Insecticidal controls aimed specifically at seed chalcid have not been successful because eggs, larvae and pupae develop within the alfalfa seeds and are protected. Insecticide applications for lygus bugs and other pest in alfalfa seed fields during bloom reduce numbers of adult seed chalcids, but continuous adult emergence from seed pods and migration from outside sources infest fields within a few days. Tiny wasps that parasitize seed chalcids help suppress the population but cannot be depended on for adequate control of seed chalcid. Cultural control practices are the only economically viable way to manage alfalfa seed chalcid.
Destroying volunteer host plants, managing the crop for uniform ripening, and by destroying infested seed and chaff reduced alfalfa seed chalcid population levels. Alfalfa seed chalcid damage has been fairly low in most Imperial Valley fields over the past several years due to good cultural control practices and crop management. Cultural control of alfalfa seed chalcid starts by clipping back alfalfa for seed as early as possible. Late seed sets receive more damage from chalcid than early sets. The percent damaged seed can exceed 50 percent in some late maturing fields. Unless alfalfa growers take certain precautions, seed losses to chalcid can become widespread.
Adult chalcids are not usually abundant in alfalfa fields until the plants come into bloom. The number of adults moving into a field depends upon the number of infested plants nearby, as well as the number of other fields in the area that are in bloom attracting adult chalcids.
Alfalfa seed chalcid control needs to be a community project to attain good population suppression. Growers in a given area should agree to let alfalfa go to seed at about the same time. Make a seed crop early since the chalcid population increases from spring to fall.
Growers need to destroy all alfalfa plants growing along ditch banks and borders through the year, but especially during winter and spring months. Only a few alfalfa plants producing seed during winter months can lead to a big buildup of chalcid wasps in the spring. Hay growers in the area must be encouraged to clip all borders, edges of fields, and ditch banks when mowing the hay crop.
Alfalfa fields are often left to set seed for a few months. The life cycle of the seed chalcid takes only three to four weeks and several generations can develop with a tremendous increase in the number of chalcids present. Produce a seed crop in as short a period as possible using good agronomic practices, good pest control practices for lygus bug, stink bugs and spider mites, and good pollination practices. Long periods of seed production and staggered seed maturity among fields in close proximity optimize conditions for chalcid population buildup.
After harvest, reclean all seed and destroy the cleanings. When planting a new alfalfa crop use only certified seed.
>> Eric Natwick is the entomology adviser at the University of California-Imperial County Cooperative Extension.
>> The Cooperative Extension serves all residents of the Imperial Valley.