Southern Pine Beetles (Dendroctonus frontalis, figure 1) are one of the most harmful pests in the world, causing hundreds of million dollars worth of damage to pine trees in the Southern United States. These beetles initiate their attack on a pine tree by having a small group of female beetles dig into the inner park (Figure 2) and phloem of the pine tree. Once this work is done these females will secrete a chemical that will draw other male and female beetles to the affected pine tree. This initial attack is followed by a massive attack by a larger number of beetles, enabling them to easily overcome the defenses of the pine trees due to the sheer number of beetles. Shortly after taking over the tree, mating begins between the male and female beetles. As part of the mating process the females fill the excavated galleries within the trees with eggs. After the eggs have been laid the adult beetles will then leave the tree and continue to attack other trees. Attacking trees and causing their death is of course a sad story and may not sound very interesting since most pests have similar attack strategies; however, one detail that we did not mention yet makes this process more intriguing. What do the larvae eat to complete their development within a gallery that is inside a dead tree? The answer is: Fungi.

Pine beetles establish a symbiotic life structure with a beneficial fungus (Entomocorticium sp. A), which is the main food source for their larvae. Adult pine beetles have a body compartment (mycangium), in which they can carry this fungus. When adult beetles dig galleries in tree barks, they inoculate these galleries with the fungi. This fungus will grow in these galleries that helps the beetles’ larvae complete their development by providing them with a source of nutrition. Fungi also benefit from this process by being transferred from one tree to another with the help of the beetles. This symbiotic life structure is threatened by the existence of an antagonistic fungus, (Ophiostoma minus, also known as blue stain fungus) and parasitic mites both of which the southern pine beetles also bring along. The blue stain fungus has no nutritional importance for larvae, this fungus can grow in the same galleries as the beneficial fungus and they can even outcompete them. In addition, the parasitic mites feed on the blue fungi and can prove harmful beetles as the amount of blue fungi increases. Therefore, beetles’ larvae cannot survive for long and thus the reproduction of the southern pine beetles can be disrupted without some form of defense.

A recent study by Scott et al. has shed some light on this complicated life structure [3]. These tiny beetles, which are only a few millimeters long, have a smart defense mechanism to prevent their larvae (Figure 3, pink arrow). As far as the history tells us, humankind started using pesticides about 5000 years ago, but these tiny beetles have been using them for preventing the growth of the blue-stain fungi long before humans started using pesticides. These studies showed that the symbiotic coexistence of southern pine beetles and the beneficial fungi (Figure 3, yellow circle) is maintained by a (actinomycetous) bacterium (Figure 3, inside the red square). This bacterium produces a previously unknown antibiotic compound (named mycangimycin), which selectively inhibits the growth of blue-stain fungus hence providing a significant advantage for the maintenance of the beneficial fungi (that is, the main food source of the larvae). How can this bacterium even be present in the freshly carved galleries in pine trees in the first place? The surprising answer to this question is that they are transferred to these galleries by the very same pine beetles. As mentioned earlier, adult pine beetles carry beneficial fungi in their body compartment (mycangium) and inoculate the galleries they carved with this fungus to provide food for their larvae. In addition to this beneficial fungi these beetles carry they also carry bacteria that can produce antibiotic compound to inhibit the growth of blue-stain fungi and consequently diminish the number of parasitic mites. This bacterium can grow inside these galleries and even in a body compartment of adult southern pine beetles. Interestingly, this antibiotic kills blue-stain fungi but does not significantly affect the growth of the fungi, which is the food source for larvae. Thus, the beneficial fungi can multiply in number and offer enough food for the development of beetle larvae. Some of these larvae manage to grow to adults and then leave to attack other pine trees carrying the same fungi and bacteria with them (Figure 4).

The interactions within the rest of the life kingdoms is not any less complicated than the symbiosis between pine beetles, fungi, and the bacteria. The interactions between animals, plants and microbes are very complex and also fragile. Removing or replacing any member of an ecosystem can often result in a serious failure in the ecosystem as was observed many times especially within the last century.

References

1. http://www.nsf.gov/news/news_summ.jsp?org=NSF&cntn_id=112319&preview=false.
2. http://entnem.ufl.edu/creatures/trees/southern_pine_beetle.htm.
3. Scott, J.J., et al., “Bacterial protection of beetle-fungus mutualism.” Science, 2008. 322 (5898): p. 63.

Ants are highly social insects and have a complex system of internal communication. It is only by this system that the colonies manage to carry out their collaborative activities like nest- construction, food-gathering, brood-rearing, and defense of the colony. The fact that ants allow some alien species full access to the benefits of their society suggests that the guests must somehow have, in the words of Hölldobler, ‘broken the ant’s code, that is, attained the ability to ‘speak’ the ants’ language, which involves a diversity of visual, mechanical and chemical cues. ’

To support this suggestion, Hölldobler focused mainly on the rove beetle and looked into its communications and relations with certain species of ants. The relations vary considerably with the beetle species. Some live along the ants’ food gathering trail, some at the garbage dump, some in the chambers within the nest and others inside the brood chamber itself.

