Chemical Mimicry in the Insect World Josh Legoza 4/6/05

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1 Chemical Mimicry in the Insect World Josh Legoza 4/6/05 Throughout the insect world, chemical signals are used for a number of survival related mechanisms. One of the primary mechanisms using chemical signals is defense. Found primarily in social insects, chemical signals are present on all members of a nest, or in many cases, all members of a species. While these chemical signals work well to protect the nest and the species, parasites and predators to these species have evolved mimicking behaviors to bypass these defenses. In most cases, this involves acquiring or producing a mimicking chemical. This is seen particularly in species that partake in brood care, whether social or solitary insects. Along with defense mechanisms, chemical cues are also used for reproduction. However predators have developed the ability to turn this species survival mechanism into a way to survive themselves. Insects of many different families use chemical signals to attract a mate. This has proven to be a very valuable characteristic that has been selected for as it enables members of a species to easily locate one another over some distance and reproduce. Along with the defensive mechanisms, predatory or parasitic insects have turned this successful mechanism against the species. This species survival mechanism had been turned into a method to attract the species to predation. A number of predators have been shown to mimic the chemical signals in order to attract their prey. An unsuspecting insect looking to reproduce instead finds itself as dinner. The mimicry of another species in order to prey on it, or rely upon it in a parasitic manner seems to fall into two categories. The first of which is chemical cues present on the body recognized by members of the prey (or host) species. The second is scent or pheromone mimicry. Chemical cues are used primarily by species preying on social insects. In order to gain

2 access to the nest, the predator mimics the structure of the epicuticular hydrocarbons found on the host species. This enables them to enter the nest without being attacked, and move freely throughout the nest without disturbance. Chemical Communication in Insects Throughout evolutionary time, a vast majority of insects have developed methods of chemical communication. Chemical communication has been selected for due to its effectiveness in a variety of species survival needs, its simplicity, and the ability to communicate over distances through pheromones. These methods of communication exist in a large number of species throughout the insect world, in social and solitary insects alike. The types of communication accomplished through chemicals are almost as numerous as the insects that use them. Some of the primary types (as well as the best understood) are chemical defense cues, chemical recognition, and mate attraction. Human knowledge of these chemical cues is limited, and research continues in an effort to understand completely the mechanisms by which this effective communication occurs. Though human understanding is insufficient, a number of other insects have evolved the capability to manipulate these chemical cues. What has proven to be an efficient means of species survival has turned (in some case) into a method to manipulate the species so that a predator or parasite species may survive. The most effective forms of chemical communication have evolved in social insects. Ants are most frequently targeted, though honey bees and termites are also manipulated by mimicking predators and parasites. In these cases, the social insects have developed a system based mainly on chemical communication. These communications have proven very effective in managing a successful colony. From determining what activities each ant is to be doing, to

3 nestmate recognition, to alarm signaling chemicals play an important role in colony survival. Though it has proven to be a successful method of managing a nest, evolution has left these nests prone to predatory or parasitic insects who manipulate one or more of these chemical cues. Though chemical communication is so widely used by social insects, it doesn t only exist in those species. A number of species use chemical cues for communication. The primary use is attracting mates for copulation. Despite the fact that this communication isn t used as frequently, and isn t as developed as social communication, it still is manipulated by predators. Manipulation of Chemical Communications: Chemical Mimicry Whether in social or solitary insects, it seems that if chemical communication is used, then there are predators mimicking the cues for their own success. A number of different species of beetles, wasps, moths and other insects infiltrate social colonies. There are a large number of examples of predators and parasites that enter colonies undetected. Lepidochrysops ignota larvae manipulate the ant species Camponotus niveosetosus in such a brilliant manner that worker ants will actually carry them into the nest where they prey on the ant brood. Aloeides dentatis can integrate themselves into the nests of Acantholepis capensis so effectively that they live inside through the day. Not only are they undisturbed inside but they come out at night to feed and are accompanied ants who provide protection. Maculinea rebeli larvae not only prey on the brood of Myrmica ants, but also simulate the brood pheromone, and as such are cared for as ant brood. Microdon albicomatus and Microdon piperi move freely in the nests of Myrmica incompleta ants and are often transported by the ants as if they were brood. Wasps of the genus Orasema often garner the same benefits out of Solenopsis invicta ants. The Myrmecaphodius excavaticollis beetle also can move freely in the nest and feed on brood, but it also can get the

