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Developmental Performance of the Milfoil Weevil, Euhrychiopsis lecontei (Coleoptera: Curculionidae), on Northern Watermilfoil, Eurasian Watermilfoil, and Hybrid (Northern × Eurasian) Watermilfoil

Sarah S. Roley, Raymond M. Newman
DOI: http://dx.doi.org/10.1603/0046-225X-35.1.121 121-126 First published online: 1 February 2006


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.

  • Euhrychiopsis lecontei
  • watermilfoil
  • development time
  • hybrid herbivory
  • insect performance

Host range expansion of herbivores to introduced species has been well documented (Solarz and Newman 1996,2001, Trowbridge and Todd 2001, Siemann and Rogers 2003). This range expansion can lead to greater herbivore fitness-faster development, larger size, and greater fecundity (Sheldon and Jones 2001, Trowbridge and Todd 2001). Herbivore host range expansion can also occur with the appearance of hybrid plant species (Fritz et al. 1994,1999, Whitham et al. 1999). Herbivore response to hybrids is variable and depends on hybrid generation (e.g., F1, F2, backcrossed), number of genes that confer resistance, expression of secondary chemicals, and environmental variability (Fritz et al. 1994, Whitham et al. 1999, Orians 2000).

Evidence for hybrid susceptibility, hybrid resistance, and intermediate resistance to herbivores has been documented (Fritz et al. 1999, Whitham et al. 1999). Most of these studies determined hybrid resistance by abundance, diversity, or density of herbivores; however, few have examined the fitness of the herbivore on hybrids. The few studies assessing herbivore fitness found that herbivores displayed equal, intermediate, or greater fitness (determined by longevity, oviposition rates, growth rate, mass, and larval mortality) (McClure 1985, Hall and Townsend 1987, Floate et al. 1993, Siemens et al. 1994, Gange 1995, Fritz et al. 1999, Ishida et al. 2004).

Euhrychiopsis lecontei Dietz, the milfoil weevil, is an aquatic, herbivorous weevil, endemic to North America, whose native host is northern watermilfoil (Myriophyllum sibiricum Komarov; Creed and Sheldon 1994). The weevil spends its entire summer life cycle submersed on watermilfoils (Sheldon and O'Bryan 1996, Newman 2004). Females lay eggs on apical meristems. On hatching, the larvae eat the meristem, then burrow through the stem, consuming the cortex. Pupation occurs in lower portion of the stem, and after eclosion, adults swim to the upper portion of the plant where feeding and mating occur (Sheldon and O'Bryan 1996).

Eurasian watermilfoil (Myriophyllum spicatum) was first documented in North America in the 1940s (Smith and Barko 1990). Fast growing and a good disperser, it outcompetes many native macrophytes (Madsen et al. 1991). As Eurasian watermilfoil spread into lakes and ponds with northern watermilfoil, the milfoil weevil expanded its range, preferentially feeding and ovipositing on Eurasian watermilfoil (Solarz and Newman 1996,2001). The milfoil weevil generally displays higher fitness (more eggs produced, larger size, shorter development time) on the exotic than on native watermilfoils (Solarz and Newman 1996,2001, Newman et al. 1997, Sheldon and Jones 2001; see Tamayo and Grue 2004, Tamayo et al. 2004 for exception). The success of the milfoil weevil on Eurasian watermilfoil has resulted in many declines of the exotic (Creed and Sheldon 1995, Creed 1998, Johnson et al. 2000, Newman and Biesboer 2000) and led to its consideration as a biological control agent for the invasive Eurasian watermilfoil (Creed and Sheldon 1995, Sheldon and Creed 1995, Newman 2004). Such declines have not been observed in northern watermilfoil populations (Tamayo et al. 2004).

Northern watermilfoil and Eurasian watermilfoil have hybridized (Moody and Les 2002), and the weevil's response to the hybrid watermilfoil is uncertain. Moody and Les (2002) suggested that hybrid watermilfoil could be more invasive than Eurasian watermilfoil, perhaps by showing resistance to herbivory while retaining the vegetative vigor of Eurasian watermilfoil. Their hypothesis is supported by Schweitzer et al. (2002), who found that hybrid plants are capable of inheriting fitness traits from both parents, and Galatowitsch et al. (1999) and Ellstrand and Schierenbeck (2000), who suggested that invasiveness can increase as a result of hybridization.

