Diversity of Insect and Arachnid Populations in Onion Companion Crop Thaddeus Gourd, Colorado State University Extension, Adams County, Brighton, CO 80601;

Joe Julian, Colorado State University Extension, Douglas County, Castle Rock, CO 80104

Abstract:Onion production across the United States has been impacted by Iris Yellow Spot Virus (IYSV) which is vectored by the onion thrips (Thrips tabaci). Reducing thrips populations can sometimes help reduce infection rate of IYSV in onions. In Colorado, companion crops (living mulch) such as barley planted with onions to reduce wind and water erosion has sometimes had an effect on thrips populations. The objectives of this study are to determine 1) which thrips predators and parasites inhabit the barley companion crop; 2) what other insects/arachnids dwell in this living mulch; and 3) if there is a relationship between predator/parasite and thrips populations. Three onion field locations were sampled during 2011 that used barley as a companion crop in Weld County, Colorado. Four randomly selected areas of each of the three field locations were sampled with the insectavac on June 14 and 15. Eight beneficial predator and parasite families were identified from the three field locations sampled. The average of all populations of insects and arachnids at all locations revealed that ladybird beetle larva was the most numerous followed by spiders, braconid wasps, syrphid flies, big eyed bugs, ladybird beetle adults, nabids and lacewings. Six plant feeding insect families and one phytophagous mite family were found in the field samples. On July 28, thrips populatiosn ranged from 0.92 to 15.4 thrips per plant. In a non-companion crop planted field, 20.5 thrips were found per onion plant. No correlation between predator/parasite and thrips populations was detected in any companion crop location. Sampling conducted in 2012 confirmed that predator and parasite populations were significantly greater in onions with living mulch compared to non-mulched onions.

Predator & Parasites Found per 10 Row Feet in Living Mulched and Non Mulched Onions on 5/16/12

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Collected on 5/16/12 at Sakata Farm Location. Data represents the average of four replicates.Specimens were collected using an Insectvac. Figure 8

Beneficials Found on 6/14/11 at Companion Crop Field Locations

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Figure 2 - Sample Prep and Counting Organisms

Figure 1 - Sampling EquipmentStihl gas powered BG56 Shredder / Vacuum

Introduction and Objectives: Across the United States, onions are often grown with companion crops such as barley (living mulch) to provide seedling onion plants protection from water and wind erosion (1, 2). Frequently, barley is planted prior to, or during the onion seed planting operation. The rapidly emerging barley protects newly sprouted onion plants from wind, sand blasting and occasionally small hail injury that commonly occurs in the early spring along the Front Range of Colorado. Barley also significantly reduces soil loss in seeded onion fields. Onion producers and researchers from many western states have sometimes witnessed a reduction in thrips populations in these companion crop (living mulch) planted fields.

This study was set up to determine if beneficial populations of insects and arachnids found in a barley companion crop grown with onions affect thrips populations at various field locations in Colorado. Objectives of this study include:

Determine which thrips predators and parasites inhabit the barley companion crop.•Determine what insect/arachnid pests dwell in this living mulch.•Determine if there is a relationship between predator/parasite and thrips populations.•

Methods: Three onion field locations were identified that used barley as a companion crop (living mulch), along with one field with no companion crop. Sakata Farm Location 1 used sprinkler irrigation. Barley was planted on March 3, 2011, and the onions were planted on March 10. Arnusch Farm Location 2 used furrow irrigation. Here, barley was planted on March 2, and the onions were planted on March 23. Arnusch Farm Location 3 used sprinkler irrigation. Here, barley was planted on March 12, and the onions were planted on March 26. Arnusch Farm Location 5 had no companion crop, used flood irrigation and was planted on March 23.

Each location had four randomly selected areas sampled with an insect vacuum. Field Locations 1 and 2 were sampled on June 4, and Location 3 was sampled on June 15. Sample size was one row (2.5 feet) wide by 10 feet long. The onions had three to four true leaves at the time of the sampling.

A Stihl gas powered BG56 Shredder/Vacuum with an insect screen liner (192 micron polyethylene monofilament 181 x 181 mesh) attached to the suction tube by rubber bands (Figure 1) was used to collect samples. The liner captured plant parts and arthropods inhabiting the barley and onions. An insect vacuum (insectavac) was created in the late 1970s by J. Ellington, New Mexico State University (3) and modified by Colorado State University Extension entomologist, Bob Hammon [Traditional hand-net samples catch only 8% of total insects, and the insectavac catch-efficiency ranges from 14 to 64%, with an average of 32.6% (3)]. The fine mesh bags were emptied into one-gallon zip lock freezer bags and placed in a cooler. Ten-foot plot samples were taken and examined in the lab using a Nikon SM2800 dissecting microscope with 6.3 x 10 magnification. Fine-meshed screens were used to separate insects and arachnids from soil debris and plant material (Figure 2).

