BEHAVIOR Male Megacyllene robiniae (Coleoptera: Cerambycidae) Use Multiple Tactics When Aggressively Competing for Mates 1 2 ANN M. RAY, MATTHEW D. GINZEL, AND LAWRENCE M. HANKS Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 Environ. Entomol. 38(2): 425Ð432 (2009) ABSTRACT Adult male Megacyllene robiniae (Fo¨rster) (Coleoptera: Cerambycidae) that are paired with a female often are challenged by conspeciÞc males that attempt to displace them. In staged laboratory bouts, challenging males used seven distinct tactics to displace defending males, including wedging their head between the defender and the female (termed wedging), straddling the mated pair and pulling the defender off (prying), pulling it with the mandibles, batting it with the antennae, or pushing, biting, or kicking the defender. Individual challengers attempted as many as six different tactics in a single bout, repeating certain tactics multiple times. They often attempted tactics that were not very effective. For example, prying was one of the most common attempted tactics but was rarely effective. However, few challengers attempted to push defenders off the female, even though that tactic often was effective. Challengers apparently were inßuenced by context in their choice of particular tactics. For example, males that approached the mated pair from the side were likely to use wedging, whereas those approaching head on were more likely to bat with the antennae. Choice of tactic apparently was not inßuenced by absolute size of challengers, nor was it strongly inßuenced by relative size of defenders. However, the effectiveness of tactics varied signiÞcantly with relative body size: larger challengers were most successful when prying or pushing, while smaller challengers were most successful when biting and kicking. By using different tactics, relatively small males were as adept as larger males at displacing rivals. KEY WORDS mating behavior, interspeciÞc competition, aggression, longhorned beetle Male animals that are competing for mates may com- not guarded by competitors, or by intercepting fe- pensate for small body size by adopting alternative males before they are discovered by larger males mating strategies (Dominey 1984). Cerambycid bee- (Hughes and Hughes 1982, Goldsmith 1987, Lawrence tles are appropriate subjects for studying body size 1987, Goldsmith and Alcock 1993, Zeh et al. 1992). effects in mating strategy because variation in nutri- In this study, we characterize the mating behavior tion of the wood-boring larvae results in great varia- of the cerambycid beetle Megacyllene robiniae (Fo¨r- tion in adult body size (Andersen and Nilssen 1983) ster), the locust borer, and present results of experi- and males compete aggressively for females (Linsley ments that determine how body size of males inßu- 1959). Large males of several cerambycid species have ences mating success. This familiar species is endemic the advantage in aggressive competition for mates, and to eastern North America (Yanega 1996). The diurnal competition among males results in size-related mat- adults are aposematically colored wasp mimics and are ing success (Hughes and Hughes 1982; McLain and active in late summer and fall (Galford 1984, Solomon Boromisa 1987; Edwards and Linit 1991; Goldsmith et 1995). Large males can be more than twice the size of al. 1996; Hanks et al. 1996a, b; Wang 2002; Wang and the smallest males (range in body length: 11.5Ð25 mm; Zeng 2004). In some species of cerambycid beetles, males of different body size adopt very different mat- Linsley 1964). Adults of both sexes feed on pollen of ing strategies. For example, smaller males may avoid goldenrod (Solidago spp.; Asteraceae) and tend to conßicts with larger males by dispersing from areas aggregate on isolated plants with conspicuous inßo- where conspeciÞc males are abundant, by searching rescences (Garman 1916, Galford 1984, Harman and for resources (food plants of adults or larvae) that are Harman 1987). There, males search for females and recognize them by a contact pheromone in their cu- 1 Current address: Departments of Entomology and Forestry and ticular wax layer (Ginzel and Hanks 2003, Ginzel et al. Natural Resources, Purdue University, 901 W. State St., West Lafay- 2003). Males remain paired with the female after mat- ette, IN 47907. ing, during which time conspeciÞc males will attempt 2 Corresponding author: Department of Entomology, University of Illinois at Urbana-Champaign, 320 Morrill Hall, 505 S. Goodwin Ave., to displace them (M.D.G., personal observation). Fe- Urbana, IL 61801 (e-mail: [email protected]). males oviposit under loose bark and in cracks in the 0046-225X/09/0425Ð0432$04.00/0 ᭧ 2009 Entomological Society of America 426 ENVIRONMENTAL ENTOMOLOGY