Although losses to the California alfalfa seed industry from the alfalfa seed chalcid are considered relatively minor, a recent study suggests insecticide sprays may harm natural enemies of the pest.
Stephen S. Peterson, a researcher with International Pollination Systems — USA in Visalia, says the tiny, black wasp that lays eggs in developing alfalfa seed pods is parasitized by two other wasp-like insects.
Peterson recently reported on his survey, funded by the California Alfalfa Seed Production Research Board, during a recent symposium held by the board in San Joaquin.
“We might be seeing here, as fields are sprayed more often for lygus, armyworms, and other pests, the wiping out of some predators of chalcid. It's a theory that might be worth looking into in the future,” said Peterson.
He surveyed 25 commercial fields in Fresno, Kings, and Imperial counties and examined seed for chalcids and parasitoids. He collected the seed in 1999 and germinated them in a laboratory and counted chalcids and parasitoids in 2000.
Peterson found chalcid damage averaged 2.1 percent and ranged from zero to 7.4 percent. No difference was detected between counties or age of stands in fields surveyed.
He separated the seed into two categories: that from fields treated with one or two insecticide applications and that from fields with three to five applications. Tank-mix applications of more than one material were considered a single treatment.
Twice the damage
More than twice as much chalcid-damaged seed was found in the group from fields treated three to five times.
At harvest, chalcid-infested seed is blown onto the ground, since it lighter than sound seed. The wasp's life cycle then continues in the soil.
Spring disking and irrigation and clipping back stands mechanically or by sulfuric acid do not completely destroy the larvae that later emerge as adults ready to lay eggs in new alfalfa seed pods.
“Contact insecticides do not appear to control alfalfa seed chalcid,” Peterson said. “This is probably due to the extended period of time in which adults can be found in the field and because once eggs are laid, the larvae are protected inside the seed pod.”
Peterson also observed that fields harvested after August 15 had more than twice the chalcid damage of those harvested earlier. He attributed the difference to seed pods being exposed to the insects for a longer time.