Effect of nitrogen fertilization levels and insecticide seed treatment on the incidence of crucifer flea beetle and cabbage seedpod weevil on canola yield and quality

Principal Investigator: Gadi V.P. Reddy

Project Personnel: Shabeg S. Briar, John H. Miller, Anamika Sharma, Govinda Shrestha, 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

1. To determine if N fertility at different rates and seed treatment affects flea beetle and cabbage seedpod weevil feeding patterns.

2. To study flea beetle and cabbage seedpod weevil feeding impacts on canola seed yield and quality with respect to N soil fertility.

Figure 1: Flea beetle damage symptoms on canola leaf, pods and stem stages of growth.

Material and Methods

Site description

Field trials were conducted at two locations in Conrad (48°18.627’N, 111°55.402’ W) at the Western Triangle Agricultural Research Center research field area and Sweetgrass (48°57.831’N, 111°40.801’ W), both representing dryland rain fed (i.e., non-irrigated) conditions of the West-Central region of Montana, USA in 2017. Previous crop before seeding canola was barley at the Sweetgrass while the field was maintained as chemical fallow at the Conrad location. Soil test from both locations were conducted in the spring of 2017 to calculate the rate of fertilizer applications while seeding canola. Based on the soil tests performed, residual N level was about 16 lbs/ac at Conrad compared to only 3.5 lbs/acre at the Sweetgrass location.

Experimental plots were seeded on May 1st, 2017 using Hy-Class® (WindField Solutions, LLC, Houston, Texas) cultivar at a rate of 12 seeds per 30 cm, using a four-row plot drill with 30 cm row-row spacing. Weed control was done by pre-plant application of herbicide RT3® (a.i. glyphosate) at a rate of 2.5 L/ha.

Air temperature (Figure 2 A) and precipitation data (Figure 2 B) for the period of study for Conrad and Sweetgrass were accessed from the USDA, NRCS weather station (NRCS, 2016) and Alberta Agriculture and Forestry weather station (Alberta Agriculture and Forestry, 2016), respectively. Average temperature at Conrad remained was slightly higher from April to September, 2017 than the Sweetgrass location (Figure 1 A). Both locations experienced low precipitation throughout the growing season especially from July to August when the dry spell prevailed until the crop harvest except for one time rain in the middle of July (Figure 1 B)

Treatments and field plot design

The treatment structure for this study consisted of four levels of N including: no external application of N (N1), 56 kg/ha (N2) (50 lbs/ac); 112 kg/ha (N3) (100lbs/ac) and 168 kg/ha (N4) (150lbs/ac) applied at the time of seeding canola. Further each level of nitrogen either received insecticide imidacloprid (Gaucho 600®) seed treatment or no treatment. All treatments (i.e. four N levels with or without insecticide seed treatment), received fungicide seed treatment prior to seeding of experimental plots. No foliar sprays of any pest control were applied to the research trials. The field experiments at both the locations were Randomized Complete Block Design (RCBD). Individual plots measured 3.6 m × 1.2 m in size. A buffer zone of 1.2 m was set up between each plot to avoid cross contamination due to spray drift. Treatments were replicated 4 times at each location.

Data collection

Each plot was rated for the flea beetle (P. cruciferae) feeding injury along one 3.6-m section of row in each plot, by sampling 10 plants at 0.3 m intervals, four times i.e. cotyledon, four leaf, pre-flowering and pod formation stage during the canola growing season. P. cruciferae injury measurements were made by visual examination, on a 5-grade scale as defined in OEPP/EPPO (2004). The visual injury ratings were converted into percent leaf area injury, where 1 = 0%; 2 = 2%; 3 = 5%; 4 = 10%; and 5 = 25% injury to the leaf area.

Seedpod weevil (SPW) damage assessments on the pods were performed at the canola pod maturation stage. Fifteen pods were randomly sampled per plot and damaged or undamaged pods were determined in the lab by visual inspection using SPW damage characteristics (Feeding holes) followed by the opening of pods with a sharp blade to check the presence of larvae inside the pods. Data is presented as percent infested pods per plot.

The canola plots were straight combined stored and air dried until the seeds were at 10% moisture. The seeds were then cleaned and weighed to determine the seed yield followed by the assessment of canola grain quality parameters including test weight, thousand kernel weight (TKW) and percent oil content.

Statistical analysis

PROC MIXED procedure (PROC MIX, SAS Institute 2017) was performed for determining the effects of insecticide, nitrogen levels and, their interactions on both flea beetle (P. cruciferae) and seedpod weevil (C. obstrictus) damage ratings and, yield and grain quality parameters including test weight, TKW and percent oil content. An alpha level of 0.05 was used for all tests. Since, interaction among N and insecticide was not significant, least square (LS) mean estimates and their SE are presented for main effects only which includes four N levels averaged over the seed treatment and, insecticide seed treatment (Treated versus Untreated averaged over the N levels).

Results

Flea beetle and seedpod weevil damage assessment

Overall comparisons among N levels, insecticide and their interactions are presented in table 1. In general, flea beetle feeding pressure based on crop injury ratings was very high throughout the canola growing season at Conrad as compared to the Sweetgrass location (Table 2). Injury rating recorded at different crop stages showed highest level of damage from flea beetle at the four leaf stage followed by the pod formation stage at Conrad whereas at Sweetgrass highest damage was noted at four leaf and pre-flowering stages of canola. (Table 2). More than 70% injury was recorded at four leaf stage in Conrad while the highest injury ratings were about 11% at four leaf and pre-flowering at the Sweetgrass location.