However, no previous studies on hybrid/herbivore interactions have found evidence for greater resistance (in terms of herbivore fitness) to herbivory by hybrids (McClure 1985, Hall and Townsend 1987, Floate et al. 1993). Because few studies of performance on hybrids have been done, and because numerous variables may affect herbivore performance on hybrids, it is not clear how milfoil weevil fitness will be affected. We hypothesized that the milfoil weevil's fitness on the hybrid watermilfoil would be intermediate between its fitness on Eurasian and northern watermilfoil. The purpose of this study was to test this hypothesis, determining fitness by development times, survival rates, mass of progeny, and stem damage.

Materials and Methods

We planted ≈100 cuttings (apical 20 cm) of each taxon (northern watermilfoil, Eurasian watermilfoil, and hybrid watermilfoil) in outdoor tanks. Because weevil performance can vary by source of watermilfoil (Tamayo and Grue 2004), we planted the cuttings in homogenized sediment and allowed the plants to root and grow for 4 wk before use. This reduced nutrient differences (all were rooted in the same sediment) and climate differences (all grown under the same temperature, water, sunlight, and rainfall) that may have occurred in watermilfoils from different lakes. We obtained Eurasian watermilfoil from Auburn Lake (Carver County, MN), northern watermilfoil from Christmas Lake (Hennepin County, MN), and hybrid watermilfoil from Otter Lake (Anoka County, MN). These locations were chosen because they had large, easily identifiable, and molecularly verified (using the methods of Moody and Les 2002) populations of the target taxa (M. L. Moody, personal communication).

Because weevil source and rearing plant can also affect weevil performance (Solarz and Newman 1996,2001, Tamayo and Grue 2004), all weevils used in this experiment came from the same source (Lake Auburn). The weevils had been reared in an outdoor tank on Eurasian watermilfoil before use in this experiment.

We obtained mating pairs of weevils from this stock, weighed them, and placed a single pair in a 1-liter glass jar, with water that contained a single, rooted milfoil plant (≥30 cm tall) collected from the outdoor tanks. The plant taxon was randomly chosen. The weevils were kept with the plant until oviposition occurred or until 3 d elapsed, at which time we placed a fresh plant of the same species in the jar. If the pairs spent >6 d without oviposition, the plant was discarded, and the weevils were returned to the stock tank. After oviposition, the weevils were placed on a different taxon (randomly chosen); most pairs oviposited on each taxon at least once. This ensured that differences in mass of weevil progeny among plant taxa were not caused by parental effects (Newman et al. 1997). A total of 17 Eurasian watermilfoil plants, 18 hybrid plants, and 20 northern watermilfoil plants were oviposited on and used in the experiment.

Plants with a newly oviposited egg were planted in 4 cm of homogenized sediment in an acrylic tube (45 cm long, 8 cm diameter) and filled with room temperature well water (Mazzei et al. 1999). The sediment was covered with 2 cm of aquarium gravel to prevent resuspension of sediment. The tops of the tubes were covered with mosquito netting and placed in an environmental chamber with a constant air temperature of 25°C and a 15-h photoperiod. Although the air temperature remained constant, the lights in the chamber (intensity of photosynthetically active radiation 60-115 μmol m-2s-1) heated the water in the tubes. Water temperature was recorded every 30 min with an Optic StowAway temperature logger (Onset Computer, Pocasset, MA). The water temperature ranged from 25 to 28°C and averaged 25.9°C.

Each tube was checked daily, and developmental stage and amount of damage (length of stem that was blackened and hollow) was recorded, following the methods and criteria of Mazzei et al. (1999). Once adult weevils emerged, they were removed from the tube, blotted dry, and weighed.

In some cases, the weevils laid more than one egg on a single plant. When possible, these extra eggs were removed. If extra eggs could not be removed or if they were not detected before hatching, the weevils were allowed to continue development. Ten of 35 plants produced multiple adults, but for statistical analyses, we used only the first weevil to emerge from each plant. Plants with multiple weevils also experienced more damage. For damage comparisons, we divided total damage by the number of observed pupae or late-instar larvae. Twenty-one of 45 plants had more than one weevil pupa, but there were no differences in mortality, mass, or development time between weevils raised on plants with multiple weevils compared with weevils raised on plants with single weevils (all P > 0.5).

