A test of sexual dimorphism in deactivating plant defense and attraction of conspecifics in the four-eyed red milkweed beetle (Tetraopes tetrophthalmus)

doi:10.21203/rs.3.rs-3637447/v1

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A test of sexual dimorphism in deactivating plant defense and attraction of conspecifics in the four-eyed red milkweed beetle (Tetraopes tetrophthalmus)

https://doi.org/10.21203/rs.3.rs-3637447/v1

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published 15 Feb, 2024

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Herbivorous insects frequently exhibit sexual dimorphism in body size and a variety of other traits. Such differences often lead to distinct behaviors, which may present themselves in insect responses to conspecifics, plant defenses, and feeding. Based on a previous study of a congeneric species, we hypothesized that sexual dimorphism may yield differences in how the host-specific red milkweed beetle Tetraopes tetrophthalmus (Coleoptera: Cerambycidae) deactivates milkweed defenses, a leaf trenching behavior in which beetles cut veins (laticifers) prior to feeding. We also predicted that beetles would display differences in their subsequent feeding patterns and attraction of conspecifics. Although beetles were size dimorphic (females were larger than males), we did not find consistent differences in the rates of trenching or feeding between males and females. Females did, however, cut more milkweed veins and attract more conspecifics than males. Sex differences in size and other traits were thus associated with some differential behaviors, but not strongly associated with the deactivation of plant defense.

Asclepias syriaca

common milkweed

host finding

latex exudation

plant-insect interactions

sexual dimorphism

Sexual dimorphism is the variability in traits between males and females of a species, including body size, pheromone production, and feeding behavior (Mori et al. 2017; Brzozowski et al. 2020; Gontijo 2013). Dimorphism is common in herbivorous insects, and often reflects sex differences in mate attraction and tolerance of certain stresses, such as avoiding predation or harsh abiotic conditions (Vinterstare et al. 2021; Gontijo 2013; Verrell et al. 2001). Here, we focus on two prominent examples of dimorphic behaviors: the attraction of conspecifics and deactivation of plant defense. Insects utilize a variety of cues, including pheromones produced by other insects, plant volatiles, and other visual or auditory signals to locate and aggregate near conspecifics (Brzozowski et al. 2020; Bengtsson 2008; Reisenman et al. 2000). In this study, we define behaviors which attract conspecifics, and may lead to mating or aggregation, as attraction. Secondly, herbivorous insects often display adaptive feeding behaviors to avoid plant defenses, including aggressive attack, leaf clipping, and vein cutting (Boone et al. 2011; Dussourd 2017). This, too, may reflect underlying sex differences. For example, research has revealed that, when available, male milkweed beetles feed opportunistically on plants which have been previously damaged (to deactivate plant defense) by female conspecifics (Gontijo 2013). This finding suggested the novel hypothesis that seemingly unrelated traits and behaviors, such as sexual dimorphism, deactivation of plant defense, and attraction, may interact.

In this study, we use latex production as a model of plant defense to investigate sexual dimorphism in defense deactivation in the red milkweed beetle, Tetraopes tetrophthalmus (Coleoptera: Cerambycidae) (Fig. 1). Latex runs through the veins (laticifers) of about 8–10% of flowering plant species (Agrawal and Konno 2009). Although it presents no primary function for plants, latex flows from damaged tissues and acts as a physical and chemical barrier against chewing insects (Agrawal and Konno 2009). In response, many insects display “trenching” behaviors, in which they sever or puncture either the midrib or veins of leaves prior to feeding. This serves to drain and depressurize latex to distal parts of the leaf, enabling safer feeding and up to 92% lower latex intake (Dussourd 1999). Insects often preferentially feed on pre-damaged leaves over leaves that have not been previously punctured (Gontijo 2013; Dussourd and Eisner1987). This finding has implications for insect aggregation and raises the question of whether plant damage and feeding may also facilitate attraction of conspecifics. Accordingly, here we measure sex differences in morphology, associated defense deactivation and feeding, and whether the sexes differentially attract conspecifics.

