- (Public Value) UCANR: Promoting economic prosperity in California
- Author: Surendra K. Dara
The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae) is an important pest of broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, and other cruciferous crops. It exclusively feeds on cultivated and weedy crucifers and has a worldwide distribution. Larvae feed on foliage and growing parts of young plants or bore into the heads or flower buds resulting in skeletonization of leaves, stunting of the plants, or failure of head formation in some hosts. In warmer areas, the diamondback moth has up to 12 generations per year. While multiple species of parasitoids and predatory arthropods provide some level of natural control, insecticidal applications are a primary means of diamondback moth management. Although several synthetic and biological pesticides are effective against the diamondback moth, resistance to Bacillus thuringiensis (Ferré et al. 1991), abamectin (Pu et al. 2009), emamectin benzoate, indoxacarb, and spinosad (Zhao et al. 2006), pyrethroids and other pesticides (Leibee and Savage 1992; Endersby et al. 2011) has been well-known from around the world. Excessive use of any kind of pesticide leads to resistance problems (Dara 2020; also see a video presentation) to an individual pesticide or multiple pesticides. Integrated pest management (IPM) strategy encourages the use of various control options both for maintaining pest control efficacy and reducing the risk of resistance development (Dara 2019). Regularly monitoring the pest populations to make treatment decisions, rotating pesticides with different modes of action, exploring the potential of biocontrol agents, and other non-chemical control approaches such as mating disruption with pheromones are some of the IPM strategies for controlling the diamondback moth. While sex pheromones effectively used to manage several lepidopteran pests and are proven to be a critical IPM tool, mating disruption is not fully explored for controlling the diamondback moth. A study was conducted in Brussels sprouts to evaluate the efficacy of a sprayable pheromone against the diamondback moth and to enhance the current IPM strategies.
The study was conducted on a 10-acre Brussels sprouts field in Santa Maria. Cultivar Marte was planted in early July 2020 with expected harvesting in mid to late December. A typical diamondback control program includes monitoring diamondback moth populations with the help of sticky traps and lures and applying various combinations of biological and synthetic pesticides at regular intervals. This study evaluated the efficacy of adding CheckMate DBM-F to the grower standard practice of monitoring the diamondback moth populations with traps and lures and applying pesticides. Treatments included i) grower standard pesticide program (Table 1) and ii) grower standard pesticide program with two applications of 3.1 fl oz of CheckMate DBM-F on 9 August and 11 September. Treatment materials were applied by a tractor-mounted sprayer using a 100 gpa spray volume and necessary buffering agents and surfactants. Each treatment was 5 ac and divided into four quadrants representing four replications. In the middle of each quadrant, one Suterra Wing Trap was set up with a Trécé Pherocon Diamondback Moth Lure. Lures were replaced once a month in early September and early October. Sticky liners of the traps were replaced every week to count the number of moths trapped. Traps were placed on 1, 12, and 24 August, 1, 11, 18, and 27 September, and 6 October and the moth counts were taken from respective traps on 8 and 20 August, 1, 11, 18, and 27 September, 6 and 15 October. CheckMate DBM-F was applied at 3.1 fl oz/ac on 9 August and 11 September. The number of larvae and their feeding damage on a scale of 0 to 4 (where 0=no damage, 1=light damage, 2=moderate damage, 3=high damage, 4=extensive/irrecoverable) were recorded from 25 random plants within each replication on 30 August and 6 and 18 October. Data were subjected to analysis of variance using Statistix software and significant means were separated using Tukey's HSD test. The retail value of various pesticides was also obtained to compare the cost of treatments.
Table 1. Pesticides, buffering agents, and surfactants, their active ingredients, rates/ac (along with the IRAC mode of action groups), and retail pricing for those applied in the grower standard diamondback moth control program.
When CheckMate DBM-F [(Z)-11-Hexadecenal (3) , (Z) - 11 - Hexadecen-1-yl Acetate (1)] was applied the first time on 9 August, Dibrom 8 Emulsive was replaced with Warrior II, the buffering agent Quest was not used, and the surfactant Dyne-Amic was replaced with Induce (dimethylpolysiloxane) to avoid potential compatibility issues. The impact of this substitution is expected to be negligible within the scope of this study. The retail cost of 3.1 fl oz CheckMate DBM-F is $45.60. The cost of lures and traps would be about $4-8 per acre for a six-month crop like Brussels sprouts.
Results and Discussion
Moth populations: Traps in replication 4 in both treatments on 8 August and replication 1 in the grower standard were missing, probably knocked down by a tractor. The day before CheckMate DBM-F was first applied, the mean number of adult diamondback moths caught were 227 in the grower standard and 271 in the plots that would receive the pheromone application. There was a gradual decline in moth counts during the rest of the observation period in both treatments. However, the decline was higher in the plots that received CheckMate DBM-F. The number of moths per trap were about 19% higher in the pheromone-treated plots compared to the grower standard before the study but were nearly 98% lower by the end of the study. The reduction in moth populations from mating disruption was significant on 18 September (P =0.039) and 15 October (P = 0.006).
