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2008 Plant Management Network.
Accepted for publication 10 November 2007. Published 8 February 2008.


Use of Neonicotinoid Insecticides Applied to Soil and Seed for Green Peach Aphid Management on Spinach


Paul McLeod, Professor, Department of Entomology, University of Arkansas, Fayetteville 72701; and Steven Eaton, Program Associate, and Larry Martin, Program Associate, University of Arkansas Vegetable Substation, Alma 72921


Corresponding author: Paul McLeod. pjmcleod@uark.edu


McLeod, P., Eaton, S., and Martin, L. 2008. Use of neonicotinoid insecticides applied to soil and seed for green peach aphid management on spinach. Online. Plant Health Progress doi:10.1094/PHP-2008-0208-02-RS.


Abstract

The green peach aphid, Myzus persicae Sulzer, a common pest of spinach produced in the Arkansas River Valley of western Arkansas and eastern Oklahoma, is currently managed with foliar applications of neonicotinoid insecticides. Data reported herein indicate that the neonicotinoid insecticdes, imidacloprid and thiamethoxam, can also be applied to spinach seed and into the soil for effective aphid management. In greenhouse studies both imidacloprid (Gaucho) and thiamethoxam (Cruiser) seed treatments were effective against adults and prevented the establishment of juvenile aphids for a period of 12 weeks after planting. Similar results were obtained in field studies. Regardless of rate, each of the neonicotinoid seed treatments significantly reduced the number of aphids when compared to aphid population levels on non-treated spinach. In addition to the seed treatments, in-furrow applications also resulted in significantly lower aphid numbers than on plants in plots not receiving the applications. Tests also demonstrated that imidacloprid (Admire) effectively managed aphid populations after being injected into the soil. Of the seed and soil applications evaluated, the only application method currently available is use of imidacloprid applied into the soil. This method currently offers the spinach producer an alternative management tool for green peach aphid control on spinach. As the labels of imidacloprid and other neonicotinoid insecticides are expanded to include additional use patterns such as seed treatments, effective alternatives to foliar sprays should prove effective for aphid management on spinach.


Introduction

The green peach aphid, Myzus persicae Sulzer, is generally the most common insect pest of spinach produced in the Arkansas River Valley of western Arkansas and eastern Oklahoma (3). Adult aphids seek newly planted fall spinach in late summer and begin to reproduce. As weather cools, the impact of beneficial insects and diseases decreases and by harvest in November, aphid populations can be sufficiently high to result in rejection of the field by the processor (1). Although aphids generally have little direct impact on spinach production in the Arkansas River Valley, they are considered a contaminate in food used for processing. A similar situation occurs with overwintered spinach, i.e., spinach planted in mid- to late fall and harvested in late winter or early spring. On overwintered spinach aphid numbers remain very low until a few weeks prior to harvest. As temperatures rise in late winter, aphid populations can again increase past the threshold level established by the processors.

Aphid management on spinach has traditionally been based on multiple applications of insecticides to foliage with aircraft and ground vehicles (2,7). In recent years the only effective insecticide against aphids and available to the spinach producer has been the neonicotinoid insecticides, primarily imidacloprid. The neonicotinoids have proven effective because they are readily absorbed into the plant, translocate within the plant and affect aphids located on the leaf bottoms (4,8,9). In spinach produced in the Arkansas River Valley, imidacloprid is often applied two or three times during the production season. Although imidacloprid is generally the most effective insecticide available, aphid management on spinach has not always been satisfactory following the foliar sprays. In other crops, especially field corn, neonicotinoids also are highly effective when applied into the soil at planting. Methods of application include seed treatments, in-furrow and side dress (5,6). Although much information is available for soil use of neonicotinoids in agronomic crops like field corn, none is available for soil use in spinach. The objective of the study reported herein was to establish the effectiveness of available neonicotinoid insecticides against the green peach aphid on spinach when applied on seed or into the soil.