The common milkweed, Asclepias syriaca, contains latex that acts as a defense against T. tetrophthalmus in eastern North America (Dailey et al.1978; Agrawal 2004). T. tetrophthalmus is size dimorphic; like many insect species, females tend to be larger than males in terms of size and mass (Mori et al. 2017). Previous research has documented sex differences in trenching behaviors of a closely related species of milkweed beetle, T. femoratus, with females trenching more frequently and earlier than males, and males opportunistically feeding on the female-damaged leaves (Gontijo 2013). Gontijo (2013) concluded that, while females disproportionately trenched, both male and female beetles preferentially fed and mated on damaged leaves. Observations of T. tetrophthalmus have indicated that they also display this trenching behavior (Dussourd and Eisner 1987), although previous research had not addressed trenching-related sex differences (Agrawal and Konno 2009). Additionally, T. tetrophthalmus has been observed to aggregate on milkweed leaves in small groups (Lawrence 1982). We were thus interested in extending the results of Gontijo (2013) to behavior of T. tetrophthalmus and further studying the relationship between defense deactivation and attraction of conspecifics.

We conducted a series of field experiments over two years to measure trenching and attraction patterns of male and female T. tetrophthalmus with relation to sexual dimorphism, initiation of feeding, and mating. Specifically, we addressed the following questions: 1) Are there sex differences in the timing and amount of trenching in T. tetrophthalmus, and are these differences driven by body size? 2) Are there sex differences in the attraction of conspecifics? And 3) Is any differential attraction to conspecifics related to amount of feeding or trenching?

Study Site

We conducted a series of experiments at two field sites surrounding Ithaca, NY. The first site was a relatively isolated hayfield, containing several discrete patches of milkweed, including several hundred plants in total (42.45302 °N, 76.428 °W). All T. tetrophthalmus were collected from this field site, and Experiments 1, 2, and 4 were conducted here. The second field site (Experiment 3) was an older field with fewer, smaller, isolated milkweed patches (around 200 stems each; 42.43021°N, 76.42518 °W). We conducted Experiments 1, 2, and 3 between 15–23 July 2022 and Experiment 4 between 3–5 July 2023.

Experiment 1 - Sexual dimorphism in feeding and trenching (when alone):

We sexed twenty-two mating pairs of T. tetrophthalmus and separated each beetle into individual 1 oz. plastic containers, measuring their length, width, and mandible width using digital calipers. The beetles were left overnight in their cups with one milkweed leaf each. The following morning, each beetle was enclosed on a single milkweed leaf at Site 1 (one beetle per plant) using a mesh bag enclosure; the enclosed leaf was young (upper third) but fully developed. We took a census of each beetle at four times: 12:30, 13:30, 16:30, and 11:30 the following morning. At each census, we recorded the number of entrenchments per leaf, whether feeding had occurred, and the sex and number of colonizing beetles present on each plant. Beetles were then put back into their individual plastic cups, frozen overnight, and freeze dried. The freeze-dried beetles were weighed to find dry mass. Additionally, we recorded data from the treatment-imposing beetles in Experiment 3 below to replicate tests of sexual dimorphism in initial feeding and trenching (total n = 134 beetles across all three trials).

Experiment 2 – Sexual dimorphism in feeding and trenching (when paired)

We collected forty-two mating pairs of T. tetrophthalmus, and males and females were placed in 1 oz. cups of two to three beetles, separated by sex. We observed the beetles periodically following collection to ensure that no mating occurred (i.e., that the beetles were sexed correctly). All beetles were left in the containers and deprived of food, either overnight (n = 37 pairs) or for two hours (n = 5 pairs). We chose two size dimorphic individuals, with a large female and a smaller male, to place in a mesh enclosure on a single milkweed leaf and observed for one hour. We recorded the following observations: time and sex of first trench, time and sex of first feeding, male/female feeding place (leaf recently fed or intact leaf), time of first mating (and whether this occurred before or after feeding), and whether males fed before or after mating.

Experiment 3 – The role of sexual dimorphism and leaf damage in attracting conspecifics

Experiment 3, Trial 1

We enclosed a single A. syriaca leaf from each of 80 stems using a mesh bag, and gave each bag one of four possible treatments: a female beetle (n = 20), a male beetle (n = 20), no beetle and manual trenching (forceps used to puncture six holes in the midline, with two each at three points in the bottom, middle, and top thirds of the leaf; n = 20), or no beetle and no trenching (n = 20). We collected 180 T. tetrophthalmus, 90 male and 90 female, from Site 1and left them with food in 6 plastic containers overnight. The following morning at 9:00 the 180 beetles were released near the center of the milkweed patch containing 80 mesh bags. We took census data at 9:30, 11:00, 12:30, 16:30, and 9:30 the following morning, taking note of the presence of trenching and feeding, and number, sex, and location of other beetles present for each of the bagged plants.

