Laboratory Evaluation of Bio-pesticides against Pea Leaf Weevil Sitona lineatus (L.) (Coleoptera: Curculionidae) Adults

Principle Investigator: Gadi V.P. Reddy Project Personnel: Govinda Shrestha and Debra Miller

Western Triangle Agricultural Research Center, Montana State University, 9546 Old Shelby Rd., P.O. Box 656, Conrad, MT 59425, USA

Aim of the Study

The aim of the study was to evaluate commercially available bio-pesticides (such as Spinosad (Saccharopolyspora spinosa), PyGanic EC® (Pyrethrins), Mycotrol ESO® (Beauveria bassiana GHA), Xpulse® OD (B. bassiana GHA + Cold pressed Neem extract) and Xpectro® OD B. bassiana GHA + Pyrethrin) against pea leaf weevil adults Sitona lineatus, which consequently help on reducing of synthetic insecticides toward this pest.

Figure 1. Pea leaf weevil adult killed by Beauveria bassiana GHA or B. bassiana GHA with Pyrethrins- dorsal (A) and frontal (B) views; pea leaf weevil adult killed by Spinosad (Saccharopolyspora spinosa)- frontal(C) and dorsal (D) views; uninfected/live pea leaf weevil adult (E).

Material and Methods

Insects

Sitona lineatus adults were collected during spring and fall of 2016 from various host plants (e.g. peas, lentils and alfalfa) at several locations in the Golden Triangle area of Montana. During the spring season, pheromone traps baited with lures 4-methyl-3, 5-heptanedione (ChemTica Internacional, Costa Rica) were installed on the soil surface in pea and lentil fields to catch S.lineatus adults. This pheromone has been known to attract both male and female of this pest (Nielsen and Jensen 1993). Adults caught in pheromone traps were collected once a week, placed in plastic deli cups (diameter = 12 cm and height = 8 cm) and transported immediately to the laboratory. Approximately 50 S. lineatus adults were placed inside a deli cup, reared with alfalfa foliage (5-6 stems) and maintained in a climate cabinet at 12 oC for 2-3 days, until enough adults were obtained to initiate bioassay experiments. In the fall season, S. lineatus adults were collected from alfalfa fields using sweep nets as the adults were found highly aggregated in alfalfa fields (Shrestha, Personal Observation). These collected adults were kept in plastic ziploc® bags with alfalfa foliage and transported immediately to laboratory. S. lineatus adults were reared under a laboratory condition in a similar method described above for spring season collected adults.

Table 1 Materials and application rates of bio-pesticides used for the laboratory bioassays against

Sitona lineatus adults.

Bio-pesticide products

Five bio-pesticide products of commercial formulations were used for the study (Table 1). Mycotrol ESO® (Beauveria bassiana GHA), Xpectro OD® (Beauveria bassiana GHA + pyrethrins) and Xpulse OD® (Beauveria bassiana GHA + azadirachtin) were obtained from Lam International (Butte, MT), Entrust WP® (spinosad 80%) from Dow Agro Sciences (Indianapolis, IN) and, PyGanic EC® 1.4 (pyrethrin) from McLaughlin Gormley King (Minneapolis, MN).

Stock solutions were prepared for each product prior each experiment by dissolving the product materials in tap water and lower concentrations were prepared by serial dilutions with tap water. The concentrations tested were 0.1, 0.5, 1.0 and 2.0 fold lowest label rate (Table 1).

Laboratory bioassay

Prior to performing experiments, the bioassay methods such as immersion and spray (bottle and perfume sprayer) were preliminarily assessed to select the most appropriate one for testing the efficacy of bio-pesticide products against S. lineatus adults. Among these bioassay methods, the perfume sprayer was selected for experiments as other methods resulted higher mortalities of S. lineatus adults (~ 80%) both in treatments and control groups after 48 h.