Because weevil larvae consume the inner cortex of stem and because developmental performance is likely related to feeding in the larval stage (Sheldon and O'Bryan 1996, Solarz and Newman 2001), we suspected that stem diameter could affect development times, survival, or mass. Before oviposition, we measured the diameter of the stem of each plant at 2 and 30 cm from the meristem. We chose these two locations because they affect key stages in weevil development: early larval development (top) and pupation (30 cm).

Survival data were analyzed using contingency tables (Moore and McCabe 2003). Correlations between stem diameter, development time, mass, damage, and host plant were tested with a pairwise Pearson test, using Bonferroni probabilities. Development times, stem diameters, and damage were compared with a one-way analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD) for multiple comparisons. Covariance between stem diameter and mass and stem diameter and damage was tested with an analysis of covariance (ANCOVA). Independent sample t-tests were used to compare mean stem diameters on plants that produced weevils and plants on which weevils died during development. All statistics, except χ2 analysis, were computed with SYSTAT 5.1 (Wilkinson 1989).


The taxon on which the larval weevils were reared did not affect development time or mass of emerged adults, but it did influence survival rate. Mean total development times (egg to adult) were 19.7 d on Eurasian watermilfoil, 20.0 d on hybrid watermilfoil, and 20.3 d on northern watermilfoil (Table 1) and were not significantly different among plant taxa (ANOVA, F = 0.5; df = 2,32; P > 0.6). Development times in each stage also did not differ significantly among taxa (egg, F = 1.01; df = 2,43; P > 0.3; larva, F = 2.48; df = 2,41; P > 0.09; pupa, F = 0.36; df = 2,32; P > 0.6). Mean mass of emerged adults did not differ among taxa (F = 0.51; df = 2,32; P > 0.6), and was 1.3 mg on hybrid watermilfoil, 1.4 mg on northern watermilfoil, and 1.5 mg on Eurasian watermilfoil (Table 1).

View this table:
Table 1.

Eighty-eight percent of the weevils reared on Eurasian watermilfoil survived from egg to adult; 61% survived on hybrid watermilfoil, and 45% on northern watermilfoil (Table 1). These differences were significant (χ2 = 7.5; df = 2; P < 0.05). Differences in survival within each stage of development were not significant among plant taxa (egg, χ2 = 4.5; df = 2; P > 0.1; larva, χ2 = 1.4; df = 2; P > 0.5; pupa, χ2 = 2.0; df = 2; P > 0.3).

These differential survival rates may be caused by taxa differences but could also be affected by stem diameter (Sheldon and O'Bryan 1996). At 2 cm, mean stem diameter was 0.4 mm in hybrid and northern watermilfoil and 0.6 mm in Eurasian watermilfoil (Table 2). These differences were significant (F = 9.6; df = 2,52; P < 0.001) and correspond to the higher survival rate and the larger stem diameter with Eurasian watermilfoil, but do not explain the difference between northern watermilfoil and hybrid watermilfoil. This pattern remained when comparing only plants on which adult weevils emerged; Eurasian watermilfoil was significantly thicker than northern watermilfoil and hybrid watermilfoil (F = 6.47; df = 2,32; P = 0.004). However, a comparison of plants on which weevils died during development showed that all three species had approximately the same diameter (mean = 0.44 mm; F = 1.48; df = 2,17; P > 0.2; Table 3). Among all plant taxa pooled, mean stem diameter was significantly smaller on plants on which weevils died (0.44 versus 0.53 mm; t = 2.03; df = 49.8; P < 0.05; Table 3).

View this table:
Table 2.
View this table:
Table 3.

At 30 cm, hybrid watermilfoil mean stem diameter (1.7 mm) was marginally larger than Eurasian (1.5 mm; F = 2.84; df = 2,52; P = 0.067); northern was intermediate (1.6 mm; Table 2). Although Eurasian had the smallest average stem diameter, it had the highest weevil survival rate. When comparing only plants that produced adults, Eurasian watermilfoil had the smallest stem diameter (1.44 mm), and hybrid and northern had the largest stem diameter (1.70 mm; F = 3.92; df = 2,32; P = 0.030; Table 3). Eurasian watermilfoil also had the smallest stem diameter on plants that did not produce adults, but differences between taxa were not significant (F = 0.269; df = 2,17; P = 0.768; Table 3). Among all plant taxa pooled, mean stem diameter of plants on which weevils died and those on which weevils survived was not significantly different (1.63 versus 1.58 mm; t = -0.43; df = 30.8; P > 0.6; Table 3).