Experiment 3, Trial 2

An additional trial was conducted for a set of 75 plants (split between the three treatments above, without the manual trenching treatment) and 210 beetles (half male, half female) were released. For this second trial, we released the beetles at 9:30, and collected census data at 10:00, 11:30, 16:30, and 9:30 the next morning, collecting the same data as Trial 1.

Experiment 4 – Size dimorphism in trenching

Experiment 4, Trial 1

We collected 26 mating pairs of T. tetrophthalmus, separating and measuring them following the procedure in Experiment 1. Individuals were enclosed in mesh bags the day after collection between 11:30 and 12:10. We did five circuits, recording if feeding had taken place and the number of trenches (measured as paired punches on vein). Data was collected at the following times: 12:13, 13:13, 14:07, 15:38, and 11:10 the next morning. On the last circuit, we also recorded the number of veins that had entrenchments.

Experiment 4, Trial 2

The second round was deployed between 12:23 − 12:56, again using 26 mating pairs collected the previous day. We did four circuits, at 13:30, 14:30, 15:30, and 9:30 the following day. We measured if feeding occurred, the number of trenches, and the number of veins that had trenches.

Statistical Analysis

Fisher’s Exact tests and logistic regressions were used to analyze sex differences in the initiation of trenching and feeding for Experiments 1 and 4. We also used a MANOVA of four traits to compare sizes of male and female beetles (and subsequent principal components analysis to generate size-related PC axes). For Experiment 2, we used sign tests to compare sex differences in the timing of trenching and feeding. Experiment 3 was analyzed using generalized linear models with a zero-inflated Poisson distribution. All analyses were conducted using JMP pro V.14.

Sexual dimorphism in feeding and trenching (when alone)

Male and female beetles fed on milkweed leaves and trenched latex canals at approximately equal rates. At the first census, 1 hour after initiating the experiment, 40% of beetles had fed and 26% had trenched the leaf. Females and males showed nearly identical probabilities of each behavior (n = 42, Fisher’s Exact test, p = 1 for each behavior). By 24 hours after initiating the trial, 93% had fed and 60% had trenched, but again there was no difference between the sexes (n = 42, Fisher’s Exact test, p = 1 for each behavior).

Size differences between the sexes did not appear to affect the probability of trenching and feeding, although males and females were size dimorphic (MANOVA including length, width, mandible width, and dry mass: exact F_4,36 =5.06, p = 0.002). All measured size components were significantly different themselves: females weighed 35% more, were 10% longer, 5% wider, and had mandibles that were 11% wider than males (all Ps < 0.01). Using PCA, a single principal component explained 72% of the variation in our four size measures. Neither this PC nor mandible width was predictive of feeding or trenching in the initial census (n = 42, logistic regression, PC1: feeding: χ² = 0.65, p = 0.42; trenching χ² = 0.03, p = 0.86; mandible width: feeding χ² = 1.38, p = 0.24; trenching χ² = 0.46, p = 0.50) or after 24 hours (PC1: feeding χ² = 0.18, p = 0.68; trenching χ² = 0.84, p = 0.36; mandible width: feeding χ² = 0, p = 0.97; trenching χ² = 2.12, p = 0.15).

We used the setup of additional experiments (Experiment 3, Trial 2, and Experiment 4, described above) to address the same question of sexual dimorphism in trenching. In Experiment 3, Trial 2, after 24 hours, beetles made substantially more trench marks than in the previous trial (range 0–25, mean 8.6, compared to a mean of 1.2 trench marks in trial 1), and females made 65% more trenches than males (females 10.68 ± 1.70 males 6.48 ± 1.19; F_1,48=4.08, p = 0.049). We did not measure beetle size in this trial. For Experiment 4, male and female beetles showed no difference in number of trench marks (F_1,98=0.56, p = 0.455), although larger beetles made fewer trenches (size PC1 F_1,98=7.03, p = 0.009), and sexes were again dimorphic in size (MANOVA exact F_3,99=21.41, p < 0.001). Nonetheless, in trial 4 females did puncture 55% more veins after 24 hours compared to males (mean ± SE, females 4.21 ± 3.45, males 2.72 ± 0.44; F_1.98=4.39, p = 0.039). Thus, although evidence for size-related dimorphism is abundant in this system, we only detected marginal differences in trenching between the sexes.

