Biological control not for all insects

Some predators no help in combating pests, Pa. researcher says

March 14, 2001|By ASCRIBE NEWS SERVICE

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 Pennsylvania State University entomologist.

Reporting in the journal Nature, the researchers said that in theory the effectiveness of biological control can be attributed to the strength of the relationship between the host and the predator.

The researchers found that predators specializing on one type of pest can, as theory predicts, increase the control of that pest as a result of the relationship, but that this result is not always predictable

"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," said Ottar N. Bjornstad, assistant professor of entomology and biology at Penn State.

"This research may potentially illuminate the enigmatic nature of biological control," Bjornstad said. "It may also help determine which specialized enemy species can serve as efficient biological controls and which, while specialized enemies, will not control populations."

For example, Bjornstad said, "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. 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.

Three insect groups

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, they found. 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 also be less than lethal, 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 affectthe host population.

Competition

The researchers suggest that the explanation for this outcome 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 relationship.

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