Stem diameter could also influence adult mass-stem diameter at 2 cm was correlated with adult mass across taxa (P < 0.03) but not within taxon (all P > 0.13). Furthermore, in an analysis of adult mass by taxon, stem diameter was a marginally significant covariate with mass (ANCOVA, F = 4.11; r2 = 0.14; df = 1,31; P = 0.051). The interaction between stem diameter and taxa was not significant (P > 0.6). Development times (individual stages and total) were not correlated with stem diameter at 2 or 30 cm (all P > 0.7).

Mean damage per pupa was 7.0 cm on Eurasian watermilfoil, 9.5 cm on hybrid watermilfoil, and 8.8 cm on northern watermilfoil. These differences were not significant (F = 2.03; df = 2,43; P > 0.1).

To check for maternal effects, we performed a Pearson pairwise correlation between mother's mass and progeny mass, development time, damage, and damage per pupa. No correlations were significant (all Bonferroni P > 0.27), indicating no significant maternal effects.


The milfoil weevil has expanded its range to include hybrid watermilfoil and successfully develops to adult on the hybrid. Weevil survival rate on the hybrid was intermediate between its survival rate on Eurasian watermilfoil and northern watermilfoil. Milfoil weevil development times were also intermediate, but these differences were not significant. The milfoil weevil thus displays additive (intermediate) survivorship (Fritz et al. 1994) and no difference in mass and development time.

Herbivore fitness studies in other systems have supported several hypotheses, including hybrid susceptibility (hybrid is more susceptible than either parent) (Messina et al. 1996), dominance (hybrid resembles one parent) (McClure 1985, Floate et al. 1993, Siemens et al. 1994, Gange 1995), additive (intermediate between parents) (Hall and Townsend 1987, Floate et al. 1993, Gange 1995), and no difference among hybrid and parents (Floate et al. 1993, Siemens et al. 1994, Gange 1995, Ishida et al. 2004). No evidence of hybrid resistance (hybrid is more resistant than either parent), when determined by herbivore fitness, has been documented (Fritz et al. 1994,1999). Our results are thus consistent with previous work, supporting the additive and no difference hypotheses. Overall, these studies show no clear trend in herbivore response to hybrids, except that hybrid resistance is uncommon.

Although the hybrid's resistance to the milfoil weevil (as determined by weevil fitness) is not greater than northern watermilfoil, as predicted by Moody and Les (2002), it is greater than Eurasian watermilfoil. Based on this, the hybrid watermilfoil could be more invasive than the Eurasian watermilfoil-lower weevil survival could result in a reduced weevil population and lower watermilfoil consumption. However, the amount of stem damage per weevil was highest on hybrid watermilfoil, which could negate the effects of lower survival and longer development times. In addition, we observed a significant decline in hybrid watermilfoil associated with the milfoil weevil at Otter Lake, MN (Newman 2004). The performance of other herbivores, such as Acentria ephemerella and Cricotopus myriophylli, could also affect the invasiveness of the hybrid watermilfoil. These herbivores are also present in Otter Lake, and a study of their response to the hybrid watermilfoil would be worthwhile. All hybrids are not genetically identical, however, and some variation in weevil response may occur between hybrid populations, as occurs between populations of northern watermilfoil (Tamayo and Grue 2004, Tamayo et al. 2004). In addition, lake-specific factors, such as presence of predators, water temperature, and overwintering conditions, can affect weevil densities and thus the amount of damage inflicted on the watermilfoil (Mazzei et al. 1999, Newman 2004).

Another possibility is that release from natural enemies is not a common mechanism for hybrid invasibility (Keane and Crawley 2002). Enemy release has been suggested as a cause of invasibility in non-native species, but there is little evidence of poorer performance by herbivores on hybrids (Fritz et al. 1999). Our results are consistent with these hybrid herbivory studies and suggest that the hybrid watermilfoil population at Otter Lake is not more invasive than Eurasian watermilfoil as a result of herbivore performance. If some populations of the hybrid watermilfoil do become more invasive, it will likely not be caused by release from herbivory, but because of superior competitive ability. The evolution of such hybrids is of concern because watermilfoils primarily reproduce clonally, so an invasive genotype would maintain fixed heterosis (Ellstrand and Schierenbeck 2000).

