Efficacy of the Insect-killing Nematode, Heterorhabditis marelatus against Black Vine Weevil in Strawberry -WSU Vancouver, Lynden Satellite Research Station Report

Start date and Duration

April through May, 1999

Investigators/ Collaborators

Dr. Lynell Tanigoshi (WSU Vancouver, entomologist), Geoff Menzies (WSU Vancouver, Lynden Satellite Station manager), Richard Sakuma (Sakuma Brothers Farms, Inc.), and Lore Walters (Sun Pacific Biological Lab, Inc.).

Project Description

Overview

The two most important root weevil pests on strawberry in northwest Washington are the black vine weevil, Otiorhynchus sulcatus and the strawberry root weevil, Otiorhynchus ovatus. They can reduce yield and stand longevity due to feeding damage to the roots caused by the larval stage, which is the predominant overwintering lifestage. There are currently no registered soil insecticides for controlling the larval stage and there is much interest in exploring alternative, biorational tactics for weevil control in this crop. In this project, the commercially available insect-killing (entomopathogenic) nematode, Heterorhabditis marelatus was applied in the spring with conventional spraying equipment to control a very dense population of black vine weevils in a strawberry field in Skagit County, Washington. Most of the weevils were in the larval stage at the time of application. Nematodes were supplied by Sun Pacific Biological Lab and the material was applied by the cooperating grower in coordination with WSU personnel.

Methods and materials

Prior to applying nematodes in mid-May, weevils were sampled by removing and screening ten soil cores from throughout the proposed site. This was done with a golf-green hole cup-cutter, which is 4 inches in diameter and extracts a 5-inch deep core. Soil and roots from each core were screened through 1/8-inch mesh to extract, identify, and count weevil larvae. Soil temperature was recorded prior to and during the experiment using a data logger (Hobo), which is manufactured by Onset Computer Company. The temperature probe was placed 4 inches below the soil surface in the center of the strawberry row.

Within the infested area, plots were set up in a randomized paired plot configuration to compare efficacy in nematode- treated and untreated plots with four replications of each. Plots were 4 rows wide and 100 feet long. Each strawberry row or hill is about 16 inches wide and hills are on 40-inch centers. Prior to application, leaf debris was removed from a 20-foot long section within one row of each of the nematode-treated plots. Previous work has suggested that this debris can restrict the nematode’s ability to reach the soil and can therefore be a major impediment to effectively controlling soil-dwelling weevil larvae and pupal stages. Ten small tea strainers, each containing 2 mature black vine weevil larvae were buried 3 inches deep in each of the nematode -treated plots prior to the spray application. These were collected 12 days after the treatment and inspected for mortality and discoloration associated with nematode infection.

Each plot was pre-watered with a tractor-mounted sprayer delivering 360 gallons of water per acre. Immediately afterwards, nematodes were applied on May 14 at 7:30 p.m. at the recommended rate of 1 billion nematodes per acre in 217 gallons of water per acre. This was accomplished with a tractor-mounted sprayer travelling at 2.3 miles per hour. Each plot was sprayed with a single pass using a boom with four 8015 fan nozzles, with each of the four nozzles centered on a strawberry row. All filters were removed from the sprayer and the sprayer pressure was 50 PSI. After the nematode application, the sprayer was used again to deliver 360 gallons of water per acre using the same nozzle configuration but at a slower speed. The nozzles were then removed from the sprayer entirely and a second and third, post-application delivery of 2500 gallons per acre was made in each of the nematode–treated plot.

Plots were evaluated on May 27 and June 3, 12 days and 21 days after treatment. This was accomplished by sampling and screening cores from each of the plots to extract and record the number of healthy and infected weevil larvae and pupae. Weevils which are infected with the nematode and the associated bacteria are reddish in color and inactive compared to healthy individuals, which are full-bodied, creamy-white, and active. Tea strainers were removed 12 days after treatment and the specimens were categorized in the same manner.

Results

The field survey conducted prior to treatment revealed a strawberry field heavily infested with black vine weevils. A total of 72 mature larvae and 3 pupae were screened from 10 core samples representing slightly less than one square foot in area. Based on this sampling, pre-treatment density across these 10 cores ranged from 23 to 138 weevils per square foot. This is an exceptionally high population. Most of the weevils were positioned in the top 3 to 4 inches of the soil profile. No adult black vine weevils were detected at this time.

