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Lab and Field Trials to Control Root Weevil Larvae in Small Fruits

Lab and Field Trials to Control Root Weevil Larvae in Small Fruits

L. K. Tanigoshi and J. R. Bergen
Washington State University
Vancouver Research and Extension Unit
Vancouver, WA 98665-9752

The residual persistence of the experimental neonicotinoid Actara (thiamethoxam) was bioassayed with black vine and rough strawberry root weevils collected from a ‘Totem’ field in Woodland, WA. Actara was applied at 0.086 lb(AI)/acre from a low pressure boom sprayer equipped with 3 Floodjet TK-SS20 nozzles calibrated to apply 114 gpa at 58 psi at 2.1 mph on 12 July 2004. Four days after treatment, 45 leaves were collected and divided into 3 sets of 15 leaves placed in water vials capped with cotton plugs. These field-weathered leaves were held under lab condition for another 6 days. Forty-five black vine weevil and 45 rough strawberry root weevil adults were placed in groups of 3 per treated leaf and observed for mortality from day 5 to day 10 after field application. A mixture of both weevil species was used as the untreated check for which no mortality occurred over the testing period.

Compared with excellent contact/stomach activity of foliar applied Brigade/Capture, mortality on 5 day old Actara residue was 29% for both species (Table 1). Maximum mortality of 76% and 84% was recorded for the black vine weevil and rough strawberry root weevil, respectively, on 10 day-old residue held under lab conditions. Data from Syngenta showed that as much as 60% of the active ingredient moved into sprayed tomato leaves by day 9 and 50% remained by day 21. Because of the translaminar movement of Actara, its extended residual control is at least 2-fold more than foliar applied Capture/Brigade.

The contact and translaminar mode of entry for Actara was measured at varying times after application on red raspberry foliage for adult, rough strawberry root weevil. Several ‘Meeker’ plants were sprayed with Actara at 0.06 lb(AI)/acre on 12 July at the Vancouver REU. Ten leaves were sampled daily for 9 days posttreatment from treated and untreated plants. Three adult, rough strawberry root weevils were placed on each leaf whose petiole was placed in a water-filled vial capped with a cotton plug and held in 6 inch Petri dishes. Daily cohorts of 30 weevils each for both treatments were held for 5 days. Excellent contact and stomach poison activity was observed within 24 hours of the application through 8 days posttreatment
(Table 2).

The contact activity of Admire (imidoclopyrid), Platinum (thiamethoxam) and Belay (clothianidin) was evaluated with a simulated test tube, soil drench bioassay for activity to both spring and fall populations of rough strawberry root weevil larvae. Maturing late-spring root weevils were placed on top of field soil contained in 3.75 inch long glass test tubes on 19 May and 24 May. These larvae migrated 1–2 inches into the soil before they were drenched with 4 ml of field rates of Admire and Platinum. Cohorts of 15 and 20 larvae/treatment at the two respective treatment dates were destructively sampled 14 days post-drench. Larval/prepupal mortality averaged 42% for Admire and 40% for Platinum (Table 3). These levels suggest that a late spring drench application(s) may not be optimal for economic control of new generation root weevils.

Similar drench tests were conducted in late August to control early instar, rough strawberry root weevil that will feed on roots over the following 8 months. From 31 August to 9 September, second instar rough strawberry root weevil larvae were collected from an infested strawberry field in Vancouver, WA. Each of four treatments consisted of 10 larvae placed in soil filled test tubes. These were treated with 6 ml of field rates of Admire, Platinum, Belay and water control one day later. Each regime was repeated at 5 different dates and larval mortality was determined with destructive sampling at 3 to 7 days posttreatment, respectively. Belay and Platinum provided excellent control from 4–7 days posttreatment; Admire was somewhat variable but significantly different from the control at the same time intervals (Table 4). General contact mortality of immatures in soil tend to be slightly delayed compared with foliar formulations when exposed to adult root weevils.

Table 1. Actara’s translaminar residual on strawberry at rennovation.

Percent Mortality
Actara 25WP 5DAT 6DAT 7DAT 8DAT 9DAT 10DAT
Rough strawberry weevil 29a 38a 67a 82a 82a 84a
Black vine weevil 29a 42a 60a 64a 73a 76a
Untreated check 0b 0b 0b 0b 0b 0b
Percentages within columns followed by the same letter are not significantly different (Tukey HSD test, P<0.05).


Table 2. 2004 residual activity of Actara to rough strawberry root weevil on red raspberry foliage

Percent Mortality
Actara 25WG 80 90 97 80 83 80 97 83 33
Untreated check 0 7 14 14 0 0 0 0 3
Cohorts fed for 5 days.


Table 3. Spring, RSRW larval drench test.

Treatment lb(AI)/acre Percent mortality
Admire 2F 0.5 53a 30a
Platinum 2SC 0.125 40ab 40a
Untreated check 0b 15a
Percentage within columns following by the same letter are not significantly different (Tukey HSD test, P<0.05).

Late instar RSRW larvae.


Table 4. Fall, RSRW larval drench test.

Percent mortality
Treatment lb(AI)/acre 3DAT 4DAT 5DAT 6DAT 7DAT
Admire 2F 0.5 10b 100a 70b 90a 50b
Belay 16 WSG 20 fl. oz. 60ab 100a 100a 100a 100a
Platinum 2SC 0.125 80a 100a 100a 100a 90a
Untreated check 20b 0b 0c 0b 0c
Percentages within columns following by the same letter are not significantly different (Tukey HSD test, P<0.05).

Fall (small) instar larvae.