A thrips population evaluation determined the number of thrips per plant on June 28 at the three companion crop (living mulch) locations and at one field location where no companion crop was used. One hundred plants were randomly sampled at each location and checked for all stages of thrips. Data was analyzed using Analysis of Variance and the Tukey HSD All Pairwise Comparisons Test.

For comparison, a similar experiment was set up in May 2012 on onions in one field, both with and without a living mulch, and sampled on May 16 with an insectavac.

Table 1. Average number of beneficial insects and arachnids per 10 foot row(25 square feet) Locations 1 and 2 sampled on 6/14/2011; Location 3 on 6/15/2011

Beneficial Arthropods

Sakata Farm Location 1

Arnusch Farm Location 2

Arnusch Farm Location 3

Avg.

Lady Bird Beetle Adult

0 1 0 0.33

Lady Bird Beetle Larvae

10.75 15.5 9.25 11.83

Braconid Wasp .25 1.5 1.5 1.08Syrphid Flies adults 0 1.75 .25 0.67Spiders 2.75 1 .75 15.0Big Eyed Bugs .5 0 1 0.50Nabid 0 .5 0 0.17Lacewing Adult 0 .25 0 0.08

Table 2. Average number of insects and arachnids per 10 foot row(25 square feet) Locations 1 and 2 sampled on 6/14/2011; Location 3 on 6/15/2011

Phytophagous Arthropods

Sakata Farm Location 1

Arnusch Farm Location 2

Arnusch Farm Location 3

Avg.

Thrips 5.25 50.75 27.75 27.92Mites 109 4.25 16.75 43.33Leafhoppers 5.5 10 15 10.17Aphids 18.25 49.25 54.5 40.67Leaf Beetles 0 18.25 1 6.42Grasshoppers 0 .25 0 0.08Boxelder Bugs 0 .25 0 0.08Misc. Flies 1.75 0 10 3.92

Results: The time required to vacuum each sample site was approximately one minute. Placing the sample in a gallon-sized bag added another minute of time. The actual process of examining just one sample for insects and arachnids required approximately two hours. Eight beneficial predator and parasite families were identified from the three field locations. The average of all populations of insects and arachnids at all locations revealed that ladybird beetle larva (Coccinellidae) was the most numerous, followed by spiders, braconid wasps (Diaeretiella), syrphid flies (Syrphidae), big eyed bugs (Geocoridae), ladybird beetle adults (Coccinellidae), damsel bugs (Nabidae), and green lacewing adults (Chrysopidae) (Table 1 and Figure 3). No statistical differences were seen between beneficial populations at any location. Six plant feeding insect families and one phytophagous mite family were found in the field samples. The average of all populations of insects and arachnids at all locations revealed that phytophagous mites were the most numerous, followed by aphids (Aphidae), thrips (Thripidae), leafhoppers (Cicadellidae), leaf beetles (Chrysomelidae), miscellaneous flies, one grasshopper and one boxelder bug adult (Table 2 and Figure 4). Other than the thrips, these insects and mites would normally be found feeding in barley and not in onions.

Thrips populations were evaluated on July 28 by sampling 100 plants from each of four locations in the four fields. Numbers ranged from 1.0 to 15.4 thrips per plant in companion crop planted fields. In one non-companion crop planted field (Arnusch 5) located near a test location, 20.5 thrips were found per onion plant. This was significantly higher than all other onion companion crop field locations (Figure 5). Because the thrips evaluation on July 28 occurred after a number of insecticide applications were made to the onion crop, no correlation between the companion crop (living mulch) fields thrips population compared to a non-companion crop planted field, could be drawn. This thrips evaluation only showed efficacy of the previously applied insecticides.

Little to no IYSV was found at any field locations sampled in 2011. Populations sampled on June 14 and 15 showed no correlation between beneficials and thrips. However, this study did document that eight beneficial predator/parasite families, six plant feeding insect families, and one phytophagous mite family inhabited one or more companion crop onion field locations in Weld County in 2011.

Results of predator and parasite insect populations sampled in May 2012 confirmed the results seen in 2011. Significantly more beneficial insect populations were found in onions that had living mulch compared to non-mulched onions (Figure 6).

References: 1. Zandstra, Bernard, Michigan State University, Horticulture, “Time to kill barley cover crops in barley,” Integrated Pest Management Resources, Vegetable Crop Advisory Team Alert, Current news articles for vegetable production, 5/12/2010.2. Stivers, Lee, Crop Profile: Onions in New York, Cornell Cooperative Extension, 249 Highland Ave., Rochester, NY 14620.3. Ellington, J.J., Kiser, K., Cardenas, M., Duttle, J., and Lopez, Y. 1984. The insectavac: A high-clearance, high-volume arthropod vacuuming platform for agricultural ecosystems, Environmental Entomology 13 (1), 259-265.

Acknowledgements: A special thank you goes out to Robert and Bob Sakata and to Marc Arnusch for allowing the use of their onion fields for this study. Funding for this project came from the Colorado Onion Association.

Thrips Populations on 7/28/11

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Companion Crop and Non-Companion Crop Field Locations

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