April through September, 1999

Dr. Lynell Tanigoshi (WSU Vancouver, entomologist), Geoff Menzies (WSU Vancouver, Lynden Satellite Station manager), Raj Bathe (Bathe Farms, Inc.), David Youngquist (Northfork Farms), and Sam Dahlival.

Several species of root weevils can be pests on raspberry. The type of damage varies with the species, but can include damage to developing floricane buds and potential yield reduction, contamination of fruit during the mechanical harvesting operation, and damage to roots from the larval stage.

As adult root weevils were detected in various projects in the field using either beating trays or ground cloth sampling methods, the most frequently encountered species on raspberry were collected and returned to the laboratory. They were individually placed in one-pint containers and supplied with fresh raspberry leaves every two to three days. Containers were checked at this same interval for weevil mortality and egg production from the time of collection in the field until all individuals had expired or through the termination of the project in late summer. Egg laying studies were carried out on spring-emerging clay colored weevil (Otiorhynchus singularis), woods weevil (Nemocestes spp.) and overwintering black vine weevil (Otiorhynchus sulcatus), as well as summer-emerging black vine weevil and obscure root weevil (Sciopithes obscurus). Other fairly common root weevils which are often detected in raspberry but not included in this study this year due to low numbers this season are strawberry root weevil (Otiorhynchus ovatus) and rough strawberry root weevil (Otiorhynchus rugosostriatus).

Having knowledge of egg laying periods for these pests allows the manager to schedule insecticide sprays prior to the onset of egg laying. If adult weevils are controlled before egg laying begins in the field, the potential for in-field increase from one season to the next should be reduced.

Clay Colored Weevil

The clay colored weevil can be a very serious early season bud-feeding weevil. During the 1999 season, adults began to emerge from the soil in late March and continued through late April. Clay weevils used in this study were collected from two sites during the first week of April. Based on these studies, it appears that this weevil has two egg laying periods. The initial period is from early May through early June. In the lab, this is followed by a non-productive period for about three months followed by the onset of egg laying again in mid-September. Of the thirty original specimens collected for this study, mortality through September 20 was limited to one specimen, which died on August 30. The charts below (Figs 1 and 2) show the average daily egg production per weevil and the average cumulative number of eggs produced per weevil throughout the season.

Fig. 1: Average daily clay weevil egg production

Fig. 2: Cumulative clay weevil egg production

The woods weevil is an early season bud-feeding weevil of secondary importance to the clay colored weevil. Overwintering adults emerge from the soil very early in the spring to feed on developing buds and leaves. Eggs are laid within a fold made by the adult in the edge of a leaf. Woods weevils used in this study were collected from one site in late March. As figures 3 and 4 show, egg laying began in mid-April and continued sporadically through late June. Weevils in this lab study began to die off during the third week of May and all were dead by the end of June. This species is quite productive, laying on average as many as 9 eggs per weevil per day during peak periods. The cumulative production over this 2 ½ month period averaged over 250 eggs/weevil and ranged from 100 to 300 eggs per weevil among the specimens in this study.

Fig.3: Average daily woods weevil egg production

Fig.4: Cumulative woods weevil egg production

Black Vine Weevil

The black vine weevil is the most common root weevil found on raspberry. The adult stage is the most troublesome as a fruit-contaminating insect during the machine harvesting operation. Most of the population overwinters in the larval stage, emerging from the soil during May and early June. A small percentage spend the winter as adults and can be found in the raspberry canopy as it develops in the early spring. The overwintering weevils used in this study were collected on raspberry in late April. The summer-emerging weevils were collected in early June. Egg production for both the overwintering and summer–emerging adult black vine weevils are shown below in Figure 5.

Fig. 5: Average daily egg production of overwintering and summer-emerging black vine weevils

Daily productivity was similar with both forms during the egg laying periods, but the average cumulative production per weevil was greater with the overwintering form due to a longer egg laying period as shown in Figure 6 below. The average cumulative production of overwintering weevils was over 300 eggs/weevil but ranged among specimens from 31 to 718 eggs. The average cumulative production of summer-emerging weevils was 170 eggs/weevil but ranged among specimens from zero to over 300 eggs.

Fig.6: Cumulative egg production of overwintering and summer-emerging black vine weevils

Obscure Root Weevil

A small number of adult obscure root weevils were collected on raspberry from one site in early July. In spite of regular scouting in this field, this was the first detection of this species for the season. Egg laying began about one week after collection but lasted for only one month due to die-off of all five specimens by the end of July. This weevil also lays its eggs within a fold made by the adult in the leaf edge. In spite of this short egg laying period in the lab, the obscure was relatively productive. The average daily production at the peak of 8 eggs/weevil/day was as high as black vine weevil. The average cumulative production was just under 100 eggs per weevil and ranged among specimens from 45 to 139 eggs. This egg laying period corresponded very precisely to the raspberry harvest period. See Figures 7 and 8 below:

Fig.7: Average daily obscure weevil egg production

In fields which are infested with either clay colored weevils or woods weevil, the normal timing for controlling the adult stage prior to excessive bud damage is early to mid-April. Based upon the findings in this study, this timing should control adults prior to or about at the time that egg laying begins for the woods weevil and well in advance of the onset of egg laying by the clay colored weevil. It also appears that pre-harvest, "clean-up" insecticide sprays, which are usually applied in late June to early July precede the start of egg laying by summer-emerging black vine weevils and obscure root weevils. However, in fields which are not infested with early season bud-feeding weevils, and typically are not sprayed in the spring, overwintering black vine weevils when present and left uncontrolled during May and June have the potential to lay down a significant number of eggs before the pre-harvest insecticide spray timing. Sampling with a beating tray during bud break and through April is recommended to determine whether overwintering black vine weevils are present. Where detected, early season control of the adult weevils should reduce egg deposition, subsequent root feeding from developing larvae, and carryover of this pest to the next season. Figures 9 and 10 below summarize the results for all root weevils tested this season.

Fig. 9: Average daily egg production and egg laying periods for all root weevils studied.