Not all insects are the enemies of crops. Many are beneficial. Ideally, insecticides should kill the bad insects without harming the good ones. Easier said than done.

New research suggests some common insecticides are more harmful than previously thought, killings bad insects as well as good insects -- predatory insects that prey on pests.

"Predatory insects contribute billions of dollars a year to agriculture through the elimination of crop pest insects," Margaret Douglas, postdoctoral researcher in entomology at Penn State, said in a news release. "We have found that neonicotinoid seed coatings reduce populations of these natural enemies 10 to 20 percent."

Like many powerful weapons, insecticides aren't always exact. But one way researchers have tried to keep pesticides from hurting helpful insects is by augmenting the delivery method.

Previously research suggested coating seeds with neonicotinoid -- as opposed to spraying onto crops -- would prevent the chemical from harming predatory insect populations. The latest findings out of Penn State University suggest otherwise.

"Applying insecticides to seeds rather than broadcasting them across a field was thought to reduce unwanted effects on natural enemies," Douglas explained. "But we found that seeds treated with neonicotinoid insecticides reduced populations of natural enemies by 10 to 20 percent in North American and European farming systems. Surprisingly, this effect was about the same as that associated with broadcast applications of pyrethroids."

Many research studies have attempted to measure the effects of neonicotinoid seed coatings on predatory insects to varying degrees of success. But results of these studies have contradicted each other.

To solve this problem, Douglas and her colleagues used a meta-analysis technique to interpret the findings from 20 field studies, comprising 1,000 observations from across North America and Europe.

Researchers hope their findings -- detailed in the journal PeerJ -- will inform the cost-benefit decisions about efficacy and safety of different pest-control measures taken by farmers, policy makers and environmental regulators.

"Several governments have restricted the use of neonicotinoids out of concern for their possible effects on pollinators," said Douglas. "But this raises the questions, 'What will farmers do without these products? If they switch to broadcast applications of pyrethroids, will those products be better or worse for predatory insects?' While our results do not speak to the pollinator issue, they do suggest that predatory insects are affected similarly by seed-applied neonicotinoids and broadcast pyrethroids."