Atemeles pubicollis, a European species of beetle, is a well-known example of the species that live inside the brood chamber. It lives in the nest of the mound-making wood ant Formica polyetena during its larval stage. Hölldobler found that the ants’ adoption of the beetle larva depends on chemical communication. The larva secretes a substance that apparently acts as an attractant for the ant. The brood-keeping ants respond to the chemical signal with intense grooming of the larvae.

A different kind of communication takes place to elicit the ant’s feeding of the larvae. Hölldobler observed that the beetle larvae imitate certain begging behaviour of ant larvae involving mechanical stimulation of the brood- keeping adults. When the adult ant touches the beetle larva with its mouth or antenna, the larva rears up immediately and tries to make contact with the ant’s head. If the larva succeeds in tapping the ant’s lip with its own mouth, the ant regurgitates a droplet of food. The beetle larvae receive more food than the ant larvae since they perform the begging behaviour more intensely than the ant larvae do.

How does the ant colony manage to survive the beetle larvae’s competition for food? The answer is a simple:The beetle larvae are cannibalistic and unable to distinguish their fellow larvae from ant larvae by odour. Thus, they reduce their own population. That is why we find the ant larvae in clusters while the beetle larvae, having devoured their neighbours, are loners in the brood chamber.

The Atemeles beetles have two different homes with ants; one for the summer and one for winter. In the autumn, the beetles migrate to nests of the dark brown insect eating ants of the genus Myrmica. The reason for their migration is that brood-keeping and the food supply are maintained in Myrmica throughout the winter, whereas Formica ants suspend their raising of young. In the spring the beetles return to Formica nests for mating and the laying of eggs. The Lomechusa beetle are also co-dwellers with Formica ants. However, they do not change their environment for the winter. Instead, after hatching they simply move on to another Formica colony of the same species and share their food supply.

How the migrating beetle find its way to a Myrmica nest is another question. We find Formica nests normally in woodlands, whereas Myrmica are found in the grasslands beyond the woods. Hölldobler suggests that when the beetles leave the Formica nest, they generally move in the direction of increasing light. This may explain how the beetles manage to reach the relatively open grasslands where the Myrmica ants Jive. When they reach open grasslands they use the odour of the host species of ant to find a nest.

The beetle obtains recognition and adoption with a ritual, involving chemical communication, when it finds a Myrimica nest. The beetle first touches the ant lightly with its antenna and raises the tips of lts abdomen towards the host. The ant responds by secretions from glands on the tip of the abdomen. Next the ant is attracted to a series of glands along the sides of the beetle’s abdomen. Hölldobler calls these ‘the adoption glands’ because the ant will not welcome or adopt the beetle unless it senses their secretion. Most probably, the odour of this secretion mimics the odour of the ant can approach, and grasp it in order to carry it into the brood chamber.

The Atemeles care not the only species capable of making themselves at home with more than one kind of ant. Xenodusa beetles also change their nests with the seasons. The larvae live in Formica nests through the summer and live in the carpenter (Campotonus) ant nests in winter time. It is interesting that the carpenter ants also maintain larvae throughout the winter. Except for above mentioned beetles do not have the command of the ant language required to gain acceptance to the brood chamber. Some species of European beetles like Dinarda are limited to peripheral chambers of the nest of their host. Dinarda offers secretions from glands similar to Atemeles’ glands, but these secretions only induce the ant to tolerate the beetle, not to adopt it and take it into the brood chamber. Therefore Dinarda can only live on such food as it can find in the peripheral chambers. Other groups of beetles have communication sufficient only to allow the beetle to feed at the ants’ garbage dumps.

Many beetles closely resemble their ant hosts in appearance. This is particularly true of guests of the army ants. Some scientists concluded that the factor inducing the ants to accept the beetles as nest-mates was the beetles’ morphological resemblance to themselves. It was even thought to be case with Atemeles, although they do not particularly resemble their hosts. Hölldobler altered the shape ond the collar of these beetles artificially and found that morphological features do not contribute to the success of their relationship with their host. Instead it appears that communicative behaviour remains the essential requirement for acceptance. The guests’ mimicry of their hosts’ appearance, probably serves as a protection against predation by birds.

There are some questions still to be answered about ants and their hosts: How did the fascinating, effective system of communication between the beetles and their hosts develop?Why do only some species of beetles have this ability while the rest do not?

REFERENCES 

• ATKINS, M. D. (1980) Introduction la Insect Behaviour, Macmillan Publishing Co. Inc. , New York, pp. 100-2.
• HÖLLDOBLER, B. (1971) ‘Communication between Ants and their Hosts’, Scientific American, January, pp. 86-93.
• WIGGLESWORTH, V B. (1964) The Life of lnsects, The New American Library, New York