Investigating New Management Approaches for Picture-Winged Flies in Sweet Corn

Investigating New Management Approaches for Picture-Winged Flies in Sweet Corn

Summary

Sweet corn IPM trials were conducted comparing pyrethroid rotations with the alternative insecticide spinetoram mixed with a protein bait for ear protection and crop destruction methods for reducing larval survivorship. Pyrethroid treated plots reduced silk fly populations and resulted in low levels of silk fly ear damage, but did not protect against fall armyworm. Spinetoram provided fall armyworm protection and intermediate silk fly protection as compared with control plots. Mowing and shallow residue incorporation did not affect first generation fly emergence from soil as compared with standing plots after harvest.  Additional application and destruction methods will be investigated in 2016.

Objectives/Performance Targets

1. Determine if silk flies demonstrate reduced pyrethroid susceptibility in field settings
2. Determine how insecticide application methods can be improved for increasing canopy penetration
3. Determine if the reduced risk insecticide spinetoram can be applied in a bait to provide adequate fly control
4. Determine most efficient and timely means of crop residue removal to reduce silk fly production

Accomplishments/Milestones

The fall trial consisted of three 20-row blocks of ‘Obsession’ sweet corn (Seminis Vegetable Seeds, St. Louis, MO) planted on 16 September 2015 using a John Deere Max Merge 4-row vacuum planter (Deere & Co., Moline, Illinois). Seed was planted 8 inches apart in rows on 30 inch centers, resulting in a plant population of approximately 24,000 plants per acre. Plots measuring 20 rows by 150 feet were established in the blocks. Plots were separated by 10 ft alleys and a 15 ft unplanted alley separated the blocks. Fertilization, disease, and weed management practices were in accordance with UF IFAS extension recommendations.

Objective 1. Before tassel-push, four strips of plastic sheeting (Clear Poly Sheeting – 4 mil, Uline, Pleasant Prairie, WI), each measuring 33 inches by 100 ft., were laid on the ground between sweet corn rows in one of the blocks that was to be treated for silk flies using a pyrethroid rotation. The plastic sheeting was laid down in an attempt to collect silk flies that might be affected by an insecticide application and had fallen off of the plants. The sheeting completely covered the soil in the furrow between two adjacent rows of corn and was held in place by metal stakes. One furrow separated each of the four plastic sheets. Plants adjacent to the plastic sheeting were observed for healthy silk flies and morbid flies.  Because the four sheets covered a combined 400 row-feet, and the plants adjacent to the sheets were examined, a total of 800 row-feet were examined for silk flies after each pyrethroid application.

Pyrethroids were applied at their maximum label rates for silk flies by a Hagie high clearance sprayer with a 40-ft boom fitted with TeeJet 8003 nozzles and calibrated to deliver 30 gal/acre at 20 psi. Ten applications were made between first-silk (October 26) and harvest (November 19). The following pyrethroids were rotated between treatements: Baythroid XL (2.8 fluid oz./acre; Bayer, Research Triangle Park, NC), Warrior II (1.92 fluid oz./acre; Syngenta, Greensboro, NC) and Mustang (4.3 fluid oz./acre; FMC Corporation, Philadelphia, PA).

At the beginning of the trial, the dominant species observed was C. massyla (90%). Midway through the trial, the species complex consisted of 50% C. massyla and 50% E. stigmatias, and by the end of the trial, the dominant species in the complex was E. stigmatias. Silk fly populations in the untreated check plots were as great as 1 fly per row-ft.

A sum total of 456 healthy silk flies were observed on the plants four hours following 8 pyrethroid applications (Fig. 1). Early in the trial, the steepest population reductions followed the first Baythroid and Mustang applications. More flies were observed on plants following the two later Baythroid aplpications (during which time more E. stigmatias were entering the field) than the treatment before. However, it is unclear if the population fluctuations were due to physiological effects of insecticide application or from silk fly movement in and out of the treated block.

A total of 67 dead or twitching flies were removed from plants and plastic sheets after pyrethroid treatments (red bars in Fig 1). Twenty four of these flies were removed from the field after Baythroid applications, 25 flies were removed after Warrior applications, and 18 flies were removed after Mustang applications. Of these flies, only a single C. massyla and a single E. stigmatias fully recovered after Mustang and Warrior, respectively.

