Linda Benedict, Leonard, Billy R., Carlton, Christopher E.
Paul P. Price III, Chris Carlton and B. Rogers Leonard
During 2007, significant foliage loss and stem injury on soybean seedlings were observed in research trials and production fields within Franklin Parish. Plant injury was similar to that previously described for grasshoppers feeding on soybean plants. However, plants that retained some leaf tissue were infested with an insect displaying the characteristics of a snoutnosed weevil. Samples of this insect population were collected and processed for identification in the Louisiana State Arthropod Museum located in the LSU AgCenter Department of Entomology. The identity of this insect was confirmed to be the pea leaf weevil.
The occurrence of this insect is new to Louisiana, and it has not been described as a pest of Louisiana soybeans. This insect was detected again during 2008 in other soybean fields located in Franklin, Tensas and Catahoula parishes. The statewide distribution of this pest has not been surveyed; therefore, these observations describe the initial indication of a potential problem and are not indicative of an annual pest.
The pea leaf weevil adult is gray-brown in color and measures about 0.2 inch, with a short, blunt-shaped beak and three light-colored longitudinal stripes on the surface of the body behind the head. Pea leaf weevil adults were observed feeding on soybean seedlings in the early stages of plant development. In April-planted soybeans, infestations initially were detected on first-leaf-stage plants and persisted for nearly three weeks. Individual plants were infested with as many as 20 adults feeding on leaf tissue and stems.
The initial stages of injury appeared as semi-circular notches producing scalloped edges on leaflets and progressed to complete defoliation and eventual main-stem feeding. A significant loss in plants was observed in areas associated with the highest infestations.
In the research plots, seedling losses from feeding injury were sufficient to reduce final seed yields by 20-40 percent. In all instances, the infestations were observed in localized areas and did not cause plant injury across the entire field. The damage was consistently more severe in soybean fields adjacent to non-cropped areas.
Larvae are reported to feed on the nitrogen-fixing nodules on legume plant roots, but no larvae were detected in association with adults feeding on soybeans in Louisiana. In addition, soybeans that were planted during May were subjected to minor pea leaf weevil infestations (less than one insect per five plants) and were much lower compared with infestations on April-planted soybeans. This observation suggests only a single generation of pea leaf weevil was observed during both years in Louisiana.
In field trials evaluating the performance of insecticidetreated seed against insect pests of soybean seeds and seedlings, the insecticides imidacloprid (Gaucho), thiamethoxam (Cruiser) and clothianidin (Poncho, Nipsit) appeared to significantly reduce damage from the pea leaf weevil. Observations on foliar insecticide efficacy indicated the highest labeled rates of pyrethroids labeled on soybeans provided more than 90 percent control at five days after treatment. However, fields must be scouted immediately upon seedling emergence to detect infestations before significant injury and plant loss occur.
This exotic species from Europe was initially discovered in the state of Washington during 1942. It is now common on numerous legume hosts across the Pacific Northwest and California and most recently documented in Virginia and Florida. The insect is an established economic pest of peas.
The origin of pea leaf weevil populations found in Louisiana may have resulted from the importation of winter legume seed grown in the northwestern United States. These plants are used as an agricultural cover crop in fallow crop fields and in wildlife food plots in Louisiana.
Paul P. Price III, Research Associate/Graduate Assistant, Macon Ridge Research Station, Winnsboro, La.; Chris Carlton, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.; B. Rogers Leonard, Professor and Jack Hamilton Regents Chair in Cotton Production, Macon Ridge Research Station, Winnsboro, La.
(This article was published in the summer 2009 issue of Louisiana Agriculture.)