Greenhouse Seed Treatments

Spinach AR380' seed treated with insecticides were planted in the Vegetable Greenhouse at the Main Experimental Station, University of Arkansas, Fayetteville. Treatments included thiamethoxam 1X and 2.5X (Cruiser 5S; 0.1 and 0.25 mg ai/seed), imidacloprid 1X and 2.5X (Gaucho 600; 0.24 and 0.59 mg ai/seed), and non-treated seed. Seeds (50 g) and the insecticide were placed into the treatment drum of a Gustafson BLT seed applicator (Gustafson Inc., Dallas, TX) and rotated and vibrated. After 10 min, seeds were removed and air dried for 24 h on paper toweling. Seeds were planted 30 October 2006 in 15-cm-diameter plastic pots that contained Miracle-Gro Potting Mix (Scotts Miracle-Glo Products Inc., Marysville, OH). Plants were watered daily and fertilized weekly with Miracle-Gro 24-8-16 fertilizer. Temperature was maintained at 75  5F under natural light. Aphids used in the study were from a laboratory culture maintained on icicle raddish, Raphanus sativus L. Ten days after planting, 5 adult green peach aphids were transferred to the first true leaves of each plant. Potted plants were enclosed in a 20  25-cm bag made from Reemay spun-bound polyester row cover. The number of adult and immature aphids were counted and removed 4 days after infestation. The procedure was repeated at weekly intervals until the termination of the study. Each treatment was replicated 20 times. Data were analyzed with GLM and when F values were significant means were separated with LSD (SAS 2000, SAS Institute Inc., Cary, NC).

Two weeks after planting, the number of adult aphids per plant was significantly reduced for each seed treatment when compared to aphid survival on non-treated plants (F < 9.34, P < 0.0001). Aphid numbers remained low on plants produced from treated seed and similar results were noted throughout the 12 week study (Fig. 1). No significant differences were detected among the seed treatments for any sample date. At the initial sample period the mean number of immature aphids produced by the adults on non-treated plants was 7.62 per plant (Fig. 2). No immatures were detected on any plants produced from treated seed (F < 28.16, P < 0.0001). No differences between the two seed treatments or between insecticide rates were detected. Similar findings were obtained with immature production throughout the study (Fig. 2).


 

Fig. 1. Effects of seed treatments against adult green peach aphids on greenhouse produced spinach.

 

 

Fig. 2. Effects of seed treatments against juvenile green peach aphid production on greenhouse produced spinach.

 

Field Trial with Treated Spinach Seed

Treated spinach seed were evaluated for green peach aphid control at the University of Arkansas Vegetable Station, Kibler, AR during the 2006 fall production season. Seed were hand planted on 18 September in plots consisting of a single 6-ft-wide, 25-ft-long bed. Five rows of spinach each separated by 1 ft were planted on each bed. Seed was placed every 2 inches within rows in each plot. Experimental design was RCB with four replications. Insecticide effectiveness was assessed on 9 November (52 days after planting) by counting the total number of aphids on each of 10 plants randomly selected from each plot. Data were analyzed with GLM and when F values were significant means were separated with LSD (SAS 2006).


Table 1. Effects of seed applied insecticides on green peach aphids on commercial spinach produced in the Arkansas River Valley, 2006.

Treatment Mean no. green peach aphids
per plant, 21November
Rormulation Rate (mg ai/seed)
Cruiser 5S 0.10 61.2 b   
Cruiser 5S 0.25 5.1 c
Gaucho 600 0.24 42.1 bc
Gaucho 600 0.59 14.4 bc
Check 195.9 a

 x Within column means followed by same letter do not differ significantly (LSD, P = 0.05)


Aphid numbers were moderate to high at the Kibler plots (Table 1). In non-treated plots the mean number of aphids per plant was 195.9. Each of the seed treatments significantly lowered the mean number of aphids per plant (F < 13.32, P < 0.0001). No significant difference was detected between the two Gaucho rates. The high rate of Cruiser significantly reduced aphid numbers when compared to the low Cruiser rate.


In-furrow Application

In addition to the seed treatments, Admire 2F was tested as an in-furrow application at planting. Plots were located at the University of Arkansas Main Experimental Station, Fayetteville, AR and consisted of a single foot-wide bed 35 ft long. Just prior to planting on 10 October 2006, insecticide treatments were applied within the planting furrow with a backpack CO2 sprayer equipped with a single TeeJet 8002E nozzle. The sprayer was calibrated to deliver 1018 ml/1000 row-ft at 15 psi (55 ml/15 sec) . Sprayer speed was 2.46 mph or 278 sec/1000 row-ft. Experimental design was RCB with four replications. Following the insecticide application AR380 spinach was hand seeded. Sampling consisted of counting the total number of aphids per plant on ten randomly selected plants per plot. Data analysis was as previously described.

