Evaluation of Trap Crops for the Management of Wireworms in Spring Wheat in Montana

Principle Investigator: Gadi V.P. Reddy

Project Personnel: Ashish Adhikari, 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 aims of the study were to examine the effect of seven trap crops pea, lentil, canola, corn, durum, barley and wheat for their attractiveness to wireworm in spring wheat crop.

Fig. Trap crops evaluated for wireworms management

Materials and Methods

Trials sites

The field trials were conducted at two sites: Ledger (48.2583° N, 111.8257° W) and Valier (48.3078° N, 112.2498° W) in the Golden Triangle region of Montana, from May to August in both 2015 and 2016. These fields are well known for being infested with wireworms. Valier soil is a sandy loam while that of Ledger is silt and clay, rich in humus. Spring wheat is the main crop grown in these fields for the past few years.

Exp. #1. Effectiveness of trap crops in field trials Experimental design

The study area of 40 ×12.6 m was established and divided into 42 experimental units each measuring 1.2 × 4.8 m. A complete randomized block design with seven treatments and six replications was used. The blocks were separated by 1 m buffers and the two plots within each block were separated by 0.45 m. Each experimental plot had four rows with row to row distance of 0.3 m. The main crop (spring wheat) was planted in the first and third rows and the trap crop was planted in the second and fourth rows. The seven treatments were Montech pea (T1), Hyeless 955 canola (T2), sweet corn (T3), Montrail durum (T4), Metealfa barley (T5), Green Land lentil (T6), and Duclair wheat as control (T0) respectively. Minimum tillage was done and a four-row seed driller was used. The rate of sowing was 22, 26, 26 and 24 seeds per 0.3 m for wheat, lentil, barley, and durum, respectively. Similarly, 7 and 53 seeds/m were panted for corn and pea, respectively. The herbicide ammonium sulphate (AMS) was broadcasted at the time of sowing at 2.24 kg /ha as were fertilizers at an N, P and K ratio of 224.2, 0, and 22.4 kg /ha.

Sampling for plant damage and wireworm density

To determine the level of crop damage from wireworms, the number of plants or seedlings in each plot was measured randomly using the 1 m line intercept method (Canfield 1941; Jonasson 1988). From two rows of wheat in a plot, three plant counts were collected. The same method was applied to the various trap crops intercropped with the wheat. Wheat plants were categorized as healthy or damaged. Healthy ones were those without any damage while plants that were wilting or yellowish in appearance or had an overall shorter plant height were considered damaged. In both fields, counts were first made two weeks after sowing, then at weekly intervals for four weeks, then at 2-week intervals for three more times, for a total of eight sampling dates.

To determine the density of wireworm larvae, destructive soil sampling was done, using a metallic sampling device, 15 cm square shape. Samples were taken at random from each row within the plots. Samples were placed in plastic bags, labelled and brought to the Research Centre, where they were processed and the number of wireworms in each sample recorded. Wireworms from samples were placed in small plastic pots filled with sphagnum moss and stored in a refrigerator at 8 °C. Later, they were identified using the keys of wireworm described by Etzler et. al (2013) and some of them were used for the shade house experiment. The soil samples were collected right after the plant count was taken. Eight soil sample readings were collected from both sites.

Exp. #2. Determination of optimal spacing for trap crops Location and experimental design

These trials were conducted in 2016 at the same two locations of Valier and Ledger. A complete randomized block design was again used, with spacing treatments of 0.25, 0.5, 0.75, and 1 m, between wheat (control), pea and lentil, each replicated four times. Pea, lentil and wheat (control) were randomly assigned to plots within each block. There were 48 plots in total, each 4 m2. The blocks and the plots within each block were separated by 1.5 m. The crop cultivar used were Montech pea, lentil and Duclair wheat, respectively. Crops were manually sown at the same rate used for the first field trials. Due to climatic reasons, plots were sown in late May; in Ledger, on the third week and in Valier, on last week of May.

Sampling for plant and wireworm density

Plant counts was taken using the 1 m line intercept method from randomly selected rows of wheat and trap crop. In each experimental unit, counts were made from one randomly selected wheat and one trap crop row for each counting event. The first count was taken two weeks after sowing. The following three counts were taken at weekly intervals at both sites. The last two count readings were taken at two week intervals, for a total of six plant counts taken from both sites.

Wireworm larvae were sampled by destructive soil sampling using a 0.15 m3 metal sampler to sample wireworm larvae in the soil. In each experimental plot, two samples were taken at random, one each from randomly selected wheat row and trap crop row. These samples were bagged, labelled, brought to the Research Centre, manually went through each bag and the number of wireworms recorded. Larvae were kept in plastic pots filled with peat moss and held in a refrigerator at 8 °C. The soil sample was collected after the plant count was taken.

