Gender- and sequence-dependent predation within group colonizers of defended plants: a constraint on cheating among bark beetles?

Abstract

Bark beetles engage in one of the most pronounced examples of group procurement of defended plants. Their aggregation pheromones attract both sexes and are essential to overcome constitutive and rapidly inducible lethal defenses. The relative benefits to senders versus receivers of these signals are only partly understood. Because the initial stage of host entry can be hazardous, there may be benefit to a cheating strategy, whose practitioners respond to pheromones but do not engage in host searching. Several disadvantages to cheating have been proposed, but the role of predators has not been considered. Predators exploit bark beetle pheromones to locate prey, accumulate at the breeding site, and consume adult bark beetles before they enter the tree. Preliminary experiments quantified arrival patterns in the field. We used a laboratory assay to investigate relative predation on pioneers (those that initially select and enter hosts) and responders (those that arrive at a host in response to pheromones) during host colonization. Our model system utilized the pine engraver, Ips pini, which exhibits male harem polygamy. We allowed male I. pini to colonize host tissue and added females 1 day later. Also 1 day later, we variably added additional males and predacious checkered beetles, Thanasimus dubius. These treatments included two densities of males and three densities of predators that were selected to emulate field conditions. Responding males experienced higher predation than pioneers. T. dubius ate more males than females, independent of the presence or absence of responding males. T. dubius affected the distribution of females per male, although the number of females that survived to construct ovipositional galleries was constant. We discuss the viability of cheating, implications for biological control, and predator-prey coevolution in this cooperative, group-colonizing herbivore.

Dangerous liaisons: the predation risks of receiving social signals

“…Checkered beetles (Thanasimus dubius), for example, are attracted to the aggregation pheromones of their pine beetle (Ips pini) prey. However, these predators consume more of the pine beetles that are attracted to the pheromones than they do on the pine beetles that emit them (Aukema & Raffa 2004). Although counter-intuitive, some alarm signals may actually increase a receiver's risk of predation.…”

Fitness consequences of pheromone production and host selection strategies in a tree-killing bark beetle (Coleoptera: Curculionidae: Scolytinae)

“…Beetles arrive at a suitable tree, and produce potent aggregation pheromones that attract more conspecifics to the tree (Wood 1982;Raffa et al 1993). Sufficient quantities of aggregation pheromones must be produced by the group to attract enough beetles to kill the tree (Raffa and Berryman 1987;Raffa 2001;Aukema and Raffa 2004). Once the tree is under ''mass attack'', antiaggregation pheromones are produced by beetles that terminate attack, prevent overcrowding, and reduce competition for phloem resource on which larvae feed (Wood 1982;Raffa et al 1993).…”

“…The other pheromones, transverbenol, verbenone and myrtenol are oxidation products of host monoterpenes (Hughes 1973(Hughes , 1975Renwick et al 1976), and may be less metabolically costly than pheromones synthesised de novo. Other costs of pheromone production include attracting predators (Reeve 1997;Aukema and Raffa 2004) and competing heterospecific bark beetles (Svihra et al 1980) that respond positively to pheromones (Ayres et al 2001;Dahlsten et al 2004). Although D. frontalis is one of the most intensively studied tree-killing bark beetles in North America, investigations on the behaviour of individual beetles have been surprisingly limited.…”

High individual variation in pheromone production by tree-killing bark beetles (Coleoptera: Curculionidae: Scolytinae)

“…Pheromones are generally assumed to have strong fitness effects on tree-killing bark beetles because they potentiate the aggregation behaviour that is necessary to overcome tree defenses and permit beetle reproduction (Raffa and Berryman 1987;Raffa 2001;Aukema and Raffa 2004). The attractiveness of a tree to dispersing beetles increases as the amount of pheromone emanating from it increases (Coster and Gara 1968;Miller et al 2005a, b).…”