March through September, 1998
Raj Bathe (Bathe Farms, Inc), Whatcom Farmers Coop., Dr. Lynell Tanigoshi (WSU Entomologist), and Geoff Menzies (WSU Lynden Research Station)
There were several mini-studies carried out during the 1998 season focusing on the clay colored weevil, Otiorhynchus singularis (CCW), an early season, bud and shoot feeding weevil.
Beginning in mid-March, numerous soil core samples (0-6" depth) were taken from throughout a known infested field to determine first emergence of adults (Biofix) as well as the presence of other stages of this insect. Beating tray samples were taken in mid and late-March to help determine when adult CCW were moving from the soil and into the plant canopy to begin feeding on buds and emerging floricane shoots. Data loggers (HOBO units) placed in the field were used to record soil and air temperature.
Egg laying studies were conducted in the laboratory on about 30 adult CCW collected in mid-April from two sites in Whatcom County The weevils were maintained from then through October on raspberry foliage in small containers in the lab. The food base was changed to blackberry foliage in late October to maintain surviving colonies and attempt to carry them over the winter. The containers were checked every 3 to 4 days and weevil mortality and egg production was recorded.
A chemical trial targeting adult CCW was applied on 4/13.The maximum fruiting lateral shoot growth at this time was 4" with most shoots ranging from 2-3" long. Shoots from some buds, mostly below the wire had not emerged by this time. Treatments included: full canopy sprays of Alert 2SC (.32# ai/acre), Asana XL .66EC(.15# ai/acre), 3 rates of Brigade 10WP(.025,.05,.1# ai/acre), Danitol 2.4EC(.20# ai/acre), Guthion 50WP(.5# ai/acre), a basal spray of Brigade 10WP(.05# ai/acre), and an untreated check. Each of the 9 treatments was replicated 5 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 D3-45 TeeJet nozzles at 200psi delivering 141 gal/acre and travelling at 2.5 mph The basal Brigade treatment was applied using only the lower 3 nozzles on each side of the row; 6 nozzles total. 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 10 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 pm. Following the 14 day counts, all plots were sprayed by the grower on May 2 in order to minimize further plant damage from surviving adult CCW. Sampling after this treatment confirmed that adequate control was achieved.
Impacts of CCW feeding on yield were measured on June 30, just prior to the onset of harvest in 5 of the treatments and on 4 replications . This was accomplished by selecting 6 floricanes randomly from each plot
and recording the number of laterals, the lateral length, and the number of berries and flowers likely to mature and be harvested. Treatments which were evaluated for yield included: Alert, full and half rates of Brigade, the basal Brigade, and the untreated check. We returned to these plots on 7/16 and 7/24 to measure the berry weight of 50 berries/plot chosen at random. In addition, qualitative ratings of fruit load (scale of 1 to 3) were made by 4 observers in the same plots on 7/24. In this rating system, 1=lesser amount of fruit and 3=greater amount of fruit. Ratings from the 4 observers were averaged for each plot over the 5 replications.
On August 7, soil was collected from 3 sites in this field just prior to it being removed and from 3 sites in an adjacent field known to be infested with CCW. CCW had been sprayed prior to the onset of egg laying in the trial field (5/2) but not until late June in the adjacent field. The purpose of this sampling was to determine the presence and distribution of CCW larvae, pupae and adults in the soil at 3 different depths (4-8", 8-12", and 12-16"). At each of the 6 sites, approximately 4.5 gals of soil were collected from each depth zone in a 3 ft. long by 4" wide trench positioned 6 inches from the center of the hill. Soil was screened through 1/8" wire mesh to extract and count weevils.
The first adult clay weevil was detected from the soil on March 11. This teneral (soft bodied ) adult weevil was just preparing to emerge from the soil. This established the Biofix for CCW adult emergence as March 11. The first hardened CCW adult was detected in the canopy with beating tray sampling on March 16. Density was approximately 1 CCW/10 trays. By March 26, counts from beating tray sampling had increased to 25 CCW/10 trays based on a random sample taken in this field. At this time, light feeding damage was underway and the most advanced fruiting lateral buds were up to 2" long.
