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Visual Active Space of the Milfoil Weevil, Euhrychiopsis Lecontei Dietz (Coleoptera: Curculionidae)

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Visual Active Space of the Milfoil Weevil, Euhrychiopsis Lecontei Dietz (Coleoptera: Curculionidae) J Insect Behav (2011) 24:264–273 DOI 10.1007/s10905-010-9252-6 Visual Active Space of the Milfoil Weevil, Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae) Justin L. Reeves & Patrick D. Lorch Revised: 22 November 2010 /Accepted: 13 December 2010 / Published online: 8 January 2011 # Springer Science+Business Media, LLC 2011 Abstract Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae), a native weevil, is used as a biological control agent for the invasive aquatic macrophyte, Eurasian watermilfoil (Myriophyllum spicatum L.). Because E. lecontei over- winters on land in the adult stage and must find plants in lakes each spring, plant finding behaviors are essential to eventually understanding and predicting long term biological control. Our research showed that E. lecontei is visually attracted to M. spicatum at up to 17.5 cm, and is more attracted to plants than other visual stimuli within 15 cm. We also showed that turbidity may affect visual plant finding at 15 cm. Using available data from this and other previous studies involving chemical cues and other life history traits, we propose a testable conceptual model for how E. lecontei finds plants each year, especially while underwater. This model may also be used to explain plant finding by aquatic phytophagous insects in general. Keywords Biological control . aquatic . host location . vision . behavior . insect Introduction The native, aquatic milfoil weevil (Euhrychiopsis lecontei Dietz; Coleoptera: Curculionidae; body length ∼2 mm), is a promising biological control agent for the highly invasive macrophyte, Eurasian watermilfoil (Myriophyllum spicatum L.; Sheldon and Creed 1995; Newman 2004). Since the introduction of M. spicatum to the U.S. in the 1940’s, it has spread to over 45 states and three Canadian provinces J. L. Reeves : P. D. Lorch Department of Biological Sciences, Kent State University, Kent, OH 44242, USA Present Address: J. L. Reeves (*) Department of Biology, Colorado State University, Fort Collins, CO 80523, USA e-mail: [email protected] J Insect Behav (2011) 24:264–273 265 (Newman 2004), causing considerable ecological and economic damage (Boylen et al. 1999; Smith and Barko 1990; Grace and Wetzel 1978). Euhrychiopsis lecontei has expanded its host range to include M. spicatum since the plant was introduced (Newman 2004), and even prefers M. spicatum over other native milfoils (Marko et al. 2005; Solarz and Newman 1996, 2001). Euhrychiopsis lecontei also develops faster on M. spicatum than native Myriophyllum spp. (Newman et al. 1997;Roley and Newman 2006; Solarz and Newman 2001), and may not significantly damage native Myriophyllum spp. (Sheldon and Creed 2003). See Newman (2004)fora more comprehensive review of weevil life history and its use as a biological control agent. Euhrychiopsis lecontei overwinters in the adult stage on land in shoreline leaf litter (Newman et al. 2001) and must relocate suitable host-plants in the spring. Being aquatic, M. spicatum grows in a completely different habitat than that in which the weevils overwintered, adding to the complexity of host location. Because biological control ideally provides long term control of problematic plants (McFadyen 1998), an understanding of how weevils find plants in the spring should be directly related to predicting the conditions under which E. lecontei can be expected to find plants, which in turn is directly related to predicting long term control efficacy. Understanding plant finding behaviors may be important not only in this system, but in any aquatic biological control system (Cuda et al. 2008). The chemical cues glycerol and uracil (general aquatic plant exudates) appear to be used by E. lecontei to find plants (Marko et al. 2005). Visual cues also can be important (Reeves et al. 2009). For instance, E. lecontei is capable of visually differentiating plant species under water (Reeves and Lorch 2009). Because the use of visual cues, in general, appears to be important for E. lecontei, a critical factor for elucidating visual cue behavior is to determine the distance at which E. lecontei can visually perceive M. spicatum. With an understanding of how far weevils can see M. spicatum underwater, we will be closer to understanding plant finding as a whole. Thus, the primary goal of the research presented here was to determine the distance at which E. lecontei become visually attracted to M. spicatum, at least while underwater. We also explored the role of turbidity in potentially reducing plant detection distance. “Active spaces” are the distances around a plant within which a stimulus (either chemical or visual) is sufficiently strong to elicit a behavioral response from a phytophagous insect (Schoonhoven et al. 