On the initial sampling date, 26 March (167 days after planting), aphid numbers were high on non-treated plants, i.e., 513.8 aphids per plant (Table 2). At this time each of the in furrow applications had significantly reduced the number of aphids per plant (F < 16.12, P < 0.0001). However, no significant differences among the three Admire rates were detected. In the subsequent samples, aphid numbers began to decline on non-treated plants. With the exception of the lowest Admire rate on 2 April (174 days after planting), aphid numbers remained significantly lower for each of the in furrow applications than those on non-treated plants. At termination of the test, the mean number of aphids per plant was only 2.6 for the high Admire rate. This was about 1% of the number on non-treated plants.


Table 2. Effects of Admire insecticide applied as a pre-plant in-furrow treatment against green peach aphids on spinach, Fayetteville, AR, 2006.

Treatment Mean no. green peach aphids/plant
Formulation Rate (mg ai/seed) 26 March 2 April 9 April
Admire 2F 0.17   80.2 a 148.9 a 90.7 b
Admire 2F 0.25   37.9 a   23.6 b 22.9 c
Admire 2F 0.34   11.6 a     1.1 b  2.6 c
Check 513.8 b 244.5 a 208.9 a

 x Within column means followed by same letter do not differ significantly (LSD, P = 0.05)


Post-harvest Side Dress Application

In order to increase profit, spinach growers in the Arkansas River Valley have been interested in obtaining multiple cuttings of spinach, as has been the common practice in the spinach production area of south Texas. To facilitate this two studies were undertaken to explore the use of Admire applied as a post-harvest side dress. The initial study was undertaken in the Vegetable Insect Greenhouse at the University of Arkansas Main Experimental Station, Fayetteville, AR. The objective of the test was to establish the time required for Admire to move from soil into the plant and produce aphid mortality. On 19 February 2007 non-treated AR380' spinach seed were planted in 15-cm-diameter plastic pots that contained Miracle-Gro Potting Mix as described previously. Plants were watered daily and fertilized weekly and temperature was maintained at 75  5F under natural light. Aphids used in the study were from a laboratory culture maintained on icicle raddish, Raphanus sativus L. On 4 April, 10 plants having 10 true leaves were selected and approximately 20 adult GPA were transferred with a camel hair brush from the culture to each plant. After reproducing for two weeks, the total number of aphids was counted on each plant (0 hour). Admire 2F was diluted in water and 1 ml of the mixture was injected one inch below the soil surface of each pot with a 50-cc cattle injection syringe. The resulting rate was equivalent to the recommended field rate of 0.38 lb ai/acre. Surviving aphids were counted on each plant at 2, 4, 6, 8, 12, 18, 24, and 48 h post application. Two weeks later, the study was repeated with 10 additional plants. Data were pooled and plotted with SYSTAT software.

Aphid mortality as illustrated by reduction in aphid numbers was rapid (Fig. 3). By 8 h post application only 44.6% of the original aphid number survived. In subsequent samples, the decline in survival continued. At 24 h post application only 7.5% of the original number was present and by 48 h after the Admire injection, no live aphids were found on the plants.


 

Fig. 3. Green peach aphid survival on spinach following injection of Admire 2F into the root zone.

 

The second Admire injection study was located in a commercial spinach field located 7 miles northwest of Spiro, OK. The field had been planted with AR415' spinach on 2 October 2006 and was harvested during mid-January 2007. Spinach harvest consists of machine cutting as much of the above ground plant as possible and transfer to waiting trucks for shipment to the processor. This process leaves about two inches of the central stem and leaf petioles along with a minimal amount of foliage. Admire 2F was applied with a John Deere 1520 grain drill on 25 January one week following the completion of harvest. The drill measured 21-ft wide and was set to cover three 6-ft beds in a single pass. For each bed 10 spray nozzles were positioned behind the disk openers and in front of the press wheels. The seeder was calibrated to deliver 6 gpa. Admire 2F was diluted in water and injected approximately 1 inch below the soil surface. Treatments included Admire at 0.17, 0.25, and 0.34 lb ai/acre and a non-treated check. Plots consisted of three 100-ft-long beds. The study was replicated in three areas within the field. Green peach aphids were sampled on 23 February, 5, 12, and 19 March by randomly selecting 10 plants per plot. Selected plants were cut at soil level with a knife, placed in zip-lock bags and transferred to the laboratory for aphid counts. Data were analyzed as described above.