Three treatments (Alert, full rate of Brigade, and the basal Brigade) reduced adult CCW populations by 85-90% 14 days after treatment compared to the untreated check. Significantly less effective treatments were the 1/2 rate of Brigade and AsanaXL (57% reduction), the 1/3 rate of Brigade (50% reduction), Guthion (47% reduction), and finally Danitol (41% reduction) when compared to the untreated check. In the untreated check, counts 14 days after treatment averaged almost 30 CCW per 10 ft of row. Results are illustrated in the graph below.
CCW started laying eggs in the lab in mid-May and ceased in mid-June, producing no eggs until the last week of September and first week of October. Although the colonies are being maintained on blackberry through the winter, no eggs have been deposited between early October and mid-December. As the graph below illustrates, there appear to be two summer egg laying periods for spring–emerging, adult clay weevils. Early season egg production was similar to the timing in 1997. Based on this lab study, CCW produces far fewer eggs than black vine weevil. The most productive CCW produced a total of 28 eggs over a 2 week period; or 2 eggs/weevil/day. Of the original 27 weevils collected for this study, 8 adults have survived through mid-December.
Performing a simple calculation (average#berries/cane X average berry wgt) resulted in the following yields relative to the untreated check: Alert (+17%), Brigade 1/2 rate (+14%), Brigade full rate (+10%), and basal Brigade (+2%). These theoretical yield estimates do not correlate well to reductions in CCW for the various treatments. However, results do support field observations that feeding by this insect significantly impacts yield. The qualitative fruit load ratings were 2.1 for each of these treatments compared to 1.5 for the untreated check.
A comparable number of CCW were found in both sites. Interestingly, most of the weevils which were found (60%) were found in the soil between 8 and 12"deep. 15% were in the 4-8" zone and 25% were in the 12-16"zone. The dominant lifestage detected (70%) were teneral CCW adults, which were most numerous in the 8-12" zone but also present in the shallower and deeper depths. The remaining detections were larvae, presumed to be CCW. No pupae (the intermediate stage between larva and adult) were found. Attempts to rear these larvae out to adults failed, and therefore, species identification of the larvae collected were not confirmed. The chart below illustrates the proportions of larvae and teneral adults in the soil at the three depths which were checked.
Unfortunately we are not able to differentiate root weevil species based on examination of the larval stage. Larvae must be reared out to adults for positive identification.
We are limited to only one registered material (Brigade) which provides adequate control of this insect.
Treatment when necessary in early to mid-April should control this insect prior to the onset of egg laying and therefore suppress buildup from one year to the next.
Based on the egg laying studies, it is possible that clay colored weevils enter an inactive resting stage during the hot summer months (July and August), possibly seeking refuge at the base of the plants. This would also explain why clay weevils are not typically a problem as a harvest contaminant, even in infested fields.
Although it was difficult to correlate yield reductions to population density, it was obvious that this bud and shoot feeding insect has the potential to dramatically reduce yield. Our estimates of yield reductions were based on damage occurring over only a 2 week period. Unchecked adult populations can feed in the raspberry canopy from mid-March through mid to late-June, a 3 month period. Feeding over a longer period would very likely reduce yield to a greater extent than was observed in this experiment.
Both teneral adults and larvae reside in the soil as deep as 16" in the late summer.
Pursue registration of Alert for early season CCW control. This will reduce our reliance on a single insecticide and minimize opportunities for CCW resistance to Brigade.
Re-consider biorational tactics (insect killing nematodes and fungi) which would target the larval and teneral adult stages present in the soil after harvest. Application at this time should favor nematode survival due to increased soil temperature in late summer.
Continue basic biological studies in the field to better determine the period of adult emergence from the soil and the year round life history of this insect. Aquiring this knowledge would streamline our selection and application of biorational control tactics.
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