The involvement of a third trophic level was detected in several cases; among them are caterpillar damaged plants, which recruit parasitoid wasps. The volatiles secreted as the interaction begins are “green leaf odors”, having a slight effect on the wasps. Accumulated damaged induce secretion of terpenoids, whose impact on the wasp is highly pronounced (Tumlinson et.al 1993). Furthermore, experiments showed that those terpenoids are evidently distinguishable from background odors, demonstrating their clarity, they are highly correlated with the plant genotype, providing them with specificity, and they happen during times the plants are most likely to be under an attack of a herbivore, demonstrating efficiency in timing (Turlings et.al 1995).
To be able to reach efficient predatory skills, the wasp was shown to develop them by learning from. While inexperienced wasp uses its innate inclination to odors, and attracted to the freshly damaged leaves, the experienced one, which had already received its first reward, is much more attracted to the terpenoids scent (Hoballah and Turlings, 2005).
Application of this topic to the agriculture field include old methods of biological pesticides control, in this case, introduced wasps for the purpose of preserving caterpillar infested corps (Shiga, 2005). Further look is given to the option of utilizing specific, artificial blend of volatiles, to increase the level of wasps’ visits (Kappers et. Al, 2003). This is an area of an increasing current research.
Caterpillar
infested plants evolved several mechanism to avoid a severe leaf
damage, among which is the call for their enemy’s natural enemy
such as a parasitoid wasps. The wasp infects a clutch of eggs
into her lepidopteran victims, and those develop into larvae,
and burst off the caterpillar’s body as adult wasps, eating
their host alive.
Nevertheless, the eggs themselves are insufficient in
combating the caterpillar’s immune system. To win this battle,
the eggs are accompanied by hordes of viruses. It is most likely
that these viruses originated as a separate entity, and
exchanged pieces of genetic material with its host. Eventually
they reached a really symbiotic relationship, in which the
survival and reproduction of both are intimately mingled.
The wasp virus, called polydnavirus targets circulating
hemocytes in a process with a striking resemblance to apoptosis.
These hemocytes, however, mainly granulocytes and plasmatocytes
are the major immune force of the caterpillar, and their
destruction marks the insect’s fate. Along with it, the
endocrine system elevates the concentration of the juvenile
hormone, keeping it developmentally arrested, to satisfy the
nutritional needs of the parasitoid larvae.
The viral genome is markedly larger than an average
virus, and it is organized in large gene families, some of which
pay a role in the parasitism process. Cystatins, for instances
harbor a reversible inhibitory property. They act upon cystein
proteases which inhibits apoptosis, thus, indirectly promote
this cell suicide, contributing to the caterpillar fatal end.
Although biological control includes the usage of wasps
as a method of pesticides for many years, current researches are
seeking to utilize the powerful weapon of the virus itself as a
method of shutting down the immune system of the insects.
Pinpointing the relevant and most efficient genes for this
purpose will bring about a cheaper and more reliable manner of
protecting plants.
