WSU-Mount Vernon Northwestern Washington Research & Extension Center

Entomology Program

Root Weevil Soil Injection Trials, 2008

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

Mixed root weevil larvae, Mount Vernon, WA.  The mature larvae used for the injection trials were obtained from a 3 year-old ‘Totem’ planting in Burlington, WA and were equally divided between the black vine weevil, Otiorhynchus sulcatus and rough strawberry root weevil, O. rugosostriatus.  Injection treatments were soil formulations of the neonicotinoids Platinum™ (thiamethoxam @ 12 fl oz/acre) and Admire Pro™ (imidacloprid @ 10.6 fl oz/acre) and untreated check.  Single weevil larvae were held in a hinged, 4 cm diameter stainless steel tea strainer that was loosely filled with soil from the trial field (Fig. 1).  Fifteen holes about 60 cm in diameter by 45 cm deep, separated by about 90 cm, were excavated along one former blueberry row from which the plants had been removed.  Exposure heights in each hole were designated at 10, 20 and 30 cm.  One strainer for the 10 cm depth was used per hole while two strainers were placed at 20 and 30 cm depths in each hole.  Each strainer was attached to cotton twine in order to hold them at the proper height, as the hole was filled-in with soil and to help remove them from the hole after 10 days.  Treatments were applied with a plot sprayer equipped with a 5 m long high-pressure hose that was attached to a modified GroCor™ feeder/irrigator injector.  Treatments were made at 40 psi and delivered 1.8 liters in 6 sec per plot.  The stainless steel injector probe was machined with 5 water jets positioned away from the user on the tip.  The probe was inserted at a depth of 20 cm into the middle of each plot containing 5 tea strainer (Fig. 2).

After 10 days, all of the strainers were pulled and taken into the lab to evaluate for insecticidal or drowning mortality.  There were significant differences at the 10 and 20 cm depths between Admire Pro and Platinum with the untreated check and no differences between the three treatments at the 30 cm depth.  The high mortality for the untreated larvae at 30 cm showed drowning symptoms when confined with a stainless steel cage compared with a free-living life style whereby the larvae migrated up and down their soil horizon around host roots.  Why larval mortality at the 30 cm depth was 2 fold less compared with the 20 cm depth for Admire Pro and less so for Plainum remains problematic.  This field trial indicates potential success with drench/injection applications of Admire Pro and Platinum to control soil populations of root weevil larva in blueberry.  These data indicate soil contact and systemicity into roots by these neonicotinoids require more than 10 DAT before evaluating their seasonal efficacy (e.g., 2-3 months).