This was purely an observational study and without replication. Methodology will be improved in the spring so that statistical comparisons will be possible. Due to the low number affected silk flies retrieved from the plastic sheets and the adjacent plants after insecticide application, spring observations will focus on healthy flies. Populations will be assessed before and after an application, and populations will be monitored in both the pyrethroid plots and the untreated check plots. 

Objective 2. Preliminary experiments with spray cards were conducted in the fall 2015 sweet corn crop. One week after silking, eight of the plots were sprayed with two different water volumes (20 or 40 GPA). The spreader Kinetic^® (Helena Chemical Company, Collierville, TN) was mixed with water and applied to four of the plots. Plots were arranged in a split plot design with the presence or absence of the spreader as the main plot factor and the two water rates as subplot factors. Each water rate was replicated twice in the main plots. In each plot, 4 yellow spray cards (Water Sensitive Paper, Gempler’s, Janesville, WI) were attached to individual sweet corn tassels via paperclips, 4 were secured to the stem two leaves below the tassel on the same plant, and 4 were secured to the sides of the plant’s primary ears.  

Cards exposed to the 20 GPA water rate received less overall spray coverage (13.5 ± 3.0 %) than the 40 GPA (30.1 ± 4.9 %) rate. Spray coverage was significantly affected by location of the spray card on the plant (ANOVA: F = 35.74, df = 2, 83, P <0.001). Cards on the ears received the least coverage (3.0 ± 1.2 %), stem-located cards recieved intermediate coverage (18.5 ± 4.0 %) and tassel-located recieved the greatest mean coverage (42.2 ± 5.9).

Card placement in relation to wind direction and spray direction was observed to have a strong impact on spray coverage, but these effects were not examined in this trial. Spray coverage for tassel-located cards was heavily dependent on orientation of the card with the direction of the sprayer, wind direction, and vertical orientation (how much the card was tilted). Very little coverage was observed on most ear-located cards.

In the spring planting, the number of plots per treatment will be increased to allow for more powerful statistical analysis. The adjuvant treatment will not be included. Spray cards will encompass the plant tissues entirely to correct for wind-direction and vertical orientation effects on coverage.

 Objective 3. Spinetoram (Radiant, Dow AgroSciences, Indianapolis, IN) was applied to three treatment plots at a rate of 6 oz per acre and tank mixed with the protein bait Nu-Lure at a rate 48 oz per acre (Miller, Hanover PA). Nulure+Radiant was applied at the same time the pyrethroid applications were applied. Plots were harvested on 19 November. Each of the 9 plots were divided into three subsamples from which 30 ears were sampled for a total of 90 ears per plot. The plot subsamples were taken from the front, middle, and back end of each plot. Ears were husked and evaluated for silk fly and armyworm damage. Silk fly damage was rated on a 0-5 scale where 0 = clean ear, 1 = maggots in the silks only, 2 = tip damage only, 3 = maggots in the top ¼, 4 = maggots in the top ½, and 5 = maggots in the bottom ½ of the ear. Maggot counts were rated on a 0-3 scale, where a 0 = no maggots, 1 = <5 maggots, 2 = 5-20 maggots, and 3 = >20 maggots present in the ear. Fall armyworm damaged ears were also tallied.

The third replicate had lower damage fly damage ratings (F_{2, 22} = 9.42, P = 0.0011), and maggot load (F_{2, 22} = 10.60, P = 0.0006). The number of FAW damaged ears was not affected by replicate (F_{2, 22} = 2.56, P = 0.1004). Subsample location did not significantly affect fly damage rating (F_{2, 22} = 0.503, P = 0.6112), maggot load (F_{2, 22} = 0.5663, P = 0.5757), or FAW hits (F_{2, 22} = 0.4220, P = 0.6609). Therefore, results presented and analyzed by replicate (Table 1).

