit alternate strategies to cope with environmental challenges. The higher concentration of carbon-based defenses in M. spicatum could provide protection from UV light (Rozema et al, 1999) and defend against generalist herbivores and algal and bacterial competitors (Gross et al, 1996;Choi et al, 2002;Cronin et al, 2002;Li et al, 2004), but have little effect on specialist herbivores (Marko et al, 2005), particularly in a new habitat. Relative to M. spicatum, the high ash content in M. sibiricum may provide a structural defense against both generalist and specialist herbivores by reducing the available caloric value of consumed material or acting as a deterrent both directly or synergistically with other chemicals (Hay et al, 1994;Yufera et al, 1997;Sharfstein and Steinman, 2001).…”

“…One closely related species that can be displaced by M. spicatum is northern watermilfoil, Myriophyllum sibiricum Komarov, a plant that is native to the aquatic habitats of the northern United States of America and southern Canada (Aiken, 1981;Moody and Les, 2002). M. sibiricum is morphologically and chemically very similar to M. spicatum (Ceska, 1977;Marko et al, 2005); however, small differences between the species may play a significant role in their ecological interactions with associated periphyton and invertebrate communities.…”

Life History and Developmental Performance of the Eurasian Milfoil Weevil, Eubrychius velutus (Coleoptera: Curculionidae)

“…Most work has focused on E. lecontei, which has expanded its host range from the native northern watermilfoil (M. sibiricum Komorov) to Eurasian watermilfoil (reviewed in Newman 2004). In addition to being effective at controlling some Eurasian watermilfoil populations, this recent host range expansion has provided valuable insights into herbivore host choice and performance (Solarz and Newman 2001;Newman 2004;Marko et al 2005;Roley and Newman 2006). Although E. velutus was identified as feeding on Eurasian watermilfoil in early surveys of potential control agents, the populations were low (Lekic and Mihajlovic 1970).…”