March through September, 1999

Dr. Lynell Tanigoshi (WSU Vancouver, entomologist), Dr. Chuhe Chen (WSU Vancouver, horticulturist), Geoff Menzies (WSU Vancouver, Lynden Satellite Station manager), Raj Bathe (Bathe Farms, Inc.), and Sam Dhaliwal.

Overview

Several studies were conducted during the 1999 growing season in order to gain a better understanding of the biology and management of the clay colored weevil, Otiorhynchus singularis, a serious early season pest on raspberry in Whatcom and Skagit counties in northwest Washington. Adult clay weevil emergence from the soil in late March and early April and subsequent feeding is well synchronized with fruiting lateral bud break in raspberry plantings. There has been significant concern that direct feeding on bud tissue, new growth emerging from buds, and small fruiting laterals by this pest can overwhelm the raspberry plant’s ability to compensate and therefore seriously impact current season yield. This pest has not historically been widespread or common throughout the raspberry growing region of northwest Washington, and therefore growers are not accustomed to monitoring or controlling it as a part of their routine practices. However, clay weevils have caused significant damage in several fields over the past few years in both Whatcom and Skagit counties, thus necessitating further study. Studies were conducted on two farms located in the Northwood area, northeast of Lynden, in Whatcom County, Washington.

Early Season Phenology and Field Sampling

In mid- March numerous soil core samples were taken from two known infested fields and screened to confirm presence of clay weevils and to provide a rough estimate of population density in the soil. At the same time on one farm, ten emergence traps were installed in the center of the hill spaced evenly along one row over a distance of approximately 250 feet. They were secured in place by wiring to a metal stake driven in to the ground and backfilled with soil at the base of the trap. These rectangular-shaped box traps were wood-framed, open on one end and covered on the sides and top with a combination of screening material and clear plastic. The clear plastic allows the observer to peer inside of the trap to determine whether weevils are present without removing the trap. An access hole with cloth cover allowed for periodic removal and counting of specimens. Dimensions of the trap are 5.5 inches by 5.5 inches by 24 inches tall. Each trap collected weevils emerging from a 0.2 square foot area. Weevils were counted and removed on a weekly interval from late March through early May.

Clay weevil activity in the raspberry canopy was evaluated regularly in two fields using a 10-ft. long by 3 -ft. wide drop cloth placed on either side of the row. After the drop cloth is placed on the ground, the top training wire at each end of the 10-ft. section is grasped firmly and shaken vigorously to dislodge adult clay weevils. Weevils knocked from the canopy drop to the sheets where they are identified, counted, and then returned to the plot. A minimum of ten plots was sampled regularly in each field from early April through as late as mid-June in one of the fields. This has been the standard procedure for evaluating chemical adulticide trials in raspberries. On occasion this sampling technique was compared to sampling with a hand held 16-inch by 16-inch square, beating tray. The beating tray has been the most convenient method for IPM field scouting. With this technique, 10 tray samples were taken along 100 feet of row. The total number of weevils detected in 10 trays was then compared to a drop cloth sample taken from the center of the adjacent row. These two techniques were compared at 10 adjacent sites at each outing. All sampling was performed in the evening usually between 9:30 and midnight.

Data loggers (HOBO units made by Onset Computer) were placed in one field for the second season running to measure soil and air temperature to support the eventual development of a predictive model for clay weevil development and management.

Egg Laying Studies

Thirty spring-emerging adult clay weevils collected from the field in mid-April were maintained in the laboratory throughout the summer. Each weevil was placed in a small container with raspberry leaves as a food source. Containers were checked every two to three days and weevil mortality and egg production was recorded.

