Seed Chalcid Damage to Alfalfa Seed in California

Stephen S. Peterson, International Pollination Systems-U.S.A, Visalia, CA

      The alfalfa seed chalcid, Bruchophagus roddi, is a common pest of alfalfa seed, but it can easily be overlooked. Not only is it extremely small as an adult (2 mm), but it's larvae are hidden inside the developing alfalfa seeds. The larvae feed on the inside of the alfalfa seed until it is completely hollowed out. When the crop is harvested, most of these light, chalcid-damaged seeds are blown out of the combine and onto the ground.

     Cultural practices are important in managing this ever-present pest. Spring discing and irrigation kills infested seeds on the soil surface. Clipping-back established stands helps to delay bloom and provide a shorter pollination window and reduces the time that green pods are available for egg laying. Eliminating volunteer alfalfa along roadsides and ditch banks is also important in minimizing seed chalcid populations.
It is important to periodically measure the amount of seed chalcid damage that is occurring so that we can judge how well our cultural controls are working and make any adjustments if needed. This experiment sought to quantify the levels of damage in California in the 2000 season.

      In this study, samples of seed were taken from 25 commercial seed fields in California in August of 2000. Samples were obtained from 12 fields in Fresno County, 9 fields in Kings County and 4 fields in Imperial County. In each field a small sample was obtained by stripping pods by hand, from four different locations. Each grower was asked to fill out a questionnaire about the age of the field, variety, cultural practices, insecticide applications, and harvest information. The samples were allowed to dry in paper bags, and then threshed by rubbing with a ribbed rubber mat. The samples were then cleaned using seed screens and a small Clipper seed cleaner. A subsample of seed from each field was glued onto a 3 x 5 inch index card and was visually inspecting under a 10x-dissecting microscope.

      An average of 517 seeds were examined from each seed field. The 25 fields sampled in 2000 had an average of 2.1% alfalfa seed chalcid damaged seed with range of 0 to 7.4% (Table 1). There was no significant difference among the three counties sampled for chalcid damage. When sorted by age of field into three groups (one-year-old, two-years-old, and three- or four-years-old) there was no significant difference among age groups for chalcid-damaged seed. This indicates that growers are doing a good job with cultural control in keeping seed chalcids from increasing year to year.

      Interestingly, when the fields were sorted into two groups based on the number of insecticide applications (1 to 2 applications versus 3 to 5 applications), there was 2.3 times more seed chalcid damaged seed was found in the 3 to 5 insecticide application group compared to the 1 or 2 application group. (Tank mixes were counted as one application.) Increased insecticide applications may be reducing predator and parasitoid populations that would normally suppress seed chalcid populations.

      Fields that were at least two years old were clipped back by various methods. When these fields were sorted into two categories (acid clip back versus mechanical clip back), there was 3.6 times more damage in fields that were treated with acid compared to those fields that were mechanically clipped back. This may be a statistical artifact, as there were only three acid treated fields in the survey.
Finally, the fields were sorted by harvest date into two groups (harvested by August 15 versus after August 15). There was 2.6 times more seed chalcid damaged seed in fields harvested after August 15 than in fields harvested by August 15. Later harvested fields allow the alfalfa seed chalcid more time to have access to seed pods than in earlier harvested fields.

      Like most pests, seed chalcids have insect enemies. There are parasitoids (also tiny wasps) that lay their eggs inside a seed chalcid larva as it is developing. I wanted to know how many seed chalcids are being parasitized. For this measurement, seed was sampled from three fields in 1999, refrigerated over the winter and then incubated starting on March 28, 2000. The seed chalcids collected in 1999 began emerging 26 days after being warmed and emergence was complete by 41 days after incubation began. A peak in emergence was noted at 33 days after incubation began. Only 2 parasitoids were collected along with 75 seed chalcids for an average of 2.5% parasitism. These parasites may be more sensitive to insecticide applications than the seed chalcid, which may explain the low levels, found in California.
The growers who assisted in this study were: David Andrada, Doug Devaney, Kevin Grizzle, Gary Lindley, Darrell Silveira, Robert Sheesley, George Wurzel and Jeff Yribarren. I also want to thank Shannon Mueller for helpful suggestions. This project was made possible with funding from the California Alfalfa Seed Production Research Board.

Table 1.� Alfalfa seed chalcid damage in California, 2000.�

n.a. = not applicable, Apps. = applications.