A One-Year Study on the Effectiveness of Oxamyl

(Vydate L^®) to Control Thrips in Onions when Injected into a

 Drip-Irrigation System

 

 

Lynn Jensen

Malheur County Extension Office

Eric Feibert, Clint Shock, and Lamont Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR, 2004

 

 

Introduction

 

Onion thrips and western flower thrips are the main insect pests on onions grown in the Treasure Valley of Idaho and eastern Oregon. In this region about 3,000 acres of onions are grown under drip irrigation. Because of the increased yield and quality of onions grown under drip irrigation, this management practice is increasing on lands that were formerly marginal for onion production. It is a common practice to inject the systemic insecticide oxamyl (Vydate L^®) into the drip lines on a weekly or biweekly basis to control thrips. Most growers also apply two to six foliar insecticide applications in addition to the oxamyl applications. Growers using conventional furrow irrigation commonly use four to six foliar insecticide applications for thrips control. The drip irrigation growers feel there is an economic advantage to the additional oxamyl applications even though the additional cost is about $150/acre. This trial was designed to determine the effectiveness of oxamyl at two different application rates and in combination with two foliar insecticide programs.

 

 Materials and Methods

 

The trial was conducted at the Malheur Experiment Station on an Owyhee silt loam soil previously planted to wheat. Onion (cv: ‘Vaquero’; Nunhems, Parma, ID) was planted on March 23 in 2 double rows on a 44-inch bed. The double rows were spaced 2 inches apart. The seeding rate was 150,000 seeds/acre. Lorsban 15G^® was applied in a 6-inch band over each double row at a rate of 3.7oz/1,000 ft of row for maggot control. The drip tape was placed in the center of the bed between the double rows. The drip tape (T-tape, T-Systems International, Inc., San Diego, CA) had a flow rate of 0.22 gal/min/100 ft of tape. Irrigation water was applied when the soil water potential reached –20 kPA. Water potential was determined by granular matrix sensors (GMS, Watermark Soil Moisture Sensors Model 200ss, Irrometer Co. Inc., Riverside, CA) installed at 8-inch depth in the center of the double row.

 

The experimental design was a randomized complete block design with four replications. The plot size was 8 double rows wide (37.5 ft) by 34 ft in length.

Oxamyl was injected into the main irrigation line by a positive displacement injector (Dosmatic Model A30, Dosmatic USA, Inc., Carollton, TX). Prior to injecting oxamyl, 95 percent sulfuric acid was diluted at a ratio of 1:6,248 acid to water to buffer the water in the soil solution to a pH of 5.0. The oxamyl was added to water buffered at the same ratio and injected immediately after the initial buffer treatment. The buffered water and buffered oxamyl treatments required 20 minutes each to inject into the treated plots. This process applied slightly more water to the treated plots compared to the untreated, but the additional water was minor compared to the overall applied water and probably did not have an overall impact on the final yield.

 

Each plot had four drip tapes supplying water to the eight double rows. Each plot was equipped with an on/off valve so that oxamyl could be applied to individual plots as needed. There were 6 treatments including an untreated check, a standard insecticide program, oxamyl at 1.0 qt/acre applied weekly, oxamyl at 2.0 qt/acre applied every other week, oxamyl at 1.0 qt/acre plus a standard insecticide program and oxamyl at 1.0 qt/acre plus the bio-insecticides azadirachtin (Aza-Direct^®) and spinosad (Success^®) (alternative program). Azadirachtin and spinosad have shown promise under conventional systems by suppressing thrips and allowing predatory insect populations to build to the point where they control thrips. Systemically applied through the drip system, oxamyl has the potential to enhance this program.  The application dates of the treatments are shown in Tables 1 and 2.

 

Thrips counts were made weekly by counting the total number of thrips on 15 plants in each plot. Onions were harvested on September 9 and10 and graded on October 5. A visual evaluation for iris yellow spot virus was taken on August 19.

 

Treatment differences were compared using ANOVA and least significant differences at the 5 percent probability level, LSD (0.05). 

 

Results and Discussion

 

Figure 1 shows the weekly thrips populations found in the different treatments throughout the growing season. There was a tendency for the oxamyl plus alternative treatments to have lower thrips pressure than the other treatments. The season average thrips populations are shown in Table 3. The oxamyl at 1.0 qt every week plus the alternative bio-insecticides had significantly lower total thrips populations than the other treatments. There were no significant differences in thrips populations between the other treatments, including the untreated check.