Chemical Trials

Insecticides targeting adult CCW were applied on April 14 at one site only. The maximum fruiting lateral shoot growth at this time was 2" , with most buds just beginning to break, particularly on the lower half of the canes.Treatments included: full canopy sprays of Alert 2SC (0.32 lb. ai/acre), Asana XL 0.66EC (0.15 lb. ai/acre), Brigade 10WP (0.1 lb. ai/acre), Guthion 50WP (0.5 lb. ai/acre), Fipronil 1.6F (0.01 lb. ai/acre)and basal sprays of Alert 2SC (0.12 lb. ai/acre), Brigade 10WP (0.04lb. ai/acre), Fipronil 1.6F (0.004 lb. ai/acre) and an untreated check. Each of the 9 treatments was replicated 4 times. Plots were single row and 30-ft long. Sprays were applied with a tractor mounted, single row over the row boom, equipped with 14 D3-25 TeeJet nozzles at 200psi delivering 92 gal/acre and traveling at 3 mph The basal treatments were applied using only the lower 3 nozzles on each side of the row; 6 nozzles total. The rationale of the basal application is that only the lower 4 ft. of the plants are sprayed, total amount of pesticide applied is reduced, and weevils come in contact with the treated zone with their frequent migration from the soil to the upper canopy. Plots were evaluated 3, 7, and 14 days after treatment. This was accomplished by placing a 3-ft wide by 10-ft. long drop-cloth on each side of the row in the center of each plot. The top training wire is grasped firmly at both ends of the plot and given five vigorous shakes. Weevils knocked from the canopy drop onto the cloth and are identified and counted; then returned to the plot. Sampling is done at night from about 9:30 to 11:00 p.m. Control plots in this trial remained unsprayed until late June when the grower applied a full canopy Brigade spray, which is the standard pre-harvest spray to reduce insect contamination of machine-picked fruit.

Cryolite bait was applied in the same field on April 13. The application was made by hand, treating a 2-ft. wide band in the center of the row using 0.9 lbs. of product per 500 feet of row. This rate of application is equivalent to 7.8 lbs. of product per acre and is well below the product label rate (40 lbs. per acre). Design consisted of four replications of paired Cryolite and control plots, each plot 5 rows wide by 100 ft. long. The middle 10 feet of the center row in each plot was sampled using the same drop-cloth method 3, 7, and 14 days after treatment.

Clay Weevil Damage Assessment in the Field

In this feeding damage evaluation, treatments included 3 weeks of continuous feeding (early April through April 24), 6 weeks of continuous feeding (early April through May 15) and minimal feeding in a nearby planting of the same age, which has no history of clay weevil infestation nor current season signs of infestation. In mid-June, six fruiting canes were harvested from each of four plots in each of the treatments. The plots were selected by evenly spacing them along a row, which was 500 feet in length. The total number of buds was counted on each cane. In addition, bud damage due to clay weevil feeding, cold injury and presence of spur blight was recorded for each cane. All fruiting laterals were measured for length and the total number of harvestable fruiting positions was recorded. Berries less than 2 mm. in diameter were not counted as they are unlikely to mature into harvestable fruit during the commercial picking period. Samples of 50 berries were randomly collected and weighed from each plot on a weekly basis during the harvest period (July 6 through August 3). Finally, the total number of fruiting canes in each plot were counted in order to establish an estimated yield per acre for each of the treatments.

Early Season Phenology and Field Sampling

The first detection of teneral (recently emerged, soft-bodied) adult clay weevils in the top of the soil profile was on March 23. Average density within the rows at the two sites in the top 4-6 inches of the soil ranged from 1.5 to 4.6 adult weevils per square foot. Light feeding damage to floricane buds was also noticed at one of the two sites on this same date. Beating tray samples in late March and early April indicated that clay weevils were beginning to climb up the fruiting canes to feed on new growth. Emergence traps confirmed the weevils were moving from the soil from early April through the end of April, with peak emergence during the second and third weeks of April. See chart below. This emergence period corresponds to soil heat accumulation of 100 to 300 degree-days (40F minimum development threshold).

Emergence density in the center 1-ft of the row ranged from 0 to 40 adult clay weevils per square foot and averaged 15 adult clay weevils per square foot. This suggests that in this site, 10 feet of row likely harbored an average of 150 spring -emerging weevils potentially feeding on 41 fruiting canes, averaging 3.6 weevils per cane. Sampling with the drop-cloth at this same site showed clay weevil feeding activity in the canopy from April 5 through May 15, when the grower was forced to apply an insecticide to arrest damage and control adults prior to the onset of egg laying. Peak densities in the canopy occurred in late April, when counts ranged from 33 to 90 adult clay weevils per 10 feet of row and averaged 62 weevils per 10 feet of row. This peak adult activity in the canopy corresponded with an increased floricane bud break and rapid growth of fruiting laterals. Drop-cloth sampling in the second site through late June showed fairly consistent presence of adult clay weevils in the canopy from mid-April through early -June. Detections dropped significantly in mid and late June. See chart below.

The first few weeks in April were unusually cool which had a delaying effect on bud break and fruiting lateral development. In spite of numerous cold nights with temperatures at or below freezing, adult clay weevils could still be found in large numbers high in the canopy.

