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 . 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.
3.Scott, J.J., et al., "Bacterial protection of beetle-fungus mutualism." Science, 2008. 322 (5898): p. 63.
Figure 1: Adult southern pine beetle (Dendroctonus frontalis). Figure adapted from Reference 1.
Figure 2: An adult southern pine beetle digging the bark of a pine tree. Image adapted from http://www.forestryimages.org.
Figure 3: Beetle larvae (pink arrow), beneficial fungi (yellow circle), and the antibiotic producing bacteria (red square) symbiotically live in galleries carved by adult beetles. Image adapted from Reference 1.
Figure 4: Beetles, beneficial and antagonistic fungi, mites, and antibiotic producing beneficial bacteria coexist within the pine galleries. Pointed arrows indicate beneficial interactions; blunt (square) ended arrows indicate suppressive interactions. Image adapted from Reference 1.