Exp. #3. Shade house bioassay

Shade house bioassays were conducted at the Western Triangle Agriculture Research Centre in Conrad, Montana in August 2016. An average room temperature of 18 to 22 °C was maintained throughout the experiment. Assay units were square plastic flower pots filled with potted soil mixture of soil, sphagnum moss and sand in ratio 4:2:1; dimension of each pot was 10.5 cm square and 9.2 cm deep. The experiment had three treatments: Montech pea, lentil and Duclair wheat (control), i.e., pea vs wheat, lentil vs wheat and wheat vs wheat with four replications of each and conducted over two time frames: 4 days and 10 days. Five grams of wheat seed were planted in the middle of two opposite sides and five grams of pea or lentil were planted in the middle of the other two sides. Nine larvae of L. californicus of similar length (1.5 cm) were then released in the center of the pot. On days 4 and 10 of the respective assays, the potting mixture in each pot was divided into nine equal sections, comprised of the four corners, the four sides and the center, the location of wireworms determined.

Statistical analysis

Analysis of variance (ANOVA) was used to analyze data in R-software version 3.1.3. For both field trials, we used date of sampling as a blocking factor and performed ANOVA. Tukey HSD post-hoc at 95% confidence interval was used for pair-wise comparison among treatments.

Values of P < 0.05 were considered significant. Paired t-test at 95% confidence interval was used to analyze the difference for wireworm numbers trapped among treatments. Chi-square tests of fitness were performed for the greenhouse bioassays and P-values < 0.05 were considered significant.

Results

Exp. #1a. Effect of trap crops on damage levels in field spring wheat

In both years, at the Valier field location significance differences in wheat plants damage (%) were found among trap crops (in 2015, F = 9.01, df = 6, and P<0.01 and in 2016, F = 15.54, df = 6, and P<0.01). In 2015, except for the barley (P>0.05) and canola (P>0.05) treatments, significance differences were found between tested trap crops and the control (Figure.1.).

Likewise, in 2016 damage levels in wheat plants intercropped with pea (P<0.01), lentil (P<0.01), and corn (P<0.05) were significantly lower than in the wheat monoculture control (Figure.2.). In Valier, in both years, pea (P<0.01) and lentil (P<0.01) treatments showed the lowest damage in wheat plants

Similarly, in both years, at the Ledger field location, significance differences were observed in damage (%) in spring wheat intercropped with trap crops (F = 59.49, df = 6, and P = <0.01 in 2015; and F = 13.68, df = 6, and P <0.01 in 2016). In 2015, except for in the barley (P>0.05) trap crop treatment, significant differences were detected in damage (%) in spring wheat plants intercropped with other trap crops (P < 0.05) compared to the wheat monoculture control.

Damage to wheat plants was significantly lower in the pea (P <0.01) and lentil (P <0.01) treatments than in the other treatments (P <0.05) (Figure. 1). In 2016, except for the durum (P>0.05) significance differences were found between trap crops and control (P <0.05). Moreover, significantly lower damage (%) in wheat plants intercropped with pea (P <0.01) and lentil (P<0.01) trap crop treatments than other (P <0.05) was observed (Figure. 2). In both years, regardless of location, wheat plant damage % was higher in first four weeks of sampling, then decreased, and leveled off. The germination of treatment corn were very low.

Exp. #1b. Effect of trap crops on wireworm densities in field spring wheat

In 2015 and 2016, at the Valier field location significance differences in wireworm numbers were found between the soil samples taken from wheat rows intercropped with different trap crops and control (in 2015, df = 6, P <0.01 and in 2016, df = 6, P <0.05). However, significantly lower numbers of wireworm were observed only for wheat intercropped with treatment pea (t = 3.41, P <0.05 in 2015, and t = 3.24, P <0.05 in 2016). While comparing the wireworm numbers recorded from the different trap crops rows with control wheat rows we found significant lower number of wireworms in corn rows (P<0.05 in both years) and canola (P<0.05 in 2016) than trap crop pea.

In both years, at the Ledger field location significance differences in wireworm numbers were found between soil samples collected from wheat rows intercropped with trap crops and control (in 2015, df = 6, P< 0.01 and in 2016, df = 6, P< 0.05). However, significant low wireworm numbers was recorded from the soil samples of wheat rows intercropped with treatment pea (t = 3.993, P <0.05 in 2015, and t = 3.16, P <0.05 in 2016). Moreover while comparing the wireworm numbers recorded from the different trap crops rows with control wheat rows we found significant lower number of wireworms in corn rows (P<0.05 in both years) than trap crop pea.

In both years, wireworm numbers in soil core samples were high until the fifth sampling date after which wireworm numbers declined. In 2015, we collected a total of 693 and 380 wireworms in Valier and Ledger respectively (Figure. 3 and 4). In 2016, we collected 301 wireworms in Valier among which 25 were Aeolus mellillus, 117 Hypnoidus bicolor, and 159 Limonius californicus. In Ledger, 262 numbers of wireworms were collected from soil sample among which 15 were Aeolus millillus, 125 Hypnoidus bicolor and 124 Limonius californicus.

Exp. #2a. Effect of trap crops and spacing on wheat plant density

At Valier, wheat plant counts showed significant differences between trap crops (df= 2, F= 158.8, P<0.01) and between different spacing levels (df= 3, F= 58, P<0.01). Significantly higher numbers of wheat plants per meter were recorded in plots intercropped with pea (P<0.01) and lentil (P<0.01) compared to control wheat plots (Figure. 5). At intercropping spacing of 0.25 and 0.5 m between the trap crops and spring wheat significantly more wheat plants were found than at 0.75 and 1m spacing’s (Figure. 6). Moreover, a significant interaction was observed between trap crops and spacing levels (df= 6, F= 131.61, P<0.01) which is presented in Table 1.