Larval populations: The mean number of larvae per 25 plants in a replication was zero on all observation dates except for 0.01 on 30 August in the plots that received CheckMate. Four insecticide applications by the time the study was initiated, and the remaining six applications effectively suppressed larval populations.
Damage ratings: Larval feeding damage ratings were consistently low (P < 0.0001) in the plants that did not receive CheckMate DBM-F. The damage was limited to the older leaves at the bottom of the plants and must have been from early feeding before the initiation of the study. The lack of larvae and the evidence of new feeding damage also confirm that the crop remained healthy and pest-free.
Since frequent pesticide applications effectively suppressed larval populations and prevented their feeding damage, the effectiveness of mating disruption in reducing yield losses could not be determined in this study. Since larval counts were not made weekly or between pesticide applications, those that were probably present between the pesticide applications could not be determined. Moths found in the traps probably developed from the larvae in the field or could have been those that flew in from other areas. However, lower moth populations in CheckMate DBM-F treatment demonstrated the overall influence of mating disruption and pest suppression.
It is common to make about 10-12 pesticide sprays during the 6-month crop cycle of Brussels sprouts. The cost of each application varied from about $73 to $192 depending on the materials used with an average cost of about $128 per application in this study. The cost of two CheckMate DBM-F applications is $91. If diamondback moth populations could be reduced with mating disruption, it is estimated that 2-3 pesticide applications could be eliminated. That results in $164 to $292 of saving for the pesticide costs and additional savings in the application costs per acre by investing $91 in the mating disruption. Since the diamondback moth can develop resistance to several chemical and natural pesticides, eliminating some applications as a result of mating disruption also contributes to resistance management along with potential negative impact of pesticides on the environment. Compared to other mating disruption strategies, a sprayable formulation compatible with other agricultural inputs is easier and cost-effective to use.
This study demonstrated that mating disruption with CheckMate DBM-F will significantly enhance the current IPM practices by reducing pest populations, contributing to insecticide resistance management, and reducing pest management costs. Additional studies, with fewer pesticide applications that allow larvae to survive and cause some damage, might further help understand the role of mating disruption where pest populations are not managed as effectively as in this field.
Acknowledgments: Thanks to the PCA and the grower for their research collaboration, Tamas Zold for his technical assistance in data collection, Ingrid Schumann for market research of pesticide pricing, and Suterra for the financial support.
Dara, S. K. 2019. The new integrated pest management paradigm for the modern age. J. Int. Pest Manag. 10: 12.
Dara, S. K. 2020. Arthropod resistance to biopesticides. Organic Farmer 3 (4): 16-19.
Endersby, N. M., K. Viduka, S. W. Baxter, J. Saw, D. G. Heckel, and S. W. McKechnie. 2011. Widespread pyrethroid resistance in Australian diamondback moth, Plutella xylostella (L.), is related to multiple mutations in the para sodium channel gene. Bull. Entomol. Res. 101: 393.
Ferré, J., M. D., Real, J. Van Rie, S. Jansens, and M. Peferoen. 1991. Resistance to the Bacillus thuringiensis bioinsecticide in a field population of Plutella xylostella is due to a change in a midgut membrane receptor. Proc. Nat. Acad. Sci. 88: 5119-5123.
Leibee, G. L. and K. E. Savage. 1992. Evaluation of selected insecticides for control of diamondback moth and cabbage looper in cabbage in Central Florida with observations on insecticide resistance in the diamondback moth. Fla. Entomol. 75: 585-591.
Pu, X., Y. Yang, S. Wu, and Y. Wu. 2009. Characterisation of abamectin resistance in a field-evolved multiresistant population of Plutella xylostella. Pest Manag. Sci. 66: 371-378.
Zhao, J-Z., H. L. Collins, Y-X. Li, R.F.L. Mau, G. D. Thompson, M. Hertlein, J. T. Andaloro, R. Boykin, and A. M. Shelton. 2006. Monitoring of diamondback moth (Lepidoptera: Plutellidae) resistance to spinosad, indoxacarb, and emamectin benzoate. J. Econ. Entomol. 99: 176-181.
- Author: Surendra K. Dara
Strawberry, a high-value specialty crop in California, suffers from several soilborne, fruit, and foliar diseases. Verticillium wilt caused by Verticillium dahliae, Fusarium wilt caused by Fusarium oxysporum f. sp. fragariae, and Macrophomina crown rot or charcoal rot caused by Macrophomina phaseolina are major soilborne diseases that cause significant losses without proper control. Chemical fumigation, crop rotation with broccoli, nutrient and irrigation management to minimize plant stress, and non-chemical soil disinfestation are usual control strategies for these diseases. Botrytis fruit rot or gray mold caused by Botrytis cineaea is a common fruit disease requiring frequent fungicidal applications. Propagules of gray mold fungus survive in the soil and infect flowers and fruits. A study was conducted to evaluate the impact of drip application of various fungicides on improving strawberry health and enhancing fruit yields.
