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Managing Weevils in Raspberries and Strawberries

Lynell K. Tanigoshi
Entomologist, Washington State University, Department of Entomology,
Vancouver Research and Extension Unit

Root weevils, especially the strawberry root weevil, Otiorhynchus ovatus, rough strawberry root weevil, O. rugosostriatus and black vine weevil, O. sulcatus, remain perennial pests for strawberry and raspberry growers in the Pacific Northwest. A basal pre-bloom application in mid-April that drenches the crown area thoroughly, as well as 24 inches of lower floricanes, with 40 to 50 gallons of water per acre for Capture/Brigade, Malathion and Asana will provide excellent control of actively foraging clay-colored weevil, O. singularis and rough strawberry root weevil causing primary bud damage. Full coverage, over-the-row application of Capture/Brigade at pre-harvest and if needed postharvest, has effectively reduced economic levels of machine harvested root weevils in red raspberries for many years. However, the rough strawberry root weevil has emerged as the key weevil pest for strawberry growers in Washington over the past 4-5 years. Weekly soil sampling from early winter and in combination with visual plant and soil searches for the remainder of the year have confirmed the seasonal occurrence, biology and behavior of this species is different from that known for the other root weevils. The current recommendations for harvest and postharvest management of the other root weevil species in raspberries and strawberries will reduce their populations below economic levels in these multi-year crop systems. This is not the case for the cool weather-adapted rough strawberry root weevil. Because this species is less active during the raspberry and strawberry harvest periods, it is rarely found foraging and notching terminal foliage at night.

From multiple years of detailed Petri dish data, we know the reproductive cycle of the rough strawberry root weevil is similar to the other root weevils species common to strawberry and cane fruit in the PNW. Over wintering females emerge in March and immediately begin laying eggs two months before strawberry harvest in early June. The major cycle for egg production occurs with the May-June emergence of summer adults that over wintered as larvae through the winter while grazing on strawberry roots. The main ovipositional cycle for the rough strawberry root weevil occurs about a month after peak egg laying by the black vine and strawberry root weevils. Our current area wide program in southwestern Washington strawberries is aimed at timing canopy application(s) of Brigade to optimize control of over wintering and summer generation adult root weevils before they begin laying eggs in the soil during strawberry harvest. Fig. 1 shows uncontrolled summer emerging rough strawberry root weevils laying eggs well into autumn unlike that observed for the black vine and strawberry root weevils. We feel this strategy of foraging and feeding activity during the postharvest period is a major factor that has affected rough strawberry root weevil’s key pest status in Washington’s strawberry culture. The rough strawberry root weevil will go undetected by scouts/growers applying traditional root weevil monitoring methods to detect notched leaves and active nighttime foraging at postharvest and renovation.

A harvest spray of Brigade at 0.1 lb(AI)/acre was applied on 17 June 2003 to control peak black vine weevil populations observed from night sampling and extensive notching of canopy foliage in a 3 year-old mixed variety strawberry field in Vancouver, WA. The low pressure boom application, applied at 50 psi with D6 45 disc core nozzles to deliver 80 gpa provided 99% adult control of an economic level of black vine and strawberry root weevils after 3 days posttreatment. This timing eliminated adults of both species into the fall months. Unexpected recovery and population increase of both soft and hard bodied rough strawberry root weevil after this harvest spray lead to three additional postharvest (D6 45 disc core nozzles) applications. Additional post-harvest (Floodjet® TK-SS20 nozzles) applications of Brigade were applied in 200 and 381 gpa with economic levels of egg laying adults commonly found in September (Fig. 2). The reported results underscore the difficulty many growers have experienced over the past 2-3 seasons to control the adult stage of this late season weevil. In addition to the traditional late evening flash light searches, we conducted daytime adult searches for adults and larvae. The cryptic rough strawberry root weevil was surveyed with 15 minute soil and 15 minute foliage and crown searches in addition to our standard golf-cup cutter to assess larval life stages and presence of adults in five core sample per field. These cores were sifted through 8 and 16 mesh sieves. We conclude that the traditional postharvest rough strawberry root weevil treatment(s) are not effective for controlling this late season weevil that we’ve shown will continue to lay viable eggs into early fall. This control failure, aimed at the preovipositing adult stage, results in fall season through spring larval feeding on strawberry roots.

Six compounds known to possess root weevil toxicity were field tested in a 3 year-old ‘Totem’ field in Woodland, WA. Applications were applied the same day the field was mowed on 12 July 2005 with a 3 row application kit equipped with 9 D6 45 disc cone nozzles at 100 psi in 114 gpa. Treatments were replicated five times and plots measured 3 rows wide by 30 feet long. Sampling consisted of 3 randomly selected areas in each plot of about 2 ft2 each. Population levels of primarily adult rough strawberry root weevil were ascertained with visual-hand searches in the soil-debris around plant crowns from the middle to the shoulder of a row, including runner foliage that escape mowing. We found congregations of the rough strawberry root weevil commonly in moistened microclimates beneath patches of green runner foliage. Renovation trials next season will include methods to cut runner foliage that extends into the strawberry rows. Hand and knee searches for populations of adult rough strawberry root weevil were taken at 3, 7 and 14 days post-renovation. Malathion provided excellent activity compared with the untreated check to 14 days posttreatment (Table 1). At 7 days after treatment, Brigade, Malathion and experimental neonicotinoid Clutch had fewer adults found with our visual search method under foliage around the crown and within the soil as well. Given the random distribution and numerical variability of these root weevils, results in part at 14 days posttreatment were not significantly different from the untreated plots at the 5% level of significance. Adult weevil suppression with foliar application of neonicotinoids in the field require 5-7 days before they impact adult mortality compared with the generally fast acting OPs and pyrethroids.

In summary, Brigade or Capture remains excellent root weevil tools for rapid clean-up and residual persistency, especially within the canopy of strawberry and cane fruits, as well. The neonicotinoid chemistries such as Provado and Actara possess good larval and adult weevil activity as alternatives to Brigade/Capture, though somewhat slower acting under field conditions. The key to summer control of the RSRW is preovipositional timing and placement/coverage of adulticides under the canopy to cover the crowns and adjacent soil as well. We’ve shown this can be physically accomplished with either drop nozzles passing through the strawberry’s canopy or by opening up the canopy with a 2″ × 6″ × 36″ board drags or 2″ diameter pipe/chain roller placed immediately in front of the spray’s interface.

Line over bar graph of eggs-weevil numbers vs. months.
Figure 1. 2004 Rough Strawberry Root Weevil Ovipositional Trend
3D line graph showing number of adult weevils vs dates for four application rates.
Figure 2. Bridage Post Harvest Renovation Study
Table 1. Rough strawberry root weevil trial
Treatment lb(AI)/acre Mean Weevils/ft2
Means within columns followed by the same letter are not significantly different (Tukey HSD test P<0.05). 12 July 04.
Actara 25 WG 0.06 4.1bc 6.9bc 1.1b
Provado 1.6F 0.04 18.6a 19.7a 2.5b
Clutch 50WDG 0.09 7.0bc 0.3c 9.9a
Brigade 10WP 0.10 8.5bc 1.6c 1.1b
Mustang Max 0.03 5.5bc 0.2c 0.1b
Malathion 8EC 1.25 0.0c 0.5c 1.2b
Untreated Check 11.5ab 13.1ab 6.8ab