At Conrad, no significant differences were among the N levels at cotyledon (F =0.88; P = 0.22), four leaf (F =1.13; P =0.36) and pod stage (F =0.66; P = 0.59) except for pre-flowering stage of canola (F =3.67; P 0.03) (Table 1). The effect of seed treatment on injury ratings at four leaf (F=30.00; P <0.0001), pre-flowering (F =9.81; P 0.01) and pod stage (F =33.1; P <0.0001) was significant, except for cotyledon stage (F =0.77; P= 0.09) (Table 1 and 2).

At Sweetgrass location where flea beetle feeding pressure was remained low during the growing season, injury ratings varied among the N levels only at cotyledon stage (F =3.89; P = 0.02) but not during the other crop growth stages. On the other hand, Insecticide significantly impacted the feeding injury ratings at cotyledon (F =7.81; P =0.01), four leaf (F =26.40; P < 0.0001) and pre- flowering (F =47.40; P < 0.0001) but not at pod stage (F =1.02; P =0.33).

Mean seedpod weevil percent infested pods were low at Conrad (range 5-13%) compared to Sweetgrass location where the percent infested pods were high and range 6-26%. No significant differences, however were among the N levels (Figure 3 A and B) and insecticide treatments at both the locations for the seedpod weevil percent infested pods.

Canola seed yield and quality parameters

Average canola seed yield ranged from to 887-992 and from 392-641 kg/ha respectively, at the Conrad and Sweetgrass locations (Table 2). Yield levels lower at Sweetgrass as compared to Conrad location. Although, yield levels increase slightly with the increase in N level, with the highest at 100 lbs/al level, differences were not significant among the N levels at Conrad location (Table 1 and 2). Similar trend of no significant differences were among the N levels for grain quality parameters. In contrast, insecticide averaged over the N levels, canola yield (F = 8.10; P= 0.01), TKW (F = 4.61; P = 0.04) and percent oil content (F = 10.70; P< 0.0001) were significantly higher in treated than the untreated plots except for the canola grain test weight (F = 0.99; P = 0.33).

At Sweetgrass location, significant differences were among the N levels for canola grain yield (F=9.55; P = <0.0001), test weight (F =3.70; P = 0.03) and oil content (F =4.55; P = 0.01) except for TKW (F =0.16; P = 0.92) (Table 1). Yield levels increase with the increase in N application at the time of seeding with highest mean yield (641 kg/ha) at the highest level of N (Table 2).

Yield (F =30.00; P <0.0001) and percent oil content were significantly higher (F =9.81; P 0.01) in the insecticide treated than the untreated plots (F =33.1; P <0.0001). No significant impact of insecticide was observed for the other grain quality parameters.

At Sweetgrass location where flea beetle feeding pressure remained low during the growing season, injury ratings varied significantly among the N levels only at the cotyledon stage (F=3.89; P = 0.02). Insecticide on the other hand significantly impacted the feeding injury ratings at cotyledon (F =7.81; P = 0.01), four leaf (F =26.40; P < 0.0001) and pre-flowering (F =47.40; P < 0.0001) but not at pod stage (F =1.02; P = 0.33).

Mean seedpod weevil percent infested pods were low at Conrad (range 5-13%) compared to Sweetgrass location where the percent infested pods were in the higher (range 6-26%). There was no trend for the N levels and no differences were observed among the seed treated and untreated plots (Figure 3 A and B).

Conclusions

It is interesting to note here that flea beetle pressure was very high at the Conrad location throughout the canola growing season. Although, seed treatment with insecticide maintained lower feeding pressure than the untreated plots, it was well above the threshold level of 20-25% as established and reported in literature. This data clearly demonstrate that if dry and warm conditions prevail over the canola growing season considerable damage can occur even at the later stages of canola growth and exhibit considerable damage during the pod formation stage thereby impacting the yield and grain quality parameters. Although, yield was highest at the 100 and 150 pounds of N/ac treatment levels, flea beetle injury pressure likely masked the differences among the N levels at the Conrad location. Consequently, no significant differences were detected between the N levels. In contrast at the Sweetgrass location where feeding pressure was low, impact of N was more pronounced as significant differences were observed among the N levels. Seedpod weevil infested pods were higher at Sweetgrass location. However, no relationship of Seedpod weevil to N levels and seed treatment were observed. No significant differences among insecticide treated and untreated plots for Seedpod weevil likely indicate that the effects of seed treatment with insecticide did not last long enough to impact this pest at pod stage in Montana dry land conditions.

Acknowledgements

This work was supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, [Hatch] project [MONB0085].

References

OEPP/EPPO. 2004. Efficacy evaluation of insecticides. Phyllotreta spp. On rape, pp. 242-244.

OEPP/EPPO Bull. Pp 1/218.

SAS Institute Inc. (2017). 9.4 In-Database Products, User’s Guide, fifth ed. SAS Publishers, Cary, NC, USA.

Table1: Effect of nitrogen level, insecticide seed treatment and their interaction, on flea beetle (FB) leaf and pod injury percent rating and, canola yield and quality at two locations in Montana, 2017.

Table 2: LS means estimates (±SE) for nitrogen levels and insecticide seed treatment, on flea beetle (FB) leaf and pod injury and canola yield and quality at two locations in Montana, 2017. F- and P-values are shown in table 1.

Figure 2: Weekly average air temperature (A) and precipitation (B) at Conrad and Sweetgrass study locations.

Figure 3: Seedpod weevil percent infested canola pods for four N levels at Conrad (A) and Sweetgrass (B) locations.