Results of the field evaluation 12 days following treatment are shown in Table 1 below. By this time in late May, the pupal stage was the dominant stage present in the soil, representing 80% of the specimens found. No adult black vine weevils were detected at this time. The combination of debris removal followed by nematode treatment provided the best weevil control, resulting in 78% larval infection and 54% pupal infection, averaging 58% infection overall for both life stages. In contrast, nematode treatment without removal of debris prior to application resulted in infection of only 13% of the recovered larvae and pupae. It is also apparent from the data that during this 12-day period, weevils were transforming from the larval to the pupal stage. This coincided with increasing soil temperatures, which were favorable for both weevil development as well as nematode activity and infection. Mortality of weevils caged within the tea strainers was similar to results from the areas where debris had been removed prior to nematode application. This was likely due to the disturbance to the soil and debris when placing them in the field, which allowed for the nematodes to reach and infect the host weevils. A final and reduced evaluation was carried out on June 3, 21 days after treatment, shown in Table 2 below. There was no increase in the degree of infection at 21 days compared to 12 days after treatment. By early June, adult black vine weevils were beginning to emerge from the soil.

Table 1: Efficacy of the Entomopathogenic Nematode, Heterorhabditis marelatus against Black Vine Weevil in Strawberries 12 Days After Treatment.

 

Treatment

 

Cores

sampled

Black Vine Weevil

% Black Vine Weevils infected by H. marelatus

no. larvae/core

no. pupae/core

Healthy

Infected

Healthy

Infected

Larvae

Pupae

Both Stages

H. marelatus

(no debris removal)

40

1.50

0.25

6.25

0.95

13.2

13.2

13.2

H. marelatus

(debris removed)

20

0.30

1.05

2.90

3.45

77.8

54.3

58.4

Untreated Check

10

3.30

0.0

 

14.0

0.0

0.0

0.0

0.0

 

Table 2: Efficacy of the Entomopathogenic Nematode, Heterorhabditis marelatus against Black Vine Weevil in Strawberries 21 Days After Treatment.

 

Treatment

 

Cores

sampled

Black Vine Weevil

% Black Vine Weevils infected by H. marelatus

no. larvae/core

no. pupae/core

Healthy

Infected

Healthy

Infected

Larvae

Pupae

Both Stages

H. marelatus

(no debris removal)

5

0.30

0.15

 

3.30

0.30

33.0

8.3

11.1

H. marelatus

(debris removed)

5

0.15

0.75

2.40

2.25

83.3

48.4

54.1

Untreated Check

5

0.80

0.0

 

9.20

0.0

0.0

0.0

0.0

Soil temperatures at this site taken four inches below the soil surface from early May through early June are shown in Figures 1-3 below. These charts are shown in the same format as is produced on a computer screen using the "Boxcar" computer program, which is the standard program provided by Onset Computer for management of data generated by their "Hobo" data loggers. Activity of H. marelatus as with most entomopathogenic nematodes is reduced in cooler soils. A minimum of 50 degrees F is considered necessary for adequate nematode activity and subsequent host infection with ideal temperature closer to

65 F. During the week or so prior to application (5/14), the soil was very cool at this site, exceeding 50F for only short periods of time. Fortunately, soil temperature increased steadily particularly during the second and third weeks after application (Figs. 2 and 3) when they were consistently above 50F and reaching almost 65F on one occasion in late May.

 

Figure 1. Soil temperature at 4-inch depth in strawberry field, May 6-20, 1999

Figure 2. Soil temperature at 4-inch depth in strawberry field, May 20-26, 1999

Figure 3. Soil temperature at 4-inch depth in strawberry field, May 26- June 3, 1999

 

Summary

This field experiment has shown that Heterorhabditis marelatus applied in the early summer with conventional spray equipment can provide on average up to almost 60% kill of mature black vine weevil larvae and pupae in strawberries. Removal of leaf debris prior to application was the key to success. As with most pest management tools and particularly biological control agents, timing and attention to environmental conditions is critical for success. The efficacy observed in this project was encouraging but well below levels of insect control normally achieved with chemical insecticides in most cropping systems. The application timing in this experiment was optimum in regard to both weevil development and soil temperature. In spite of this, only 60% control was achieved long after damage from the root-feeding larval stage was completed. Most farmers cannot be expected to achieve such precise timing to control soil dwelling insects such as root weevils.

Considering these encouraging results more experimentation should be undertaken to evaluate other methods of application and alternative treatment timing. Late summer treatment is likely preferable in order to control the damaging larval stage before most of the damage to roots is underway. Soil temperatures during September and October in most years should be favorable to nematode activity. Practical methods for leaf-debris removal prior to application, or direct delivery of nematodes to the soil in the hill via drip irrigation or mechanical shanking need to be resolved to optimize the utility of entomopathogenic nematodes for root weevil control in strawberries.

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