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L. K. Tanigoshi, B. S. Gerdeman and G. Hollis Spitler
Washington State University
Mount Vernon Northwestern Research and Extension Center
Mount Vernon, WA 98273-4768

Clay colored root weevil, Otiorhynchus singularus (L.).

Clay colored weevils (CCW) were collected from the Lynden area on ‘Meeker’ red raspberry on 26 June, 2008. Individual trifoliate leaves were placed in water-filled vials capped with a cotton roll plug. Each treatment consisted of 30 weevils placed on 6 individual leaf arenas held in 5 inch diameter Petri dishes held at room temperature. These leaf arenas were each dipped in respective deionized water-insecticide solutions for approximately 5 sec and air-dried (Table 1). After 1 day posttreatment, Brigade™ (bifenthrin) and two rates of experimental BAS 320 I provided complete mortality of CCW through contact and ingestion under lab conditions. Under these ideal lab conditions, the results provided no evidence for the onset of CCW resistance to Brigade as was suggested from 2006 results from a field population collected at the same location. BAS 320 I represents a new class of chemistry (Group 22) that controls insects by ingestion, blocks the flow of sodium ions and does not require metabolic bio-activation to become insecticidal. CCW exposed to BAS 320 I were in a moribund state after 1 day posttreatment. Symptoms observed were cessation of feeding, metabolic stress (e.g., diarrhea) and uncoordinated movements that result in prolonged morbidity and death that extended to 12 DAT (e.g., 63% low rate, 100% high rate). These post exposure responses are similar for other species of root weevils when also exposed to neonicotinoids such as Actara™. Though the target site of BAS 320 I differs from the neonicotinoids, population mortality upon exposure to Actara often is variably prolonged for 3-5 days as well in adult root weevils. We scored the moribund CCW’s as dead because they were incapable of pest status and population survival under field conditions given their responses in the laboratory. The insecticidal effect on root weevils is irreversible, as we have observed for the pyrethroids and neonicotinoids, and slower acting compared with the mode of actions of the old carbamate and OP chemistries. BAS 320 I is pending registration in blueberry and we will suggest it as an ‘A’ priority next year for strawberry and caneberry IR-4 residue projects.

Table 1. Clay colored root weevil bioassay, 2008.
Treatment lb(AI)/acre Percent Mortality
Mean within columns followed by the same letter are not significantly different (Fisher’s protected LSD, P<0.05), PRC ANOVA SAS.
BAS 320 I 0.18 100a
BAS 320 I 0.25 100a
Brigade WSB 0.10 100a
Untreated check 0b

Other than spotty clay colored weevil infestations in the north Lynden area, the black vine, strawberry and rough strawberry root weevils were generally economically managed throughout Whatcom and Skagit counties this past red raspberry growing season. These empirical observations suggest the root weevil complex still remains susceptible to Brigade/Capture and Malathion, especially if timed for evening application when the adults are known to be actively feeding during their pre-egg laying period. Growers are learning and feeling more confident with application/timing of the neonicotinoid chemistries registered for foliar control of root weevils in caneberries (i.e., Actara, Assail™).

Two Clay-colored weevils.
Figure 1. Clay-colored weevils resting at base of old canes.
Man kneels visually examining cane bases.
Figure 2. Visual search for early emerging adult clay-colored weevils.
Figure 3. Clay-colored weevil feeding on a primary bud.
Figure 4. Notched and flagged fruiting lateral shoots.