Sexual dimorphism in feeding and trenching (when paired)

Paired male and female beetles showed comparable timing of trenching and feeding when introduced together to a milkweed leaf in the field. In 15 of the 29 pairs that showed trenching (total n = 42 pairs), the female beetle trenched first (sign test, p = 1) and 12 of 26 pairs had a female beetle feeding first (sign test, p = 0.84). Mating occurred in 13 pairs, and of these 11 mated before feeding or trenching.

The role of sexual dimorphism and leaf damage in attracting conspecifics

In a natural population, our 135 treated stems were colonized by 68 female and 68 male T. tetrophthalmus across all census times. We found significantly higher average colonization rate on stems with a caged female beetle than on stems with a male beetle or on controls (ZI Poisson glm, Wald χ² = 10.02, p = 0.007; pairwise comparisons, control-female p = 0.085, control-male p = 0.12, female-male p = 0.003) (Fig. 2). These pairwise comparisons were similar when we separately considered male vs. female colonizers, as the only significant difference was that females attracted more beetles than males.

The role of sexual dimorphism in attraction of conspecifics

Our strongest result is that caged female beetles were more attractive to both male and female conspecifics in the field compared to male beetles. This differential attraction may be linked to a variety of factors. Previous research has indicated that many beetles typically do not use long-range signaling from either pheromones or sound production (Brzozowski et al. 2020; Alexander 1957). Nonetheless, these may be important at short distances or for maintaining an aggregation once it is formed. Despite their lack of long-range signaling, T. tetrophthalmus do tend to form moderately sized, often sex-biased groups (Reagel et al. 2002; Matter 2009). This may be due to differential movement patterns and durations of stay for male and female beetles: male beetles tend to travel more frequently and for longer distances than females (Lawrence 1982). Males appear to land arbitrarily on plants but stay for longer durations on plants already occupied by females than on plants with other males or no other beetles (Reagel et al. 2002). This preference for occupied plants may explain the formation of aggregations, regardless of whether the beetles themselves have a mechanism for attracting conspecifics. Finally, cuticular hydrocarbons may play a role in differential aggregation. These pheromones are found predominantly in females and are thought to facilitate short-range formation and maintenance of groups in several species of insects (Blomquist and Bagnéres 2010). Although there are little data on the presence of cuticular hydrocarbons in female T. tetrophthalmus, the role of these compounds in other longhorn beetles suggests that they may have contributed to differential attraction in our study (Silliman 2014).

Physical features of milkweed flowers may also yield differential attraction to certain plants. Plants with large inflorescences tend to attract beetles more than non-flowering plants or plants with small inflorescences. Furthermore, there is a positive correlation between plant height and size and number of umbels per plant with density of beetle aggregation (Reagel et al. 2002; Agrawal 2004). Though we expect the effects of plant characteristics to be minimal due to randomization of our treatments, it is nonetheless possible that these factors may have been involved. More likely, plant damage may have been a contributor to differential aggregation. Given Gontijo’s (2013) findings, we had expected damaged plants to be more attractive to beetles. Although we found no difference in trenching and feeding frequency for male and female beetles, female trenching did tend to be more extensive, making cuts to more veins than males. This widespread trenching could serve as a visual cue for conspecifics, indicating the presence of a food source or potential mates. In this way, increased trenching of veins in females may provide an alternative explanation for dimorphic attraction of females over male beetles. Previous work had not documented a link between these two phenomena, and the interaction of these two findings is an interesting direction for further research.

The role of sexual dimorphism in feeding and trenching

We had predicted that, following Gontijo’s (2013) work, females would broadly trench more than males (or at least more quickly), and that this may be related to dimorphism in size. After repeated experimentation on wild caught beetles over two years, however, we found that dimorphism in this behavior is not common in T. tetrophthalmus despite differences in size. Although closely related, T. tetrophthalmus and T. femoratus tend to occupy different regions of the USA (Rice 1998). Here, we studied T. tetrophthalmus in central New York state, while T. femoratus were observed in Washington state by Gontijo (2013); it is thus possible that these distant populations evolved different patterns with regards to male and female trenching. Furthermore, small size is not a barrier to trenching in other species of insects. Danaus gilippus larvae, for example, trench less frequently as they grow; similarly, Theroa zethus caterpillars utilize acid secretions to enable trenching even at small sizes (Ferreira and Rodrigues 2015; Dussourd 2015). In monarchs, caterpillars trench throughout development but in different patterns as they grow (Agrawal 2017). These findings are consistent with our result and suggest that size may not be a strong predictor of trenching in many systems. As noted earlier, we did find that, while total trenching rates were comparable, females tended to make trenches in more veins than males. This approach may be more effective in deactivating latex over an entire leaf, despite similar rates of the behavior overall.