4 host ants of Solenopsis and Iridomyrex to feed it! Although it is surprising that these insects can take advantage of such highly organized ant colonies, it is perhaps more surprising that there are ant species that take advantage of other ants. The cuckoo ant Leptothorax kutteri is a social parasite that relies on the closely related Leptothorax acevorum because the queen has lost the ability to feed or produce her own workers. Even more surprising, the ant Harpagoxenus sublaevis makes slaves out of its hosts (either Leptothorax acevorum or Leptothorax muscorum). But it is not only ants that are being manipulated. The beetle Trichopsenius frosti freely infiltrates the nest of its host termite Reticulitermes flavipes. The moth Acherontia atropos occasionally integrates itself into the nests of honey bees. (All above examples are from Dettner & Leipert, 1994.) The social parasitic wasp Polistes semenowi take over the nests of their host Polistes dominulus (Turillazzi et al. 1999; Lorenzi, et al. 2004). The salticid spider, Cosmophasis bitaeniata enters the nest of Oecophylla smaragdina freely, makes its own nests, and preys on its nestmates (Allan, et al. 2001; Elgar, Allan; 2004). Despite the high level of organization and communication within social insects, there are still many cases of inquilines entering their colonies. With so many examples of parasitism and predation within such well defended social insects, many questions are brought to mind. How Do Inquilines Successfully Infiltrate And Inhabit Nests? If ants and other social insects communicate so well through chemical cues and have good defensive mechanisms, one must wonder how so many other insects can successfully overcome these obstacles and prey on their hosts. When ants come across another being in the nest, they quickly examine the being, and determine if it is a nestmate or not. This is accomplished by a chemical signature of sorts. Epicuticular hydrocarbons are present on the bodies of each ant in a nest. They are deposited on the cuticle, not synthesized in it (Lucas, et al.

5 2004). These are specific to species, and in some cases, specific to each nest of a particular species. When the chemical signature matches, and the other being is recognized as a nestmate, the ant moves on. If the chemical signature is wrong, the ant attacks and kills the being. In order to enter and freely move in a host nest, the predator or parasite must be able to convince the ants that it is a nestmate. To do that, the chemical signature of the inquiline must mimic that of the ants in the nest. Chemical analyses of all of the examples presented earlier in this paper show that in each case the epicuticular hydrocarbons on the host and inquiline very closely match. Almost all of the important chemicals are present, in the right quantities, and it has been determined that those missing are not important to the recognition process of the host. However, in no case was the signature correct. There is never an absolutely identical match between the epicuticular hydrocarbons of the inquiline and the host. The inquilines then must be mimicking the chemicals. They are a mimic in all senses of the word. They do not produce the actual chemical signature, or even have all of the elements of the signature present; there is simply enough done to mimic the chemicals, and therefore mimic a nestmate. How do the Inquilines Mimic the actual chemicals? There are a number of different strategies for mimicking chemical cues in order to take advantage of social insects. In fact, it seems that some don t even necessarily mimic the chemicals, but simply have a chemical signature that tells the investigating ant nothing. It has been proposed that some parasites use an odorless strategy to conceal their identity (Lorenzi, Bagneres; 2002). For those who try to appear as if they belong, rather than just conceal their identity, the epicuticular hydrocarbons are critical. As stated earlier these differ between species, and in some cases differ between nests of the same species. In these cases, it is important to the invading insect to have the right signature and be accepted into the nest. So where do the

6 chemicals come from? Though rare, in some cases such as those of M. Piperi and Camponotus modoc, the inquiline synthesizes its own chemical signature that is sufficient enough to enter the nest without attack, and move around freely enough to prey on the brood or hosts (Dettner & Leipert, 1994). In these cases, it is not necessary to acquire the correct chemicals from the host. More commonly, the inquiline will mimic the chemicals closely enough to enter the nest and then acquire the right chemical signature. Even this is done differently from species to species. C. bitaeniata acquire their hydrocarbons through contact with nest material (Elgar & Allan, 2004). D. nevermanni enters the nest then grooms the head and thorax of worker ants in order to obtain the proper chemical cues (Dettner & Leipert, 1994). Still more inquilines don t even carry the proper identification to enter the nest. For example M. excavaticollis is attacked when it enters the nest. It endures attacks over a couple of days, but through the process eventually has the epicuticular hydrocarbons matching those of the host species, ending the attacks (Dettner & Leipert, 1994). A little more active is the Cuckoo ant queen, who is attacked upon entering the nest, but acquires the hydrocarbons during the battle and is never treated aggressively inside the nest (Dettner & Leipert, 1994). In these cases, the inquiline modifies its own epicuticular hydrocarbon profile (Sledge, et al. 2001). These recent studies have shown how it is possible for well organized colonies to be taken advantage of by predators and parasites mimicking nestmates. Though recognition by epicuticular hydrocarbon obviously plays a big role in the process of recognition, it is believed that this is not the whole story. It is already known in some cases that behavior also plays an important role. Even if the chemical signature matches, the behavior of the inquiline must be in line with that of the host species. The role that pheromones play in recognition in social insects is uncertain. It is also unclear if these are being mimicked also or if they are not as important in

