Evaluation of Reduced Risk Insecticides for Management of Wireworms (Coleoptera: Elateridae) on Spring Wheat

Principle Investigator: Gadi V.P. Reddy

Project Personnel: Frank Antwi, Govinda Shrestha, John H. Miller and Julie Prewett 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 this study to examine the several commercial or non-commercial reduced risk insecticides for their potential to manage wireworms’ pest problem in a spring wheat crop in the Golden Triangle area of Montana.

Fig. Life cycle of wireworm

Materials and Methods

Study sites

Before starting experiments, we extensively sampled wireworms at each farm site using soil digging and bait traps (Reddy et al. 2014) to confirm the presence of adequate densities of wireworms for the study. The experiments were carried out in two growers’ fields at Ledger (N48°18’26.9244 W111°51’34.4376) and Valier (N48°18’37.4148 W112°25’19.0956) in the ‘Golden Triangle’ area of Montana from April-September, 2015 and 2016. Experimental plots were seeded on 16 April and 16 May in 2015 and 2016 respectively at Ledger, and on 28 April and 31 May in 2015 and 2016 respectively at Valier location.

The hard red spring wheat variety ‘Duclair’ (using certified foundation seed) was seeded at a rate of 22 seeds per 30 cm with a four-row plot drill spaced 0.3 m at both locations. Before seeding, the herbicide glyphosate (RT3®, Monsanto Company, St. Louis, MO) was applied at the rate of 2.5 L/ha for weed control, following regional farming practice. Fertilizer (N, P, and K) was applied at a ratio of 224.2, 0, and 22.4 kg/ha by broadcast application during planting, and an additional fertilizer application (N, P, and K at a ratio of 12.3, 25.2, and 0 kg/ha) applied through the seed plot drill. The experimental plots received 5 cm of water via overhead irrigation whenever needed. The first irrigation was done 30 days after treatments.

Reduced risk insecticide application

The insecticides and rates used were based on manufacturer’s recommendations (see Tables 1 and 2, for 2015 and 2016 respectively). Some of the treatments and formulations that failed to reduce wireworm numbers or to protect plants stand in 2015 were not included in 2016 study (Table 2). For treatments application methods in 2015, imidacloprid (Gaucho ® 600) was applied as a seed treatment. No fungicide was added to the seeds treated with Gaucho. Gaucho + Beauveria bassiana GHA (Mycotrol ESO®) and Gaucho + Metarhizium brunneum (Met52 EC®) were applied in rows by spraying Mycotrol and Met52 as soil drenching at the base of the seed treated Gaucho plants. The granular formulation of the entomopathogen B. bassiana ANT-03 (BioCeres G®) was applied by placing 21.6 g of product in gelatin shot glasses (66 ml) and applied to each row by hand. In 2016, Gaucho and a heat-killed formulation of the bacterium Burkholderia spp. strain A396 (Venerate XC®) were applied as seed treatments. A formulation of the bacterium Chromobacterium subtsugae (Grandevo SC®), Met52 formulated as microsclerotial and corn grit granule, and the organophosphate insecticide phorate (Thimet 20-G®) were applied as in-furrows. All other treatment combinations used in 2015 and 2016 (see Tables 1 and 2) were mixed in the tank and applied in rows as soil drenches. About 92 ml of water was applied per row with sprays. Spray treatments were applied to plots with a SOLO 4 gallon backpack sprayer # 425 (SOLO, Newport News, VA) with flat spray nozzle, 144.8 kPa (21.0 psi) valve, and calibrated at 816.89 L/ha. The spray applications were made 14 days after seeding.

Experimental design

The experimental design was a randomized complete block design (RCBD) with four replications. The plot sizes were 3.6 m × 1.2 m separated by 0.60 m buffer zones to avoid cross contamination of treatments. The numbers of standing plants, larval wireworm populations and seed yield in each plot were recorded to assess effectiveness of the treatments.

Plant stand count sampling

Emerged wheat seedlings were counted in a 1 m strip in the middle of the centermost two rows of each plot. The starting and ending points of the sample areas were marked with plastic labels so that the same seedlings could be recounted just before and again after treatments. In 2015, wheat seedlings were counted at 7, 14 or 28 days after treatments and for 2016 at 28 days after treatments.