Vol. 38, no. 2 bark of stressed or weakened black locust trees (Ro- arena. Body size of challengers (C), relative to the size binia pseudoacacia L.; Solomon 1995). of defenders (D), was calculated as (C/D) ϫ 100. We observed during preliminary studies that male Challengers and defenders were selected to represent M. robiniae attempted a variety of tactics to displace a broad range of relative body sizes. We assigned rival males that already were paired with females (see challengers to three categories based on their relative below). The great variation in body size of the males size: small challengers (Ͻ95% the size of the de- suggested that individuals may compensate for small fender), similarly sized challengers (from 95 to 105% body size by adopting different tactics. In this study, the size of the defender), and large challengers we examine the inßuence of absolute and relative (Ͼ105% the size of the defender). We also explored body size on one component of competition for mates, the data by using more extreme categories (with 10 the ability of unpaired males (challengers) to displace and 20% differences in relative body size), but these rivals (defenders) that are paired with a mate. We test reanalyses did not alter the Þndings signiÞcantly and the following predictions that are based on the current therefore are not reported here. We also examined the literature on aggressive competition in cerambycid data to determine whether choice of mating tactic was beetles: inßuenced by absolute body size (i.e., small challeng- Prediction 1: relatively large challengers will dom- ers choose different tactics than larger challengers, inate in aggressive competition for mates, displacing regardless of the size of the defender). smaller defenders. In each bout, the challenger was allowed enough Prediction 2: challengers of different relative size time to come into contact with the mated pair and one use different tactics. opportunity to separate them (the bout was termi- Prediction 3: challengers choose tactics that are nated when the challenger broke off the attack). This most effective for displacing defenders. limited opportunity for gaining access to a female is representative of natural conditions (see Mating Be- havior and Aggressive Competition). Individual females Materials and Methods were arbitrarily selected and reused in trials. Individ- Mating Behavior and Aggressive Competition. We ual males usually were used in bioassays only once per characterized the mating behavior of M. robiniae and day, although we reused some males as challengers if aggressive interactions among males by observing and they had not mated (but the same two males were videotaping Ϸ20 pairs of male beetles that were com- never used in more than one trial). Males often were peting for females on goldenrod inßorescences. This not cooperative, the Þrst male not responding study was conducted at Lodge Park (Piatt County, IL) to the female or the second male not challenging the on sunny and warm afternoons in late September and defender. In some cases, challengers apparently did early October 1997. In many cases, males were ob- not Þght because they had not detected the female, scured by vegetation or their behaviors were not but it usually was not possible to conÞrm this behavior. clearly discernible because of the camera angle. Males On some days, all males were consistently uncooper- were clearly visible in Þve cases, and slow-motion ative. We have noted in studies of the behavior of playback of videotapes allowed us to determine which many species of cerambycids that beetles apparently tactics were attempted by challenging males. are inßuenced by weather conditions, despite the fact Influence of Body Size on Choice of Tactic and that they are under laboratory conditions. For exam- Competitive Ability. We tested our predictions by ple, males often become inactive when storm fronts staging bouts between pairs of male M. robiniae in are approaching, perhaps because of changes in baro- three independent laboratory experiments. Adult bee- metric pressure. We included in our analyses only tles were collected from inßorescences of goldenrod bouts in which the Þrst male mated with the female into glass vials at Allerton Park, Piatt County, IL, and the second male challenged him. within a few days of each study. Body size of beetles In study 1, we used nine female (mean elytron was estimated by measuring the length of the right length Ϯ SD: 11.9 Ϯ 1.5 mm) and 21 male (10.6 Ϯ 1.1 elytron using a caliper. Beetles were housed individ- mm) M. robiniae in 49 bouts during 15 September to ually in Ϸ300-cm3 cages of aluminum window screen 8 October 2004 (Table 1). Bouts were videotaped, and with fresh goldenrod inßorescences and feeder