Our results are consistent with previous studies in eastern North America, which found equal or significantly greater weevil mass and equal or significantly faster development times on the Eurasian watermilfoil compared with other watermilfoils (Newman et al. 1997, Solarz and Newman 2001, Sheldon and Jones 2001). Our development times and survival rates are also consistent with other studies conducted under similar conditions. Specifically, Mazzei et al. (1999) found that, at 25°C, the average egg-adult development time on Eurasian watermilfoil was 20.6 d with 56% survival. Newman et al. (1997) found that, at 24.8°C, average egg-adult development time was 22.5-24.5 d, with 20-70% survival.

Tamayo and Grue (2004) found that performance varied by weevil and watermilfoil population in Washington, with some weevils performing better on northern watermilfoil than Eurasian watermilfoil. All of our weevils came from the same population, so weevil population differences probably did not influence our results. However, because differences in weevil populations exist (Solarz and Newman 2001, Tamayo and Grue 2004), further studies to compare responses of different weevil populations are warranted. In addition, the progeny of weevils reared on northern watermilfoil could respond differently than the progeny of weevils reared on Eurasian watermilfoil (Solarz and Newman 2001), and studies addressing this difference with hybrid watermilfoil would be worthwhile.

Weevil performance is affected by taxon-specific host differences, but also by factors that vary within taxa, such as stem diameter. In this study, weevils experienced greater mortality and smaller mass on plants with a smaller stem diameter at 2 cm. This may occur for several reasons. (1) The larva must be able to fit inside the stem. Small stems may limit the growth of the larva, increasing mortality. (2) Less tissue will be present in smaller stems, forcing the larva to burrow further to obtain adequate nutrition. This energy trade-off could adversely affect growth and survival. (3) Instead of burrowing further in smaller stems, weevils may feed on more tissue in the upper plant, including the less nutritious components (e.g., epidermis, outer stem). A less nutritious diet could result in greater mortality and lower mass.

Although the weevils experienced highest mortality in the pupal stage on all taxa, development on plants with a smaller stem diameter at 30 cm did not result in greater weevil mortality. This suggests that stem diameter in our plants (≥1.4 mm) was adequate for pupal development. Stem toughness, which could be related to stem diameter, may also affect weevil development, and deserves investigation.

In 2003, we performed a similar study, which also found that development time was not affected by taxa (E. A. Paulson, M.D., Marko, and R.M.N., unpublished data). That study found significantly lower mass and survival in weevils reared on hybrid plants, but those differences were likely caused by stem diameter: the hybrid plants had a significantly smaller stem diameter 20 cm from the meristem. In addition, most of the hybrid mortality occurred during the pupal stage, which would be affected by the smaller stem diameter at 20 cm (Sheldon and O'Bryan, 1996).

The milfoil weevil displays additive survivorship on the hybrid watermilfoil, but we found no differences in development times or mass between the three taxa. Weevil survival may be influenced by taxon-specific plant qualities, such as nutrient differences or concentration of secondary compounds; however, stem diameter in the upper 2 cm of the plant also affects survival and mass. Reduced weevil fitness relative to Eurasian watermilfoil could affect invasiveness of the hybrid watermilfoil, but our results and the control of the hybrid at Otter Lake (Newman 2004) suggest that Eurasian x northern hybrid watermilfoil is not more invasive, at least in terms of herbivore resistance. Furthermore, these results may indicate that enemy release does not explain invasiveness in hybrids. Comparisons of different hybrid populations' growth and dispersal abilities and their response to the milfoil weevil, as well as other weevil fitness traits (e.g., oviposition preference, fecundity) will be necessary to determine the relative invasiveness of hybrid watermilfoil.


We thank M. Marko for assistance with experimental set-up and D. Andow, B. Vondracek, M. Marko, the editor, and two anonymous reviewers for helpful comments on the manuscript. This research was supported in part by the Minnesota Agricultural Experiment Station, Project 74.

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