Larval wireworm sampling

“Stocking bait” traps, described here below (Reddy et al. 2014), were used to detect wireworms and to estimate their relative abundance. The stocking bait traps were evenly placed along the center of each plot, spaced 1 m between baits. To make the baits, about 90 g wheat seed was placed in a nylon stocking, which was then tied shut with a string, leaving a tail end of about 30 cm. These traps were immersed in water for 24 h for the grain to start germinating before using being placed in the field, to make them attractive to wireworms. The baits were placed in holes 7- 15 cm deep and positioned so as to maximize the contact of the grain mixture with the soil as much as possible. The strings were left above the soil surface to help relocate the traps later. The traps were then covered with about 3 - 5 cm of soil. A 12 × 12 cm piece of black polythene was then placed on the covered holes and 4 metal pegs were used to secure these piece of polyethene to the soil.

In 2015, three stocking traps, spaced 1 m apart, were placed in the middle row of each plot. These traps were deployed one week before the spray applications. Just before treatments were applied, one trap from each plot was removed just to estimate the pre-treatment wireworm density in plots (one bait per plot, with 4 baits per treatment, for four replicates of the pre-treatment sampling). The second and third traps were removed 14 and 28 days after treatments. Larvae found in traps were counted in the laboratory. Similar procedure was used for 2016 wireworm samplings, except that two stocking bait traps per plot were used with one sampling before treatments and another at 28 days after treatments. Furthermore, identification of wireworm species composition was performed in 2016 by using morphological keys described by Etzler et. al (2013)

Yield and protein assessment

A Hege 140 plot combine was used to sample the plots for yield assessment. Wheat seeds were cleaned with a seed processor (Almaco, Nevada, IA) and weighed on a scale to determine yield at the WTARC seed laboratory in Conrad, MT. The protein content of seed was determined with NIR grain analyzer IM 9500 (Perten Instruments, Springfield, IL).

Statistical analyses

The data were analyzed using SAS 9.4 (SAS 2012). Data on number of plant and larval numbers were analyzed using ANCOVA (analysis of covariance). Treatment differences were tested using Fisher’s Least Significant (LSD) Test.

Results

Plant stand count

Irrespective of treatments, locations or years, plant stand counts generally decreased with time as the growing season progressed (Table 3 - 4). Pre-treatment plant stand counts per meter for 2015 varied from 26.4 to 56.6 and 20.1 to 36.1 respectively at the Valier and Ledger locations (Table 3) and the corresponding value for 2016 being 14.4 to 26.8 and 41 to 50.9 respectively (Table 4).

In 2015, the reduced risk insecticide treatments had shown significant effect on plant stand counts of wheat seedling at 28 days after treatments at Ledger and Valier locations. Among treatment plots, the significantly higher plant stand counts was observed for the seed treatment with Gaucho (20.3) and rest of the treatments with no significant difference(Table 4), when compared to the water control (14.6) at Ledger Location. Interestingly at Valier location, not only the Gaucho treatment (20.3) but also Mycotrol + Met52 (20.3), and Mycotrol + Gaucho (20.0) treatments had significantly stand counts when compared to the water control treatment (13.6).

In 2016, reduced risk insecticides that previously (2015 study) lacked significant effects on plant stand counts over water control treatment were discarded from this study and, new other reduced risk insecticides along with Gaucho and Entrust were tested for study. Overall, the study depicted that treatments had a significant effect on plant stand counts per meter at Ledger location (F = 1.92; df = 33, 239; P = 0.00), while non-significant effect of treatments at Valier location (F = 0.98; df = 33, 239; P = 0.50). However, when plant stand counts from reduced risk insecticide treatments were compared over water control treatment in post application sampling dates at Ledger location (LSD test), no significant difference or even a significantly lower plant stand counts in some of reduced risk insecticide treatments were observed over water control treatment. Consequently, the study of 2016 indicated non-significant effect of reduced risk insecticides on plant stand count at both field locations.

Wireworm populations

Wireworms were successfully captured in baited stocking traps in all treatments regardless of locations with exception of no wireworms recorded in Met52 + Entrust treatment at Ledger location in pre-treatment sampling of 2015. The mean number of wireworm populations per baited trap varied from 0.8 to 4 and 1.8 to 5.3 at Ledger and Valier location respectively in 2015. In 2016, similar mean number of wireworms (0.3 to 4.3) (with exception of no wireworms noticed in Thimet 20-G treatment) was recorded at Ledger location, while very low number (< 0.8) of wireworms recorded at Valier location.

Overall significant differences were observed in wireworm populations at both locations: Valier and Ledger in 2015. Across the treatment levels, as an unexpected, the significantly higher population of wireworms were observed in some of the reduced risk insecticide treatment plots over water treatment plots at 14 or 28 days after treatment at Ledger location. Treatments- Met52 (5.5) + Entrust, M-1 low (5.0), and Entrust (4) had significantly higher wireworm population over water control treatment (1.6) at 14 days after treatment. Similarly, at 28 days after treatment, Mycotrol treatment had significantly higher wireworm population of 4.0 when compared to the water control (1.0).

