The silverleaf whitefly, Bemisia argentifolii, is only about one-sixteenth-inch long. Click image for additional information.

Scientists Identify Wasp's Chemical Cue for Marking Whiteflies

A tiny wasp that parasitizes silverleaf whitefly nymphs has surprised scientists with its chemical communiqué.

Agricultural Research Service scientists found that female Eretmocerus mundus wasps produce specialized lipids for marking whitefly nymphs they've chosen as suitable egg hosts. The discovery lends biochemical evidence to earlier observations on E. mundus' reproductive behavior and could foster greater success in mass-rearing it to battle silverleaf whiteflies, a worldwide crop pest.

Insecticide use problems, including whitefly resistance, have necessitated alternative measures such as biocontrol, according to James Buckner, an insect biochemist at the ARS Red River Valley Agricultural Research Center in Fargo, N.D.

E. mundus' heat tolerance, host-specificity and fecundity are appealing biocontrol attributes. It's also a team player. After choosing a nymph, a female wasp chemically marks its back with lipids, alerting her compatriots: "I put my egg beneath this one; find another!" Such cues avoid double-parasitizing of the host, making the wasp a proficient whitefly parasite, according to Walker Jones, who leads the ARS Beneficial Insects Research Unit in Weslaco, Texas.

These lipids, known as marking pheromones, are highly unusual, according to Jones. The female detects the marks with her antennae. That's how she knows whether a nymph has been touched by another wasp.

In Weslaco, Jones designed an experiment enabling Buckner to use gas chromatography (GC) techniques to identify lipids extracted from the cuticles of whitefly nymphs in four groups: those with wasp eggs beneath them; those without; 10-day-old nymphs with wasp larvae feeding inside them; and an older group, called fourth instars, with eggs beneath them.

In the first group of whitefly nymphs, Buckner's GC analysis revealed two foreign lipids, known as C31 and C33 dimethylalkanes. Neither lipid is produced by whiteflies. When Buckner checked for these lipids in the wasps, however, he found a match. No dimethylalkanes appeared in the third group—even though those nymphs had been parasitized—because they had shed their skins before GC testing. Lipids also appeared in fourth instars that had been parasitized after molting.

ARS is the U.S. Department of Agriculture's chief scientific research agency.