This study was conducted in an experimental strawberry field at the Shafter Research Station during 2019-2020. Cultivar San Andreas was planted on 28 October 2019. No pre-plant fertilizer application was made in this non-fumigated field which had Fusarium wilt, Macrophomina crown rot, and Botrytis fruit rot in the previous year's strawberry planting. Each treatment was applied to a 300' long bed with single drip tape in the center and two rows of strawberry plants. Sprinkler irrigation was provided immediately after planting along with drip irrigation, which was provided one or more times weekly as needed for the rest of the experimental period. Each bed was divided into six 30' long plots, representing replications, with an 18' buffer in between. Between 6 November 2019 and 9 May 2020, 1.88 qt of 20-10-0 (a combination of 32-0-0 urea ammonium nitrate and 10-34-0 ammonium phosphate) and 1.32 qt of potassium thiosulfate was applied 20 times at weekly intervals through fertigation. Treatments were applied either as a transplant dip or through the drip system using a Dosatron fertilizer injector (model D14MZ2). The following treatments were evaluated in this study:
i) Untreated control: Neither transplants nor the planted crop was treated with any fungicides.
ii) Abound transplant dip: Transplants were dipped in 7 fl oz of Abound (azoxystrobin) fungicide in 100 gal of water for 4 min immediately prior to planting. Transplant dip in a fungicide is practiced by several growers to protect the crop from fungal diseases.
iii) Rhyme: Applied Rhyme (flutriafol) at 7 fl oz/ac immediately after and 30, 60, and 90 days after planting through the drip system.
iv) Velum Prime with Switch: Applied Velum Prime (fluopyram) at 6.5 fl oz/ac 14 and 28 days after planting followed by Switch 62.5 WG (cyprodinil + fludioxinil) at 14 oz/ac 42 days after planting through the drip system.
v) Rhyme with Switch: Four applications of Rhyme at 7 fl oz/ac were made 14, 28, 56, and 70 days after planting with a single application of Switch 62.5 WG 42 days after planting through the drip system.
Parameters observed during the study included leaf chlorophyll and leaf nitrogen (with chlorophyll meter) in February and May; fruit sugar (with refractometer) in May; fruit firmness (with penetrometer) in April and May; severity of gray mold (caused by Botrytis cinereae) twice in March and once in May, and other fruit diseases (mucor fruit rot caused by Mucor spp. and Rhizopus fruit rot caused by Rhizopus spp.) once in May 3 and 5 days after harvest (on a scale of 0 to 4 where 0=no infection; 1=1-25%, 2=26-50%, 3=51-75% and 4=76-100% fungal growth); and fruit yield per plant from 11 weekly harvests between 11 March and 14 May 2020. Leaf chlorophyll and nitrogen data for the Abound dip treatment were not collected in February. Data were analyzed using analysis of variance in Statistix software and significant means were separated using the Least Significant Difference test.
Results and Discussion
Leaf chlorophyll content was significantly higher in plants that received drip application of fungicides compared to untreated plants in February while leaf nitrogen content was significantly higher in the same treatments during the May observation. There were no differences in fruit sugar or average fruit firmness among the treatments.
The average gray mold severity from three harvest dates was low and did not statistically differ among the treatments. However, the severity of other diseases was significantly different among various treatments with the lowest rating in Abound transplant dip on both 3 and 5 days after harvest and only 3 days after harvest in plants that received four applications of Rhyme. Unlike the previous year, visible symptoms of the soilborne diseases were not seen during the study period to evaluate the impact of the treatments. However, there were significant differences among treatments for the marketable fruit yield. The highest marketable yield was observed in the treatment that received Rhyme and Switch followed by Velum Prime and Switch and Rhyme alone. The lowest fruit yield was observed in Abound dip treatment. Unmarketable fruit (deformed or diseased) yield was similar among the treatments. Compared to the untreated control, Abound dip resulted in 16% less marketable yield and such a negative impact from transplant dip in fungicides has been seen in other studies (Dara and Peck, 2017 and 2018; Dara, 2020). Marketable fruit yield was 4-28% higher where fungicides were applied to the soil.
Although visible symptoms of soilborne diseases were absent during the study, periodic drip application of the fungicides probably suppressed the fungal inocula and associated stress and might have contributed to increased yields. The direct impact of fungicide treatments on soilborne pathogens was, however, not clear in this study due to the lack of disease symptoms. Considering the cost of chemical fumigation or soil disinfestation and the environmental impact of chemical fumigation, treating the soil with fungicides can be an economical option if they are effective. While this study presents some preliminary data, additional studies in non-fumigated fields in the presence of pathogens are necessary to consider soil fungicide treatment as a control option.
Acknowledgments: Thanks to FMC for funding this study and Marjan Heidarian Dehkordi and Tamas Zold for their technical assistance.
Dara, S. K. 2020. Improving strawberry yields with biostimulants and nutrient supplements: a 2019-2020 study. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=43631
Dara, S. K. and D. Peck. 2017. Evaluating beneficial microbe-based products for their impact on strawberry plant growth, health, and fruit yield. UCANR eJournal of Entomology and Biologicals. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25122
Dara, S. K. and D. Peck. 2018. Evaluation of additive, soil amendment, and biostimulant products in Santa Maria strawberry. CAPCA Adviser, 21 (5): 44-50.