Analysis of the Spatio-Temporal Distribution of Major Insect Fauna of Alfalfa by Using Geostatistical and Spatial Analysis by Distance Indices: Implications for Pest Management

Principle Investigator: Gadi V.P. Reddy1 Collaborator: Jhalendra Rijal2

Project Personnel: Govinda Shrestha1, Debra Miller1, Ramadevi L. Gadi1 and Ashish Adhikari1 

1Western Triangle Agricultural Research Center, Montana State University, 9546 Old Shelby Rd., P.O. Box 656, Conrad, MT 59425, USA

2University of California Cooperative Extension, 3800 Cornucopia Way, Suite A (Corner of Service and Crows Landing), Modesto, CA, USA

Aim of the Study

The aims of this study were: 1) to determine the spatio-temporal distribution of major insect fauna of alfalfa crops in the Golden Triangle area of Montana and 2) to assess the destructive and beneficial insects of alfalfa crops in the Golden Triangle area of Montana.

Material and Methods

Study sites

The research stated here was carried out in 2016 field season. Commercial alfalfa fields were selected from Ledger (two fields), Bullhead (one field), Conrad (one field), and Valier (one field), the areas in Pondera County. These sites are located in the Golden Triangle region of Montana.

Sampling

The sampling of major insect fauna of alfalfa field was conducted in an area of each field (45 × 45 m) about 5 m away from the field edges. The insects sampled for this study were alfalfa weevils (Hypera postica), lygus bugs (Lygus spp.), alfalfa weevil parasitoids (Bathyplectes spp.), aphid parasitoids (Aphidius spp.), damsel bugs (Nabis spp.) and coccinellids/ lady beetles, (Coccinella spp.). The sampling area contained 81 sampling points distributed at every 5-m distance in a square gird (i.e. 9 sampling points across X-coordinates and 9 sampling points across Y-coordinates) demarcated by using 1 m tall marking red painted wooden sticks. Sweep net sampling was used in which sweeping was conducted using a standard sweep net (180o arc), and 20 sweeps (five sweeps per direction) were made per sampling point. The collected insects were kept in plastic ziploc® bags and transported immediately to laboratory. The insect samples were either processed immediately or frozen for insect identification and counting under a microscope/naked eye at a later time. Four samplings were carried out: two samplings were performed before and two samplings after the first alfalfa cutting. After second sampling (before first cutting), wooden sticks were removed from study sites and were replaced with plastic ear tags. Prior to the start of the third sampling, plastic ear tags were removed and wooden sticks were relocated in the study sites. The sampling was conducted at 10 day intervals: first sampling from June 1-3; second sampling from June 16-17; third sampling from July 27-29; and fourth sampling from August 5-8, 2016.

Data analysis

Two major statistical analysis (Geostatistics (semivariogram) and Spatial Analysis by Distance Indices (SADIE)) are underway. The preliminary results (Mean ± Standard Error) of this study were presented for the interim.

Results

Pests

Alfalfa weevils

Alfalfa weevil larvae were present throughout the season (i.e., in all 4 samplings) in all sites. However, especially before the first cutting, higher variation of weevil larval population was observed in study sites. Among the five study sites, higher infestation levels of alfalfa weevil larvae were observed at the Bullhead and Valier study sites, moderate infestation level at Conrad study site and very low infestation level at Ledger study sites (Table 1). However, after first cutting, very low infestation of alfalfa weevil larvae were noticed in all study sites and the mean number of alfalfa weevil larvae were 2-3 per 20 sweeps. With respect to alfalfa weevil adults, they were found only at first or last two sampling in all field sites but no presence recorded at second sampling (Table 1). The mean number of alfalfa weevil adults caught in 20 sweeps was relatively low, with mean value varied from 0.30- 0.70 (Table 1).

Lygus bugs

Lygus bugs (Lygus spp.) were observed throughout sampling period irrespective of study sites (Table 1). Similar levels of lygus bug infestation were seen in all study sites (Table 1). However, infestation level was very low with mean value ranged from 0.20 – 2.00 and, it thereby indicating no threat to alfalfa crop.

Natural enemies

Parasitoids

Two parastioid species were recorded from all study sites: 1) alfalfa weevil parasitoids Bathyplectes spp. and 2) aphid parastioids Aphidius spp. (Table 2). Overall, higher number of aphid parasitoids were observed in all study sites as compared to alfalfa weevil parastioids.

Although very low number of alfalfa weevil parastioid adults were found in study sites, there was a tendency to find more parasitoids before the first alfalfa cutting rather than after first cutting (Table 2).

Table 1. Mean (± SE) number of larvae and adults of Hypera postica and Lygus spp. recorded per 20 sweeps in alfalfa fields in Pondera County, MT, 2016

Table 2. Mean (± SE) number of Bathyplectes spp. adults and, Aphidius spp. adults and mummies recorded per 20 sweeps in alfalfa fields in Pondera County, MT, 2016

Predators

Two predators were recorded from all study sites: 1) damsel bugs (Nabis spp.) which prey on aphids and alfalfa weevil larvae, and 2) coccinellids (Coccinella spp.) that prey mostly on aphids but also on alfalfa weevil larvae. Nearly similar level of damsel bugs population were seen in all study sites with relatively high population after first cutting (Table 3). Similar trend was also recorded for coccinellids (Table 3).

Table 3. Mean (± SE) number of Nabis spp. adults and, Coccinella spp. adults and larvae recorded per 20 sweeps in alfalfa fields in Pondera County, MT, 2016.

Acknowledgements

We would like to thank Connie Miller, Gabby Drishinski and Dawson Berg for assistance with field work. We would like to thank cooperator growers (Zane Drishinski, Jeremy Curry, Tony Verstrate and John Balkenbush) for allowing their fields for the study. This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch project under Accession# 1009746.