7 recognition as the epicuticular hydrocarbons. Lastly, it seems that some inquilines may use pheromones to effectively distract the host so that it performs other duties such as brood care instead of be concerned with the stranger present. Such is the apparent case in A. dentatis where a secretion released evokes a behavioral sequence that is elicited by substances on the ants heads (Dettner & Leipert, 1994). Another possible component whose existence has been neither confirmed nor denied is a symbiotic relationship between the host and inquiline. In some of the cases listed previously, it is suspected that some of the secretions may be rewards to the ants and they are not necessarily oblivious to the fact that there is a non-nestmate present. Chemical Mimicry in Solitary Insects Chemical mimicry does not necessarily need to be a means of being recognized as a nestmate in order to infiltrate a nest of social insects. Many insects prey on solitary insects by simply drawing them near through chemical deception. By mimicking a chemical signal that is used for two members of a species to communicate, a predator can attract prey via chemical mimicry. The prey goes to a place it is not likely to leave, thinking that it is safely going to another member of its species, rather than into its certain doom. Behavior similar to this is seen in many cases. Several examples can be seen in the females of several spider taxa (Araneinae, Celaeneae, Mastrophorinae), where the sex pheromones of female moths are mimicked in order to attract a male moth for prey. Another such example is that of Leistotrophus versicolor which deposits odorous material on leaves and waits for drosophilid and phorid flies when its typical prey are not available (Dettner & Leipert, 1994). Perhaps more interesting than the previous deceptions is intra-specific chemical mimicry. In this mimicry mechanism, one member of a species mimics chemicals present in the opposite sex. It seems odd that insects would trick their own species. One would think that over

8 evolutionary time nature would select members of the species that are more cooperative with others, rather than develop the ability to trick members of the same species. Species survival however is the very reason that intra-specific mimicry has developed and stayed. In many cases, the mimicry is used to spare the life of the mimic, or to make a situation more beneficial for the mimic in another way. One example is the case of Cardiocondyla obscurior. There are both winged and wingless males in this species and the winged are docile while the wingless are aggressive and often kill one another. In this species, a very strange form of chemical mimicry has evolved. A somewhat passive form of chemical mimicry now exists where young winged males have an epicuticular hydrocarbon profile similar to those of the females of the species. Males of the wingless variety tolerate and even try to mate with the young winged male. In fact, they are as attractive as young virgin queens. What makes this mimicry so peculiar is the fact that it does not lessen mating success of either of the two morphs. Instead, it provides a new evolutionary context for chemical deception; it is deployed irrespective of condition by all young winged males and renders them simultaneously attractive to both sexes. This example of chemical mimicry enables two alternative reproductive strategies to exist in the male sex (Cremer, et al. 2002). In contrast to this more interesting mimicry, the chemical mimicry of the rove beetle Aleochara curtula causes lowered mating success in order to avoid competition. The rove beetle is very aggressive to competing males when on a carcass (where they look for blowfly maggots and mates). When many are competing for the area and few females, young, starved, or previously copulated males produce the female sex pheromone in order to avoid aggression from competing males. It is not only effective in preventing attacks; it also often causes the males to attempt to copulate with the pseudo-female. This chemical mimicry may allow a young male to

9 survive, but it also prevents them from mating. Females are repulsed and even behave aggressively to males bearing the female sex pheromone (Dettner & Leipert, 1994). Although this mimicry does not seem as effective as that seen in C. Obscurior it still must be effective enough to have been selected by nature. That possibly implies that males who effectively removed themselves from mating competition in order to save their lives will have the chance to mate. Without the female pheromone, they could have died before having the chance to mate at all. A final example of intra-specific chemical mimicry exists in Drosophila melongaster females. After mating, the females deceive courting males by releasing the male sex pheromone. This hydrocarbon profile is basically not present in a virgin female, however, it is transferred during copulation, and the female then begins to synthesize the pheromone herself. Not having to be bothered by courting males is an effective genetic strategy because it allows the female more time for feeding to support her young. Although deceiving one s own species seems like it would be against species survival and accordingly be selected against, it has been shown that these mimicries are effective means of increasing species survival. Others Involved in Chemical Mimicry Insects aren t the only ones that manipulate other insects through chemical signals. Even plants have evolved capabilities to use insects to further their species. Many flowers count on insects to pollinate them. Though it is a more symbiotic relationship than the predator/parasite interactions discussed earlier, chemical mimicry still is used by the plants in order to make the symbiotic relationship happen. If fact, within the family Orchidacae approximately 50% of the species attract pollinators by mimicking female sex pheromones. Ophyrs has about 30 species that mimic specific sex pheromones to attract specific insects. Various epiphytic plant families

