Visual Active Space of the Milfoil Weevil, Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae)
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- Reeves, J.L. & Lorch, P.D. J Insect Behav (2011) 24: 264. doi:10.1007/s10905-010-9252-6
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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 overwinters 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.
KeywordsBiological controlaquatichost locationvisionbehaviorinsect
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 (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) for a 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.
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).
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. The bulbs (General Electric brand F40 Plant and Aquarium bulbs with Ecolux® Technology) were 40 W and emitted 1,900 lm. White copy paper (21.6 × 27.8 cm) was taped over the length of the bulbs to diffuse the light from the banks, as E. lecontei is positively phototactic (Reeves et al. 2009). The weevils responded to plants more strongly in the presence of ambient laboratory lighting, so lab lighting was left on for all trials. Finally, in all trials, a wall of aluminum foil was erected at 70 cm from the trough’s center in either direction.
The weevils for this experiment (and those described below) were housed en masse (∼50–100) in 37.9 L aerated aquaria under a 14:10 L: D cycle. Unrooted and unweighted M. spicatum biomass was placed into the aquaria (filling half of aquaria) and replaced as necessary (i.e., when plants were extensively damaged and collapsing/dying) throughout the experimental period. No weevils were used more than once for a single distance or for different distances on the same day (except as noted below). Weevils may have been used in more than one experiment across days, however, as they were haphazardly picked from the aquaria. For all experiments, weevil age was unknown, and sex was not determined because sex did not matter in Reeves et al. (2009). Also, for all experiments, injured weevils that could not move easily throughout the trough or weevils that did not make a choice were excluded from the experiments and analyses. Between three and ten weevils were excluded from any set of trials (distances). Weevils for all experiments (collected in various locations around Michigan, U.S.A.) were reared and donated by EnviroScience, Inc. (Stow, OH; www.EnviroScienceinc.com) and all plant stems were collected in Portage County, OH, USA. These experiments were performed between 26 June 2009 and 20 August 2009. All trials were preformed between ∼10:00 a.m. and 2:00 p.m. to control for any potential behavioral differences at different times of day.
For this visual active space experiment (replicated twice), 15 cm was arbitrarily chosen as the starting point because previous work clearly showed that E. lecontei is visually attracted to plants from at least 8.5 cm (Reeves et al. 2009). Based on initial success of 20 weevils at 15 cm, new sets of 20 weevils were tested at 25 cm, 20 cm, and finally 17.5 cm from the trough’s center. In each trial, weevils were singularly released into the trough at the center and their choice (i.e., swimming toward or away from plant stem) was recorded. During this first replicate of distance experiments, weevils from the 20 cm trials were immediately used in 8.5 cm trials to confirm that weevils were attracted to the M. spicatum stems at this distance as they were in Reeves et al. (2009) even if they were not attracted at 20 cm. This was done to test whether or not a lack of response was due to general unresponsiveness, and also to show that the results from these methods are comparable to Reeves et al. (2009). This was the only instance in which a weevil was used more than once in a day for any of the experiments. The entire experiment described above was replicated a second time using a different set of weevils (no individuals from first replicate were used again in second replicate and weevils from 20 cm trials were not re-used for the 8.5 cm trials in replicate two).
Turbidity was shown not to affect visual plant detection by E. lecontei in Reeves et al. (2009) at a distance of 8.5 cm. To test whether turbidity effects occur over larger distances, turbidity trials were performed to see how plant detection would be affected at 15 cm (the first distance beyond 8.5 cm as above). Two turbidity levels were used, 0 and 41.5 ± 2.0 NTU (Nephelometric Turbidity Units) that were created by suspending bentonite clay in water (as in Reeves et al. 2009). A smaller, ∼36 cm long section of the same type of trough as above was used to more easily ensure getting all water and clay out of the trough when water was changed. For each turbidity value, 25 weevils were individually used with sealed plant stems randomly placed 15 cm from the center of the trough. To remain consistent with the methods above, the series of turbidity experiments were performed on different sets of weevils on different days, rather than each weevil being run in each turbidity level successively as in Reeves et al. (2009).