Comparison of the drop cloth method of sampling to the hand-held beating tray on two occasions during April showed that the drop cloth method on average detected twice as many weevils as a 10- beating tray sample. This 2:1 ratio was quite consistent and allows the IPM practitioner to convert from beating tray results to drop-cloth, which is the standard research tool. This in turn allows for an improved assessment of yield impacts based upon the more practical beating tray method.

Egg Laying Study

As was observed in 1998, clay weevils have two summertime egg laying periods. During 1999, they initially laid eggs from early May through early June. This is followed by a non-productive period for three months and then the onset of egg laying again in mid September and through mid October. See chart below. As of September 20, the most productive weevil produced 87 eggs and the least productive produced none. Average production for this period was 46 eggs per weevil. Of the thirty original specimens collected for this study, mortality through September 20 was limited to one specimen which died on August 30.

Chemical Trials

Basal application of Brigade as well as full canopy sprays with Brigade and Alert provided excellent initial knockdown of clay weevils and sustained control 14 days after treatment. Although the initial control with full canopy applications of Guthion and Fipronil were marginal both materials performed well 14 days after treatment. Basal applications of Alert and Fipronil as well as the full canopy Asana provided less than desirable control. See chart below.

The single application of Cryolite reduced clay weevil populations by 25% 3 days after treatment, 40% 7 days after treatment, and 45% 14 days after treatment This level of control was unacceptable given the potential impact of continued feeding on yield. For this reason, the grower chose to apply a basal Brigade in late April. Cryolite performance may have been compromised due to a day-long, light but steady rain five days after the treatment was applied. This rain may have caused the active ingredient to be leached from the bait, thus reducing its effectiveness.

Damage Assessment in the Field

Feeding by the clay colored weevil for three and six-week periods resulted in 20% to 32% destruction of fruiting lateral buds respectively. Feeding damage reduced the number of fruiting laterals per cane compared to the control, but there was no significant difference between the two feeding periods. However there was a significant reduction in average fruiting lateral length and lateral length per cane with the longer feeding period (see Table 1). The damage from this population density exceeded the plant’s ability to compensate. Fruit weight and number of berries per cane were both significantly lower in the clay weevil -infested plots. Finally, three and six-week feeding periods reduced yield per cane by 20% and 38% respectively and reduced the estimated yield per acre by 1 ton, which represents a 25% yield reduction. (see Table 2)

Means with different letters were significant at P=0.05 level by Tukey’s test

Means with different letters were significant at P=0.05 level by Tukey’s test.

Cold damage to buds was more severe in the control plots (18 %) compared to the weevil-infested plots (6% to 9%), most likely due to site variation. All plots had similar levels of spur blight bud infection ranging from 13% in the control to 14% to 16% in the weevil-infested sites. Spur blight-infected buds are often capable of producing normal fruiting laterals, though sometimes they are delayed in emerging.

These field studies have documented that the clay colored weevil can severely impact raspberry yield through early season feeding on floricane buds and developing laterals. Adult emergence from the soil occurs within a four-week period during bud break and early fruiting lateral growth. Weevils migrate rapidly into the raspberry canopy soon after emergence in spite of sub-freezing nighttime air temperatures. Where damage to buds is noticed, nighttime beating tray samples will confirm presence of the pest, which will assist in decision making. This work has shown significant yield reduction with 3 weeks of continuous feeding in the early spring. This supports the need for properly timed scouting and insecticide application when needed. The critical period for control is likely within a week or so of the first adult emergence from the soil and as weevils are moving into the canopy to begin feeding. This was early to mid-April in the 1999 season when soil heat accumulation was approximately 150-200 Degree-days (based on 40F minimum development threshold).

Only two currently registered insecticides (Brigade and Guthion) provide adequate control and due to the faster knockdown, Brigade applied to the lower 4 feet is the preferred tactic. Cryolite may be useful as a supplementary treatment two weeks after initial treatment for high populations, and possibly as the sole tactic in fields with low clay weevil populations to prevent buildup from year to year. More experimentation will be needed to best evaluate how Cryolite may provide an alternative to insecticide sprays. Based on lab egg-laying studies, it is assumed that with this suggested timing, adult weevils are controlled prior to the onset of initial egg-laying in the field. Weevils which escape control during the summer may survive in the field for several months and have the capacity to lay eggs again in the early fall.