10 have seeds that release a mimic of brood pheromones of Camponotus femoratus and a Crematogaster species. This tricks the ants into taking the seed into the brooding chamber of their nest, giving the seed a greater chance of germination (Dettner & Leipert, 1994). Plant mimicry of insect chemicals has is of large importance to scientific understanding of insect behavior. Use of mimicry, especially that of plants points to the importance of chemical communication over sight or touch in insects. F.P. Schiestl et al. demonstrate this importance in the interaction of Andrena nigroanea and the orchid Ophrys sphegodes. In fact, they suggest that there is a superior importance of olfactory stimuli over visual or tactile stimuli. This importance is the reason for success of chemical mimicry against insects, and leads to questions about where evolution will lead over time. It seems that in order to avoid being taken advantage of, eventually insects (especially social ones) will develop more discriminating requirements for actions to be put into effect. Making visual or tactile stimuli more important will help protect the species from the manipulations discussed here, however there may not be enough selective pressure for these changes to happen, and there may never be.

11 References: Akina T., Knapp J.J., Thomas J.A., Elmes G.W. (1999) Chemical mimicry and host specificity in the butterfly Maculinea rebeli, a social parasite of myrmica ant colonies. Biological Sciences. Allan R.A., Capon R.J., Brown W.V., Elgar M.A. (2002) Mimicry of host cuticular hydrocarbons by salticid spider Cosmophasis bitaeniata that preys on Larvae of tree ants Oecophylla smaragdina. Journal of Chemical Ecology. Cremer S., Sledge M.F., Heinze, J. (2002) Male ants disguised by the queen s boquet. Nature. Dettner K., Leipert C. (1994) Chemical Mimicry and Camoflage. Annual Reviews. D Ettorre P., Mondy N., Lenoir A., Errard C., (2002) Blending in with the crowd: social parasites integrate into their host colonies using a flexible chemical signature. The Royal Society Elgar M.A., Allan R.A. (2004) Predatory spider mimics acquire colony-specific cuticular hydrocarbons from their ant model prey. Naturwissenschaften. Elmes G.W., Akino T., Thomas J.A., Clarke R.T., Knapp J.J. (2002) Interspecific differences in cuticular hydrocarbon profiles of Myrmica ants are sufficiently consistent to explain host specificity by Maculinea (large blue) butterflies. Ecophysiology. Lorenzi M.C., Cervo R., Zacchi F., Turillazzi S., Bagneres A.G. (2004) Dynamics of chemical mimicry in the social parasite wasp Polistes semenowi (Hymenoptera: Vespidae). Parasitology. Lorenzi M.C., Bagneres A.G. (2002) Concealing identity and mimicking hosts: a dual chemical strategy for a single social parasite? (Polistes atrimandibularis, Hymenoptera: Vespidae). Parasitology. Lucas C., Pho D.B., Fresneau D., Jallon J.M. (2004) Hydrocarbon circulation and colonial signature in Pachyondyla villosa. Journal of Insect Physiology. Schiestl F.P., Ayasse M., Paulus H.F., Lofstedt C., Hansson B.S., Ibarra F., Francke W. (2000) Sex pheromone mimicry in the early spider orchid (Ophrys sphegodes): patterns of hydrocarbons as the key mechanism for pollination by sexual deception. Journal Comp. Physiology A. 186: Sledge M.F., Dani F.R., Cervo R. Dapporto L., Turillazzi, S. (2001) Recognition of social parasites as nest-mates: adoption of colony-specific host cuticular ordours by the paper wasp parasite Polistes sulcifer. The Royal Society. Rasa O.A.E., Heg D., (2004) Individual variation and prior experience affect the discrimination of a brood-parasite by its subsocial beetle host. Behavioral Ecology Sociobiology.

12 Turillazzi S., Sledge M.F., Dani F.R., Cervo R., Massolo A., Fondelli L. (2000) Social Hackers: Integration in the Host Chemical Recognition System by a Paper Wasp Social Parasite. Naturwissenschaften.

The function or adaptive value of signals has been broken down into the following classes:

The function or adaptive value of signals has been broken down into the following classes: Communication notes.doc 1 Communication and signals an action on the part of one animal that alters the behavior of another (Wilson 1975). The essence of communication is the relationship between signaler