Though we generally found little evidence of dimorphism in trenching behaviors, we did note that for one out of sixteen measured timepoints, female beetles made significantly more trenches than males. This finding occurred 24 hours after the start of Experiment 3, Trial 2, the trial in which we observed the highest overall trenching rates for both males and females. Although our analysis of dimorphism comes from an average of trenching rates from males and females, we noted that for this trial, a few female beetles had made particularly high numbers of trenches. Given this finding, and the results of our other fifteen trials, we conclude here that this datapoint is likely an outlier based on a few individuals and not reflective of true dimorphism in trenching. A second limitation of our experiment comes from the timing of our data collection. We observed each plant during discrete rounds across 24 hours, with several hours between each circuit. It is thus possible that differences in timing of trenching across long time periods may not have been captured. Nonetheless, our observations of timing of trenching and feeding at shorter timeframes have indicated that there is no difference between males and females in this regard.

Our findings have demonstrated that sexual dimorphism is an important driver of differential behavior with respect to some, but not all, behaviors in T. tetrophthalmus. While we provide empirical evidence of differences in attraction and veins trenched, we also note that some predicted differences in trenching and feeding were not present. Future work studying this, as well as the broader mechanisms of beetle aggregation, is needed to fully understand the results we report here.

We studied milkweed and its insect specialist T. tetrophthalmus as a model to understand plant-insect interactions and sexual dimorphism of adaptive behaviors. Existing literature on a related species, T. femoratus, had suggested that beetles are dimorphic in both size and trenching frequency. While we did not find strong evidence for sex-specific differences in trenching in T. tetrophthalmus, we note that female beetles were larger in size, more attractive to male and female conspecifics, and trenched more veins than males. The latter two findings are novel observations, and we propose here that extensive vein trenching may serve as a visual cue to facilitate differential attraction. These findings raise interesting questions regarding the evolution of behavior in closely related beetle species, the significance and interaction of differential vein trenching and attraction, and the broader mechanisms of coevolution between milkweed and its herbivores. Our work highlights the complexity of sexual dimorphism in this system.

Funding

This work was supported by National Science Foundation grant IOS-2209762 to AAA.

Competing Interests

The authors have no financial or non-financial interests to disclose.

Acknowledgments

We thank Amy Hastings for assistance in beetle collection and National Science Foundation grant IOS-2209762 to AAA for financial support.

Author Contribution

AAA and EJC conceptualized the study and designed the experiments. EJC wrote the first draft of the manuscript and all authors participated in additional writing and editing. The bulk of field work and data collection was done by EJC (Experiments 1-3) and MBG (Experiment 4), and data was analyzed by AAA.

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No competing interests reported.

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You are reading this latest preprint version

A test of sexual dimorphism in deactivating plant defense and attraction of conspecifics in the four-eyed red milkweed beetle (Tetraopes tetrophthalmus)

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Materials and Methods

Study Site

Experiment 1 - Sexual dimorphism in feeding and trenching (when alone):

Experiment 2 – Sexual dimorphism in feeding and trenching (when paired)

Experiment 3 – The role of sexual dimorphism and leaf damage in attracting conspecifics

Experiment 3, Trial 1

Experiment 3, Trial 2

Experiment 4 – Size dimorphism in trenching

Experiment 4, Trial 1

Experiment 4, Trial 2

Statistical Analysis

Results

Sexual dimorphism in feeding and trenching (when alone)

Sexual dimorphism in feeding and trenching (when paired)

The role of sexual dimorphism and leaf damage in attracting conspecifics

Discussion

The role of sexual dimorphism in attraction of conspecifics

The role of sexual dimorphism in feeding and trenching

Conclusion

Declarations

Funding

Competing Interests

Acknowledgments

Author Contribution

References

Additional Declarations

Status:

Journal Publication

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