On the initial sample date, 23 February (29 days after treatment), aphid numbers on non-treated plants were moderate with a mean of 57.3 per plant (Table 3). Each of the Admire injections had produced significant reductions in aphid numbers per plant (F < 14.7, P < 0.0001). While aphid numbers continued to increase during the next two sample dates in the non-treated plots, numbers in plots receiving the Admire injections remained significantly lower [F < 17.60, P < 0.0001 on 5 March (39 days after treatment), and F < 88.18, P < 0.0001 on 12 March (46 days after treatment)]. Non-treated plants had about 200 times the number of aphids as the 0.34 lb ai/acre Admire rate on 12 March. Although non-treated aphid numbers decreased the following week, each of the injections continued to significantly reduce aphid numbers (F < 88.28, P < 0.0001).


Table 3. Effects of Admire insecticide applied as a post-harvest injected side dress on green peach aphids on commercial spinach produced in the Arkansas River Valley, 2007.

Treatment Mean no. green peach aphids per plant
Formulation Rate
(mg ai/seed)
23 Feb 5 March 12 March 19 March
Admire 2F 0.17 3.7 a 3.8 a 7.9 a 2.7 a
Admire 2F 0.25 13.7 a 2.8 a 10.3 a 2.3 a
Admire 2F 0.34 10.1 a 14.0 a 2.1 a 3.3 a
Check 57.3 b 89.2 b 403.2 b 235.2 b

 x Within column means followed by same letter do not differ significantly (LSD, P = 0.05)


Conclusions and Recommendations

Currently, aphid management on commercial spinach produced in the Arkansas River Valley relies on foliar sprays of insecticides, primarily imidacloprid. Although concern with unwarranted insecticide use always exists with preventative applications at planting, historically, green peach aphid populations are highly predictable in certain locations like the Arkansas River Valley in Arkansas and eastern Oklahoma. In these areas, it is likely that the preventative insecticide applications for aphid management on spinach are justified. Although not currently labeled as a spinach seed treatment, this method of application offers a useful alternative to the foliar sprays. Also, the neonicotinoid insecticide, imidacloprid is now labeled for "in furrow" and "side dress" application. Imidacloprid, applied in these manors, can provide the spinach producer with an effective alternative to the foliar sprays.


Literature Cited

1. McLeod, P. 1987. Effect of low temperature on Myzus persicae on overwintering spinach. Environ. Entomol. 16:796-801.

2. McLeod, P. 1987. Influence of temperature on contact and volatile toxicities of aphicides against the green peach aphid, Myzus persicae. J. Entomol. Sci. 22:362-366.

3. McLeod, P. 2006. Identification, biology and management of insects attacking vegetables in Arkansas. Sirena Press, Santa Cruz, Bolivia.

4. Nauen, R., Tietjen, K., Wagner, K., and Elbert, A. 1998. Efficacy of plant metabolites of imidacloprid against Myzus persicae and Aphis gossypii (Homoptera: Aphididae). Pestic. Sci. 52: 53-57.

5. Palumbo, J. C., and Kerns, D. L. 1994. Effects of imidacloprid as a soil treatment on colonization of green peach aphid and marketability of lettuce. Southwestern Entomologist. 19:339-346.

6. Pike, K. S., Reed, G. L., Graf, G. T., and Allison, D. 1993. Compatibility of imidacloprid with fungicides as a seed-treatment control of Russian wheat aphid and effect on germination, growth and yield of wheat and barley. J. Econ. Entomol. 86:586-593.

7. Sorensen, K. A., and Baker, J. R. 1994. Insect and related pests of vegeTables. Bulletin AG-295, North Carolina Coop. Ext. Serv., Raleigh, NC.

8. Thomson, W. T. 2000. Agricultural Chemicals - Book I: Insecticides, Acaracides and Ovicides. Thomson Pub., Fresno, CA. 249p.

9. Westwood, F., Bean, K. M., Dewar, A. M., Bromilow R. H., and Chamberlain, K. 1998. Movement and persistence of [14C] imidacloprid in sugar-beet plants following application to pelleted sugar-beet seed. Pestic. Sci. 52:97-103.