7.2 The real deal?: behavioral mimicry in hymenoptera  (Page 6/7)

Chemical mimicry allows for hymenopteran insects to serve as model organisms for interspecific co-evolutionary arms races

We have established in this chapter that the chemical mimicry Hymenopterans utilize to modify their cuticular hydrocarbon profiles allows for social parasitism. This phenomenon has also been recognized in avian systems for years (Cervo 2006). When social parasites exhibit cleptoparasitic behavior in order to raise their young, it is known as brood parasitism. Birds and brood parasitism have long been considered together as a driving mechanism for co-evolutionary arms races between the host and parasite. In these arms races, host and parasite species continuously adapt and counter-adapt to one another via positive feedback loops, with both species trying to gain the upper hand in their interaction. Up until relatively recently, social insects, such as the Hymenoptera, were largely only considered as model organisms for studying sociobiological interactions; however, they also make suitable models for co-evolutionary arms race studies (Cervo 2006). Polistes paper wasp species are phylogenetically similar, rare, present in high local populations, and are capable of exerting strong selective pressure on each other, all key factors in serving as a model organism for co-evolution. Most importantly, and what distinguishes paper wasps as possibly better models than avian brood parasites, is that Polistes parasites and hosts have similar population sizes and generation times (Cervo 2006). Thus, Hymenoptera can serve as good models for co-evolutionary arms races and sociobiological studies.

Another example of interspecific co-evolution in Hymenoptera and their hosts is facilitated by chemical mimicry. In Central America, acacia trees play host to an ant, Pseudomyrmex ferrugineus (Espelie&Hermann 1988). The social wasp Parachartergus aztecus chemically mimics the ant’s cuticular hydrocarbon profile and the acacia tree’s thorn wax lipids in order to nest peacefully in the acacia with the ants. This strongly suggests that biochemical co-evolution occurs between the two Hymenopterans, the ant and wasp, and their host plant (Espelie&Hermann 1988). Both of the examples discussed here illustrate convincingly that Hymenoptera can serve not only as good models for social behavior, but also for co-evolution.

Mimicry as self-defense

An example of Müllerian mimicry in Hymenoptera is elucidated by a protective mechanism in Parachartergus colobopterus , a social wasp species. Workers of P. colobopterus sting only in self-defense, not for the protection of the colony (Strassmann et al. 1990). Workers have stingers, but do not sting the antagonist unless individually threatened. Instead, if the colony is disturbed, workers simply fly around and heavily beat their gasters , the bulbous portion of the Hymenopteran abdomen. One explanation for this behavior that is being debated is that these wasps mimic more aggressive species, such as vespid wasps, to achieve their protection. P. colobopterus do have alternating stripes of yellow and brown, which could serve as aposematic signals to deter predators. In this regard, the P. colobopterus wasps act as Müllerian mimics because they use the signaling to avoid predation and possess effective stingers, however, they seldom use their stings in colony defense. Other possible explanations for this interesting behavior include that it is a warning to predators that the larvae are distasteful or that nestmates have a low level of relatedness, suggesting that it is not within an individual’s interest to fight aggressively to defend the colony. Though all of these theories could explain this mimetic behavior, it is still widely unknown as to why P. colobopterus uses its sting sparingly in defense of the colony. It is possible that this occurs because vertebrate predation of these wasp species is rare in areas where P. colobopterus is found (Strassmann et al. 1990).