SHAFTER, Calif. (AP) _ Biological pest control seems simple enough: Just find good bugs that like to gobble up the bad bugs that have been chomping on your crops.

The reality is a whole lot more complex, as scientists are learning at the Shafter Cotton Research Station near Bakersfield, Calif.

For example, they're trying to find non-native parasites that can survive in the San Joaquin Valley and kill aphids that thrive in cotton fields.

``We have found parasites that kill aphids year around. Now, we're testing species to see which are compatible in our area,'' says Kris Godfrey, a researcher for the state Food and Agriculture Department.

The research is pinpointing two species of tiny wasps. One type imported from Florida is just 1.5 millimeters long; the other, from Japan, is 2 millimeters long. A millimeter is about the thickness of a paper clip.

Tests of those wasps last year were inconclusive, says Godfrey. She ran into a perplexing problem this spring.

``We can't find them,'' she says. ``Maybe they don't like the San Joaquin Valley or they survived (winter) in only small numbers.''

John McLaughlin of the U.S. Agriculture Department says the imported wasps may just be taking a long time adjusting to California's climate.

``On the other hand, biological control reduction is a bit of a crapshoot,'' McLaughlin says. ``So, you don't release just one species; you try to introduce several.''

With that concept in mind, Godfrey says parasites that eat cotton aphids in other parts of the world also will be brought to Shafter for testing.

Her husband, Larry Godfrey of University of California-Davis, is looking at aphids from a different perspective _ that of the plant.

``We're trying to see what effects aphids have on plant physiology,'' he says.

There is some indication that low levels of aphids may not damage the plant extensively, but it also is possible aphids inject a toxin or directly affect physiology or photosynthesis.

Larry Godfrey says a study in Fresno County last year showed that a field heavily infested with aphids suffered a drop in the photosynthesis rate after the plant had more than 500 aphids per leaf. The leaf temperature also increased about 2 degrees.

In another attempt to find good bugs, predator mites have been released in hopes of control damaging spider mites. However, releasing 2,000 beneficial mites per acre didn't reduce the spider mite population below that of a control plot, says Rami Colfer of UC-Davis.

``We're trying to find what conditions predator mites work best under,'' Colfer says. ``We're studying different release rates. Do they work? And if so, is it economically feasible?''