Biological basis of biological control

In order for a species to maintain constant population numbers, every pair (male and female) of individuals in that population need to produce only one pair of offspring, as long as those offspring are able to reach maturity and, subsequently, produce their own offspring. So, why does a gypsy moth produce 800 eggs? Or a house fly lay as many as 2000 eggs? In our living world, virtually everything is eaten by someone else. Predators. Parasites. Disease-causing microbes. These terms are all part of our every-day vocabulary because we see these biological processes in action. Insects tend to produce lots of offspring because most of those offspring will die before they reach sexual maturity. Some may die as a result of adverse weather, or lack of food. But many die because they become someone else's food.

Figure 1. This is a hypothetical example of a successful biological control project. The graph depicts nine years of data. In each year, the pest population was sampled eight times. You can see that the population fluctuated yearly, but generally rose to quite high levels each summer. In the fifth year, a new natural enemy species was released and established in the area. In the following year, and the years after, the new natural enemy substantially reduced the peak population densities of the pest. In some cases, it may take more than one year for a new natural enemy to become established and exert this degree of control.

Scientists who study the ecology of populations have developed a very good understanding of how different species interact in both natural and agricultural ecosystems. We know, for example, that no population can increase forever to infinitely high numbers (although in some years soybean aphid appears to try!). Ultimately, bad weather, lack of food, predators, or disease will stop population growth and, usually, cause population decline. Therefore, most animal populations tend to go through some sort of cycle of increase and then decline.

In the case of soybean aphid, our soybean fields provide an abundance of aphid food until senescence, so the aphids can build to high numbers. Once their food is lost and winter comes on, aphid populations crash to very low numbers and the winter is survived only in the egg stage. The following season the cycle starts over again. In its native Asia, soybean aphid goes through the same seasonal cycle, but rarely shows the dramatic population increases we see here in North America. We think the reason for this is that the aphid has many more types of natural enemies in Asia where it originated.

In many ecosystems, as the population of a particular species increases, it provides more food for its natural enemies. As the natural enemies have more food, they are able to produce more offspring, and these offspring have a better chance of survival. As natural enemy numbers increase, they eventually have a dampening effect on the population of their prey. The prey population then decreases, resulting in less food available for the predator population, which, without sufficient food resources, also declines.

In other words, the population density of the prey has a direct effect on the population of the predator, and vice versa. Population ecologists and biological control scientists refer to this reciprocal action between predator and prey as "density dependent population regulation" because the numbers of one species have a direct impact on the numbers of the other species.

When a new pest species is introduced to an area, the local natural enemies often are not "evolutionarily familiar" with the new organism and are not adapted to be highly effective predators of the new pest. So even though there are lots of types of natural enemies of aphids in North America, many do not "recognize" a soybean field as a place to find food, nor do they "recognize" soybean aphid as a possible type of food. Therefore, they do not provide an adequate density dependent response to increasing soybean aphid populations.

The objective of our biological control research is to identify natural enemies that evolved with soybean and soybean aphids, and which recognize and have the biological capabilities to utilize soybean aphids as a source of food. When classical biological control programs are successful, the pest population continues to cycle, but the new natural enemies dampen the degree of oscillation; in other words, the peak population densities are not as great as when the natural enemies are not present (as in Figure 1).