Similar sized of pea leaf weevil adults (length = 5-5.5 mm) were placed in groups of 7 adult individuals in a Petri dish lined with a filter paper (diameter = 9 cm). Placement of the weevils was accomplished with the help of a fine camel paint brush. The Petri dishes were then maintained in a cold climate cabinet (5 oC) for 1 h to reduce the adults’ activity. The adult individuals of each group were then placed in the center of Petri dish. They were then topically treated with 1 ml of bio-pesticide product materials. Controls were treated with 1.0 ml of tap water. After the spray applications, a fresh alfalfa stem about 5 cm long with 9-12 leaves, was placed close to the treated adult individuals inside a Petri dish as a source of feeding materials. Feeding materials were replaced within 2-3 day intervals. Dishes were incubated in a climate cabinet at 22 ºC ± 1 oC, 16:8 L: D and 75% RH. The bioassay experiment was performed in two seasons- spring and fall. The numbers of replicates (one replicate equals one Petri dish) per treatment were 8 and 5, respectively, in the spring and fall experimental run. However, in the fall season, only bio-pesticide products that were effective in spring experimental run were tested.

Starting one day after the treatment, pea leaf weevil adults’ mortality was checked daily for 9 days. S. lineatus adults have a specific characteristic since they act like dead weevils with minimum disturbance (Shrestha, Personal Observation) and are known as “dead play” insects (Jackson and Macdougall 1920). By gentle prodding with a camel paint brush the individual adult mortality was determined. Any adults that lacked any movement were considered to be dead. Dead weevils, particularly from insect pathogenic fungi or mixture with other product, were removed and placed on moist filter paper in a Petri dish to check for sporulation.

Statistical analysis

The statistical analysis part of this study (such as calculations of LC50 and LT50) is underway. The raw results (Mean ± Standard Error) of this study were only presented for the interim.

Results

Mortality of spring population of pea leaf weevil adults

Five bio-pesticide products were evaluated against adults of pea leaf weevil. Overall, this study showed that all tested bio-pesticide products have abilities to cause mortalities on pea leaf weevil adults (Table 2). However, the difference in pea leaf weevil adult mortalities were observed across products or at their concentration levels. Among the five tested products, Spinosad (Entrust WP®) seems to be most effective product (Fig 1), Mycotrol ESO® and Xpectro OD® as moderately effective (Fig 1) and the Xpulse OD® and PyGanic EC® as less effective products.

The total mean mortality of pea leaf weevil adults caused by bio-pesticide products ranged from 16 to 100% for Spinosad product, 9 to 64% for Mycotrol ESO® product, 9 to 63% for Xpectro

OD®, 7 to 36% for Xpulse OD® and 5 to 21% for Pyganic EC® (Table 2).

Table 2. Total mean percentage mortality of spring population of pea leaf weevil adults treated with different concentrations of bio-pesticides. Mean percentage of mortality (± SE) 9 days post inoculation.

Mortality of fall population of pea leaf weevil adults

Three bio-pesticide products were only evaluated against fall population of pea leaf weevil adults since Spinosad (Entrust WP®), Mycotrol ESO® and Xpectro OD® were found effective against spring population of pea leaf weevil adults. The similar results were also found for fall population of pea leaf weevil adults (Table 3) but with slightly lower mortalities (from 8-10%) for Mycotrol ESO® and Xpectro OD® bio-pesticide treatments.

Table 3. Total mean percentage mortality of fall population of pea leaf weevil adults treated with different concentrations of bio-pesticides. Mean percentage of mortality (± SE) 9 days post inoculation.

Acknowledgements

We would like to thank Connie Miller for assistance with field work. This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Multistate Project S-1052, and the Working Group on Improving Microbial Control of Arthropod Pests Covering Research in Montana under Accession # 232056.

References

Jackson, D. J. and K. S. Macdougall (1920). "Bionomics of weevils of the genus Sitones injurious to leguminous crops in Britain. Annals of applied Biology 7: 269-298.

Nielsen, B. and T. Jensen (1993). Spring dispersal of Sitona lineatus: the use of aggregation pheromone traps for monitoring. Entomologia experimentalis et applicata 66: 21-30.