For statistical analyses, we used logistic regression to test for differences between replicates and distances in the visual active space experiment to determine if pooling the data between the two replicates was appropriate. The replicate by distance interaction term in the model was not significant (L-R Chi-Square = 0.54, P = 0.46), nor was the replicate term (L-R Chi-Square = 0.0003, P = 0.98). The reduced model with just distance yielded a significant logistic regression (L-R Chi-Square = 6.38, P = 0.01). Because of this (and because both replicates were from two different sets of weevils with no overlap of individuals between replicates), we pooled the data between the two replicates for analysis of each distance using Chi-Squared tests. For each distance in the active space experiment and each turbidity level in the turbidity experiment, Chi-Squared tests were used to determine whether significantly more than 50% of weevils swam toward the plant, or if the weevils contacted the vial before the trough wall.
Based on our data, E. lecontei has a visual active space of at least 17.5 cm. Furthermore, E. lecontei has the visual capability to become more attracted to plant stems than other attractive visual stimuli (i.e., the trough walls) within 15 cm. Although these distances may be relatively small compared to some terrestrial insects (2–10 m; Table 6.2 in Schoonhoven et al. 2005), it is worth noting that these previous visual active space studies involved flying adult insects searching out host plants (including trees) that were relative much larger and seemingly easier to locate than the single M. spicatum stems used here.
Because even at 25 cm the weevils were still trending towards the vial, our estimate of the visual active space of E. lecontei may be conservative, especially because the trough walls were at least somewhat attractive to the weevils. Perhaps the trough walls may have been perceived as structure (safety), or the walls reflected or bent the light at the water’s surface, which may have attracted the weevils (E. lecontei is positively phototactic; Reeves et al. 2009). The successful methods here involved using the foil-covered walls which eliminated the ability for the weevils to interact with the ridges at the bottom of the trough (Fig. 1). They were attracted to these ridges and responded to them during initial experimentation (potentially because they were perceived as plant stems, or because weevils may crawl along lake-bottoms while searching for plants). Also, the foil may have removed any strong directional light cues via reflection. Even though the trough walls were closer than the vials to the weevils in all cases, at up to 15 cm, the plant-filled vials were more attractive than the trough walls. The visual acuity of E. lecontei is strong at short distances such as these, especially considering that at a distance of 8.5 cm, similar looking plant species can be visually differentiated (Reeves and Lorch 2009).
Besides distance from the plant, turbidity seemed to have an impact on plant finding by E. lecontei. At a distance of 15 cm, turbid water at 40 NTU significantly reduced visual plant detection. Furthermore, the proportion of weevils swimming toward the plant that touched the vial first was highly reduced. At a shorter distance of 8.5 cm, however, turbidities up to 100 NTU did not affect visual plant location by E. lecontei (Reeves et al. 2009), so turbidity may only affect E. lecontei at distances beyond 8.5 cm. If glycerol and uracil (the phytochemicals attractive to E. lecontei; Marko et al. 2005) can be detected beyond 8.5 cm from a plant in turbid conditions, perhaps E. lecontei would use these chemicals to perceive plants and track them to within their visual active space so they could then finally locate them using visual cues.
Whereas some of the steps in Fig. 4 are understood, some parts about how such a small weevil can find plants in large lakes each spring remain unanswered. For instance, the relative roles of flying over vs. crawling into lakes in spring are mostly unknown, though limited flight dispersal appears to be possible (Newman et al. 2001). Lake finding in general remains unexplored (Step 1 in Fig. 4), along with the behaviors associated with finding appropriate on-shore over-wintering sites (Step 6 in Fig. 4). These habitat finding behaviors also will be important to understanding long term control efficacy, and both are requiring more research. Once all the elements of host finding are examined, the variation in efficacy of E. lecontei in controlling M. spicatum can be more fully understood. We will have to know how E. lecontei finds lakes and plants in the spring before we can predict when E. lecontei will find and thus damage/control M. spicatum. The study presented here, along with the conceptual model and corresponding literature in Fig. 4, contribute to our understanding of plant finding by E. lecontei. Much of the conceptual model of plant finding in Fig. 4 may also apply to aquatic phytophagous insects in general (see Harms and Grodowitz 2009 for a comprehensive list of aquatic phytophagous insects).
We thank EnviroScience, Inc. (www.EnviroScienceinc.com) for donating the weevils used in these experiments and their continual support of our research.