In 2016, overall treatments had no significant effects on number of wireworms trapped on baits among plots at both locations: Valier (F = 1.12; df = 11, 35; P = 0.37) and Ledger (F = 1.20; df = 11, 35; P = 0.32). Across the treatments, the mean number of wireworm populations per baited trap varied from 0 to 0.8 and 0.3 to 1.3 at Ledger and Valier location respectively at 28 days after treatment.

Wireworm species composition

The identification of wireworm species composition was performed in 2016 at both research locations- Valier and Ledger. Overall, three wireworm species- Limonius californicus, Hypnoidius bicolor and Aeolus mellilus were observed regardless of study locations. However, in both locations, H. bicolor was the most predominant species followed by L. californicus and A. mellilus at both sampling times- before and 28 days after treatments. The total number of H. bicolor, L. californicus and A. mellilus recorded at Ledger location were 57, 8 and 2 respectively and the comparing value for Valier being 24, 12 and 4 respectively.

Yield

Wheat yield for 2015 varied from 2448 to 3541 kg/ha and 3436 to 4743 kg/ha respectively at the Valier and Ledger locations (Table 5) and the corresponding value for 2016 being 514 to 762 kg/ha and 1017 to 1867 kg/ha respectively (Table 6). The treatments had shown significant effect on wheat yield for 2015 at Valier and Ledger locations, while for 2016 only significant effect of treatments was observed at Ledger location. There was a tendency for relatively higher grain yield when wheat plots were treated with Entrust followed by Mycotrol ESO + Entrust WP, BioCeres GR, Met 52 +Gaucho 600 and M-1 at Valier 2015, but no significant differences were observed when compared over water control treatment (Table 5). In contrast, at Ledger location in 2015, yields in Xpulse (4743.7 kg/ha) and Met52+ Gaucho (4420 kg/ha) treatment plots were significantly higher over water control treatment (3498 kg/ha). Similarly at Ledger location in 2016, Xpectro treatment improved wheat yield over water control treatment and rest of the treatments with no significant difference over control treatment.

Acknowledgements

This work was supported by Montana Wheat and Barley Committee. This material is also based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Multistate Project W3185, The Working Group Biological Control of Pest Management Systems of Plants under Accession # 231844.

References

Etzler, F. E (2013) Identification of economic wireworms using traditional and molecular methods. M.S. thesis dissertation, Montana State University, Bozeman, Montana

Reddy GVP, Tangtrakulwanich K, Wu S, Miller JH, Ophus VL, Jaronski ST (2014) Evaluation of the effectiveness of entomopathogens for the management of wireworms (Coleoptera: Elateridae) on spring wheat. J Invertebr Pathol 120: 43–49.

SAS Institute (2012). SAS/STAT, version 9.2 user’s guide, 2nd ed. 2012; SAS Institute, Cary, NC.

Table 1 Materials and rates of application in each treatment, 2015.

Table 1. contd.

a, Gaucho 600, seed treatment application rate unit (ml/45.35 kg seed).

b, Entrust WP, application rate unit (g/L).

c, Gaucho 600, seed treatment application rate unit (35.49 ml/45.35 kg seed).

d, BioCeres GR, application rate unit (20 g/m2)

Table 2 Material, rate, and method of application in each treatment, 2016

Table 3 Plant stand count of wheat seedling treated with reduced risk insecticides at Ledger and Valier, 2015

Means within a column followed by the same letter are not significantly different at P < 0.05.

a, PT, pre foliar and granular application (21 days after planting).

b, 28 DPT, days after foliar and granular application (49 days after planting).

Table 4 Plant stand count of wheat seedlings treated with reduced risk insecticides at Ledger and Valier, 2016

Table 4 Plant stand count of wheat seedling treated with reduced risk insecticides at Ledger and Valier, 2016

Table 5 Wireworm catch per baited trap on wheat seedling plots treated with reduced risk insecticides, 2015

Table 6 Wireworm catch per baited trap on wheat seedling plots treated with reduced risk insecticides, 2016

Means within a column followed by the same letter are not significantly different at P < 0.05.

a, PT, pre foliar and granular application (28 days after planting).

b, 28 DPT, days after foliar and granular application (45 days after planting).

Table 7. Yield of wheat seedlings treated with reduced risk insecticides at Valier and Ledger 2015

Table 8. Yield of wheat seedlings treated with reduced risk insecticides at Valier and Ledger 2016