Feb. 28, 2001 University Park, PA -- An enemy is an enemy is an enemy, but some natural enemies are better than others at controlling prey populations and some enemies are ineffective, even though they are specialized, according to a Penn State entomologist.
"Indian meal moths are a serious stored-food pest, and pathogens such as the virus Plodia interpunetella granulovirus and parasitoids, such as the wasp Venturia canescans, are prime candidates for its biological control," says Dr. Ottar N. Bjornstad, assistant professor of entomology and biology. "However, while they are both specialist enemies, the parasitoid wasp serves to depress host densities greatly, but the virus is completely ineffective."
Bjornstad, working with Steven M. Sait, David J. Thompson, and Michael Begon, University of Liverpool and, Nils Chr. Stenseth, University of Oslo, studied populations of Indian meal moths, alone and infected by both the virus and the wasp over a two-year period.
Reporting in the Feb. 22 issue of the journal Nature, the researchers note that the difference in biological control can be attributed to the strength of coupling between the meal moth host and the parasitoid.
"Theory predicts that strong coupling between a prey and a specialist predator/parasite should lead to an increase in the dimensionality of the prey's dynamics, but weak coupling should not," says Bjornstad. "When coupled host-enemy dynamics occurs, the abundance of the host species, is affected by the abundance of the enemy which, in turn, feeds back on the abundance of the host. This is the mechanism responsible for the successful control of the moth by the wasp."
The researchers studied three groups of insects. The control group of uninfected Indian meal moths, a group infected by the virus and a group infected by the predator wasp. The life cycles and infectious mechanisms of the parasitoids play an important role in the success of the parasitoids in controlling the meal moth population. The virus infects the moth larvae either when they eat infected food or the carcasses of infected moth larvae. However, resistance to disease increases with age so that the older larvae are immune to the virus.
Also, viral infection while sometimes fatal can be sublethal allowing infected individuals to reach the reproductive stage. In the wasp/meal moth system, the researchers found that the number of adult wasps emerging depended on the numbers of susceptible larvae and the number of adult wasps that were present three weeks before when the eggs were laid. The system was strongly coupled with a lag of three weeks.
For the meal moth/virus system, the number of infected larvae was also dependent on the number of infected and susceptible larvae present three weeks before, but the number of adult hosts did not decrease with the abundance of previously infected meal moth larvae. The meal moth/virus system is not fully coupled. Increased meal moth abundance does lead to increased virus infection, but this increase does not negatively impact the host population.
"The lack of host-virus coupling is surprising since the virus is a highly specialized enemy that induces significant mortality in the early larval instars of the host," says Bjornstad. The researchers suggest that the explanation for this lack of coupling lies in the strong competition between large larvae. While the wasps attack older larvae, the virus infects younger larvae and the virus-induced mortality is partially compensated for by meal moths that survive to adulthood. While the virus does affect the meal moth, it does not form a strong enemy host coupling. "We found that the specialist enemies can, as theory predicts, increase the dimension of host dynamics through complete coupling, but also that the increase in dimensional complexity can be counterbalanced if coupling is weak," says Bjornstad. "In theory, there is a direct connection in ecological systems between the number of identifiable interacting groups and what is referred to as the dimensions of the dynamics. We are now able to calculate this quantity from time series data. To our surprise, we found that in natural systems, there may be such a connection, but it is not inevitable."
The researchers believe that this might explain why certain keystone species embedded in rich ecological communities apparently exhibit low numbers of interactions with other species. "This research may potentially illuminate the enigmatic nature of biological control," says Bjornstad. It may also help determine which specialized enemy species can serve as efficient biological controls and which, while specialized enemies, will not control populations."
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