At the Ledger field location, significant differences in wheat plant density was found among trap crops (df= 2, F=33.66, P<0.01) and spacing levels (df=3, F= 4.36, P<0.01). While pea and lentil treatments did not differ significantly from each other, wheat density in the plots intercropped with pea and lentil was significantly higher than that in the control (P<0.01) which is shown in Figure. 5. Wheat density was significantly higher at 0.75 m spacing than at 0.25 m (P<0.01), but wheat plant numbers were not significantly different among the 0.75, 0.5 and 1 m spacing levels (Figure. 6). Moreover, there was significant interaction between of spacing and trap crop (df=6, F=3.53, P<0.01) (Table 2).

Exp. #2b. Effect of trap crops species and spacing on wireworm density

At the Valier field location, the number of wireworm collected from wheat rows intercropped with different trap crops showed significant differences (df= 5, F= 5.87, P<0.01), but no differences were observed for the different spacing levels used and interaction between trap crops and spacing level was non significance. Wireworm numbers in wheat rows intercropped with pea (P=0.05) or lentil (P<0.01) were significantly lower than in rows of the control monoculture. From the soil samples, we collected 209 numbers of wireworms which were 11 Aeolus millillus, 70 Hypnoidus bicolor and 128 Limonius californicus.

At the Ledger field location, there were no significant differences among trap crops and spacing levels for the numbers of wireworm recorded from the soil samples in wheat rows intercropped with different trap crops at different spacing levels, P value more than 0.05 for both factors.

There was no any significant interactions between trap crops and spacing levels. We found 178 numbers of wireworms from the soil samples among which 15 were Aeolus millillus, 73 Hypnoidus bicolor and 90 Limonius californicus.

Exp. #3. Effect of trap crops on feeding habits in shade-house potted plants

In a shade-house bioassay, wireworm distribution within pots found by sampling on the 4th day after seeding showed significant differences between wheat intercropped with pea, lentil or wheat as a control grown in pots. Pea (P<0.01) and lentil (P<0.01) seeded with wheat both trapped significantly more wireworm than the control. Similar results were obtained on the 10th day, with both pea (P<0.01) and lentil (P<0.01) trapping more wireworms than the control (Table. 3). We never found wireworms evenly distributed across all nine sections of the pot, confirming that wireworm are not randomly distributed in the soil.

Acknowledgements

This project is supported by the 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, Hatch under award Accn# 1009746. We would like to thank Dr. Bob Vernon, Agriculture and Agri-Food Canada, Dr. Kevin Wanner and Dr. Mike Ivie from Montana State University, Bozeman for their valuable suggestions and summer interns Javan Caroll and Gaby Drishinski for their assistance in the field.

References

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

Table 1. Interaction between intercropping spacing and trap crops in Valier, Montana in 2016 y ns = not significant, * and ** indicate significant interactions at P <0.05 and 0.01, respectively according to three way ANOVA, Tukey HSD.

z P = Pea, L = Lentil, and W = Wheat.

Table 2. Interaction between intercropping spacing and trap crops in Ledger, Montana in 2016

y ns = not significant, * and ** indicate significant interactions at P < 0.05 and 0.01, respectively according to three way ANOVA, Tukey HSD.

z P = Pea, L = Lentil, and W = Wheat.

Table 3. Mean number of wireworms found in different regions of soil from plastic pots sown with wheat and trap crops in the bioassay. Chi-square values are at α 0.05.

Figure. 1.

Fig. 1. Effect of trap crops on wheat plants damaged (%) by wireworms in Valier and Ledger locations in 2015. Different letters above bars indicated significant differences by two way ANOVA, Tukey HSD α=0.05.

Figure. 2.

Fig. 2. Effect of trap crops on wheat plants damaged (%) by wireworms in Valier and Ledger locations in 2016. Different letters above bars indicated significant differences by two way ANOVA, Tukey HSD α=0.05.

Figure. 3.

Fig. 3. Total number of wireworms collected from wheat rows and trap crops rows during soil sampling in Valier and Ledger locations in 2015.

Figure. 4.

Fig. 4. Total number of wireworms collected from wheat rows and trap crops rows during soil sampling in Valier and Ledger locations in 2016.

Figure. 5.

Fig. 5. Mean number of wheat plants/m when intercropped with different trap crops at spacing level of 0.25, 0.5, 0.75 and 1 m between wheat and different trap crops. Different letters over bars represent significant differences according to three way ANOVA, Tukey HSD α = 0.05.

Figure. 6.

Fig. 6. Mean number of wheat plants/m, when spring wheat is intercropped with pea, lentil and wheat at spacing levels of 0.25, 0.5, 0.75 and 1 m. Different letters over bars represent significant differences according to three way ANOVA, Tukey HSD α = 0.05.