The silk fly damage ratings between Nulure+Radiant treatments and pyrethroid treatments did not significantly differ from each other (P <0.05), although the pyrethroid treatments did result in numerically lower damage ratings. The Nulure+Radiant treated ears had intermediate maggot loads compared to the pyrethroid and untreated check plots. Nulure+Radiant plots had the fewest number of ears damaged by fall armyworm, and an intermediate number of silk fly damage-free ears overall. Check plots had high silk fly damage ratings, maggot counts, and armyworm damaged ears and very few silk fly damage-free ears (Table 1).

The spring planting was planted 18 February 2016 and will consist of four treatments: Untreated check, Pyrethroid, Nulure+Radiant, and Radiant-only to test how the addition of Nu-lure affects the efficacy of the Radiant treatment.

Objective 4. The untreated check plots contained the greatest larval silk fly population and were mowed, disked once (disc1x), disked twice (disc2x), or left standing immediately after the harvest samples were taken from the field. An emergence cage covering 6.25 ft² was erected over the soil to intercept new flies as they exited their pupae in the soil. Silk flies were removed and tallied from each cage three times a week beginning 10 days after harvest and continuing until no more silk flies emerged from the soil. Treatments were replicated three times.

A total of 2,582 silk flies were removed from the 12 cages (Fig. 2). There were no significant differences between the residue destruction treatments and the standing corn treatment on fly emergence from the cages (ANOVA; F = 0.37, df = 3, 176, P = 0.77). Extrapolating the number of silk flies captured from the emergence cages by the total area that the cages covered to a full acre would mean that one acre of infested sweet corn could potentially produce 1.5 million silk flies.

Due to apparent inefficacy of mowing or shallow incorporation, the mow and disc1x treatment will not be repeated in the spring 2016 trial. Deeper incorporation will be examined by ploughing crop residue and comparing ploughing with the disc2x treatment. Treatments will be applied to both the check plots and the pyrethroid plots. Treatment replication will be increased from 3 to 6 replicates per treatment.

Table 1. Mean (± SEM) insect damage per replicate (n=3) to sweet corn ears grown at EREC Belle Glade, Florida and harvested Nov 19-22, 2015

Means within a column followed by the same letter are not significantly different (Tukey’s HSD, P > 0.05).

Figure 1. Total number of live healthy flies (blue line) and insecticide-affected flies (red bars) observed from 800 row feet of sweet corn four hours after each insecticide application.

Figure 2. Mean silk fly emergence from soil underneath cages erected over the control plots that were left standing, mowed, disked once or disked twice.

Impacts and Contributions/Outcomes

A grower meeting was held on February 15 to present results from this trial to 47 attendees, including local growers and crop consultants. Our data show that spinetoram used for armyworm management might have some added silk fly efficacy if mixed with a feeding stimulant. This would allow additional insecticide mode of actions to target silk flies, thus reducing pyrethroid resistance selection pressure. This can improve long term management of silk flies in sweet corn in Florida.

It is extremely important to demonstrate insecticidal management efficacy on research plots, which this project was able to achieve in the 2015 research trial. The methods used in this trial will help guide further field insecticide management research trials for silk fly control. The 1.6-3.2 point difference on the silk fly damage rating scale among the treatments is the best treatment separation achieved by any insecticide trial for these flies to date.  

Crop residue destruction can be an important means of silk fly control if done soon after harvest and before silk flies can infest unprotected ears. Research to determine the extent of post-harvest ear infestation and the effect soil incorporation has on larval survivorship is ongoing. Data indicate that shallow incorporation does not reduce silk fly larval survivorship. Therefore, crop residue should be destroyed as soon as possible to limit the number of eggs silk flies are able to deposit in unprotected secondary ears after harvest. We anticipate that as a result of this work, grower awareness of the importance of timely crop residue removal will result in greater vigilance and timeliness of crop destruction.

Preliminary spray card data indicate that a small proportion of a spray is reaching the ear, and research is ongoing to determine if ear coverage can be increased when insecticides are applied by ground. Modifying application procedures to increase canopy penetration could improve silk fly management and potentially reduce the number of applications necessary for adequate fly control. Finally, insecticide efficacy monitoring of pyrethroid treated plots will allow for a baseline efficacy in the field.

Collaborators: