Predatory insects such as spined soldier bugs and green lacewings eliminate crop pests without noxious sprays. Too bad they require $400-a-pound moth-egg meals to reach maturity. Or at least they did. Allen Cohen, an entomologist with the U.S. Department of Agriculture, has concocted a gruel of chicken egg, beef liver, and fatty hamburger that satisfies the dietary needs of beneficial bugs for just $2 per pound. Cohen also devised a blood test to make sure his recipe keeps the insects fertile so they can get the next generation going.