2005, p.144). Few studies have quantified the active space for either visual (four total) or chemical (six total) cues. None of these prior studies were aquatic systems (reviewed in Table 6.2 in Schoonhoven et al. 2005), so the research presented here is intended to add to the list of insects for which the size of active spaces has been estimated, especially for aquatic systems where no such data have yet been published. The variability seen both within and across lakes in the effectiveness of E. lecontei at controlling M. spicatum (Reeves et al. 2008) may be related to factors affecting plant finding in the spring. The work presented here and by Marko et al. (2005), Reeves et al. (2009) and Reeves and Lorch (2009) all help to understand plant finding by E. lecontei. The results of these (and other) studies are used to provide a conceptual model for how E. lecontei, and likely other aquatic phytophagous insects, may find underwater plants in the spring. 266 J Insect Behav (2011) 24:264–273 Methods To determine how far weevils are visually attracted to M. spicatum stems underwater (i.e., their visual active space), we constructed 3.1 m long troughs (Fig. 1)to incrementally move sealed plant stems to different distances from the starting point and record how far the weevils can be from the plants and still be significantly visually attracted to them. A small (∼4.5 cm) portion of apical M. spicatum stem [sealed inside a one dram (∼3.7 ml) glass vial to prevent detection of any chemical cues; Reeves et al. 2009; Reeves and Lorch 2009] was placed in a random side relative to the mid-point of the trough. Individual weevils were released in the middle of the trough and given 5 min to choose which direction to swim (either toward or away from the plant stem). A choice was defined either by vial contact or swimming past the 8.5 cm mark from the center release point in the trough. This distance was chosen as the weevil choice point because 8.5 cm was the distance used to determine choice in Reeves et al. 2009 (weevils can at least see this far), and also because 8.5 cm represents over 40 body lengths of the weevil, making it unlikely that such a large movement from the release point was random (especially because weevils are poor swimmers). These methods also allowed us to judge the instantaneous weevil response to the plant stem (weevils swam directly to plants when they entered their field of view in Reeves et al. 2009 and Reeves and Lorch 2009). Especially at distances beyond 8.5 cm, weevils could have randomly swam in the plant’s direction (without initial detection) until they saw the plant and eventually swam to it. We chose not to use a specified/standardized distance from the plant stem as the choice point for this reason, as we wanted to ensure that weevil response to the plant stems was direct and not a result of happening to swim in the plant’s direction. Almost all weevils that swam past the 8.5 cm mark in the direction of the plant stem reached the vial and bumped against the vial for several seconds, apparently trying to get to the plant. For each distance examined in this study (described below), 20 weevils (both male and female) were tested individually. If a significant number of weevils (using Chi-Squared tests) swam toward or contacted the vial, we considered weevils to have been visually attracted to the plant at that distance. Empty vials were not used as controls for these experiments, as it was shown that weevils were not attracted to the same empty vials in Reeves et al. (2009). Fig. 1 Cross section and dimen- sions of trough used for these experiments. End caps of essen- tially the same size and shape were attached to the ends of the trough using clear 100% silicone caulking, creating a water-tight seal. The asterisk shows where weevil was released relative to sides, and horizontal dotted line indicates approximate water level J Insect Behav (2011) 24:264–273 267 For each trial, the trough was filled with about 2 cm of dechlorinated tap water. Because chemical cues were not a large concern for these experiments with sealed vials, and because the volume of water required to fill the trough was substantial and difficult to completely remove, water was changed after every ∼five successful trials. For all experiments, multiple sealed plant stems were used both within and across experiments. For this visual active space experiment, considerable preliminary experimentation was performed before weevils would start positively responding to the sealed plant stems. The successful methods involved lining the inside of the trough with aluminum foil (duller side up) and placing the fluorescent light banks that were used 60 cm above the troughs. The light banks were placed parallel to and behind the trough so the back side of the trough was in line with the front side of the light bank. Two light banks were used, mounted end to end. Each light bank consisted of two 122 cm fluorescent bulbs spaced ∼3 cm apart.
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