WEEVIL PROBLEM — From the left, Brooke Bohannon from the MSU-Northwestern Ag Research Center in Kalispell and Dr. Bob Stougaard, far right, show the damage by a pea leaf weevil to Dr. Gadi V.P. Reedy, center, Superintendent/Entomologist at the Western Triangle Ag Research Center.
Special to the I-O by Dr. Gadi V.P. Reddy
We already have been confronted with the wheat stem sawfly and wireworms on wheat and barley, and flea beetles on canola.
Now there is another insect causing foliage damage on field peas this year: the pea leaf weevil. Damage is more severe throughout Flathead County and the Golden Triangle areas of Montana.
Pea leaf weevil larvae (Sitona lineatus) feed on the root nodules of field peas, but the adults will feed on leaves of a wide range of cultivated and wild legume species. This year we are seeing 90 percent seedling damage (on clam leaf) in some fields.
Adult beetles feed on leaf margins and growing points of legume seedlings. Feeding on the leaf margins produces a characteristic scalloped (notched) appearance. Larvae feed on the nitrogen-fixing nodules of legume species thereby reducing nitrogen fixation by the plant. Nominal economic thresholds in the Pacific Northwest suggest that a value of 0.3 percentseedlings with terminal leaf damage) is considered optimum time to initiate control methods.
Fields with high levels of residual nitrogen in the soil are less likely to suffer yield losses from PLW larvae and should not require control measures because the pea plant can rely on soil nitrogen instead of its own nitrogen fixation for adequate growth.
Foliar applications of insecticide are recommended if one (or more) feeding marks occur per three clam-leaf pairs (the most recently emerged leaves). According to Dr. Hector A. Cárcamo, Agriculture and Agri-Food Canada, Lethbridge, the foliar applications do not seem to work on this pest because of re-invasion by new waves of the insect. However, they have seen that despite heavy feeding sometimes yield losses are not bad especially if the soil had enough nitrogen. They have recommended seed treatments over foliar insecticide applications despite mixed results with them.
He suggested that trap crops of winter peas have potential because they concentrate the weevils and they can be sprayed repeatedly there in 10 percent of the field. Earlier planted peas may also work.
Dr. Reddy suggests that pheromone baited traps will help in monitoring and mass trapping the weevil population. The aggregation pheromone has been identified as 4-methyl-3.5-heptanedione but requires studies on the optimization of the trapping technique for the field use.
Dr. Maya Evenden at the University of Alberta has been researching the pheromone lures in southern Alberta. Dr. Reddy, Vickie Ophus, Dr. Kim Tangtrakulwanich, Dr. Stougaard and Brooke Bohannon, in collaboration with Dr. Cárcamo are planning on developing a research project dealing with integrating control tactics for tracking this pest.