 

Table 4 shows the breakdown in yield and quality between the different treatments. There was a significant increase in colossal-sized bulbs with the three foliar-applied insecticide treatments versus the untreated check or the oxamyl alone treatments.

 

Iris yellow spot virus (IYSV), which is thrips transmitted, appeared in the trial during August. A visual evaluation of the onions for IYSV showed significantly less infection in the oxamyl plus azadirachtin plus spinosad treatment compared to the oxamyl alone treatments or the untreated check (Table 5).

 

Conclusion

 

The oxamyl plus alternative insecticides (azadirachtin plus spinosad) treatment significantly controlled thrips better than any other treatment and had the highest yield of colossal, super-colossal, and total yield. All of the treatments with foliar insecticides gave significantly higher colossal yields compared to the oxamyl only and the untreated check. Oxamyl treatments applied as 1.0 qt/acre weekly or 2.0 qt/acre every other week were no better than the untreated check. The lack of thrips control by oxamyl may be due to the late initial application on June 3. This application was about 2 weeks later than growers would typically start. There was also the possibility that the oxamyl was not applied with enough irrigation water to allow movement to the onion roots during the early onion growth period when the root zone was small.

 

 

 

 

Table 1. Application dates for the different treatments in the drip-irrigation/oxamyl trial, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

Date	Oxamyl 1.0 qt/wk	Oxamyl 2.0 qt every other week	Standard insecticide	Alternative insecticide
6/03	X	X
6/04			X	X
6/11	X	X
6/16	X		X	X
6/23			X	X
6/25	X
7/02	X	X	X	X
7/08	X		X	X
7/19			X
7/20	X	X
7/29			X	X
8/06	X

 

 

 

 

 

 

 

 

 

Table 2.  Application dates for foliar insecticide applications for thrips control on drip-irrigated onions, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

	Product	Rate/acre	Product	Rate/acre
June 6	Warrior MSR	3.84 oz. 2.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
June 16	Warrior MSR	3.84 oz. 2.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
June 23	Warrior Lannate	3.84 oz. 3.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
July 1	Warrior Lannate	3.84 oz. 3.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
July 8	Warrior MSR	3.84 oz. 2.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
July 19	Warrior Lannate	3.84 oz. 3.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.
July 29	Warrior Mustang Lannate	3.84 oz. 4.0   oz. 3.0   pt.	Aza-Direct Success	20.0 oz. 10.0 oz.

 

Table 3. Average thrips counts for the 2004 season, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

Treatment	Average thrips/plant
Untreated	47.9
oxamyl 2.0 qt - every other week	51.8
oxamyl 1.0 qt - every week	50.7
oxamyl 1.0 qt + alternative	36.2
oxamyl 1.0 qt + standard	49.6
Standard treatment	50.1
LSD (0.05)	9.6

 

Table 4. Total yield of oxamyl-treated onions grown under drip irrigation, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

	Onion Yield
Treatment	Medium	Jumbo	Colossal	Super-colossal	Total yield
	----------------------------------cwt/acre-------------------------------
Untreated	26.4	676.3	198.3	11.9	912.9
oxamyl 2.0 qt (every other week)	30.4	642.3	210.8	22.8	906.3
oxamyl 1.0 qt (every Week)	22.8	708.1	193.5	13.3	937.7
oxamyl 1.0 + Alternative	19.6	630.5	326.4	46.1	1022.6
oxamyl 1.0 + Standard	17.4	633.7	307.4	34.7	993.2
Standard only	21.3	655.6	310.9	28.0	1015.8
LSD (0.05)	ns	ns	91.3	ns	ns

Table 5. Iris yellow spot virus (IYSV) evaluation in oxamyl-treated onions grown under drip irrigation, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

Treatment	IYSV rating 1 = no virus, 5 = severe virus
Untreated	3
oxamyl 2.0 qt (every other week)	3
oxamyl 1.0 qt (every Week)	3.3
oxamyl 1.0 + Alternative	1.8
oxamyl 1.0 + Standard	2.5
Standard only	2.5
LSD (0.05)	0.9

 

 

 

 

 

 

Figure 1. Weekly thrips populations, 2004 oxamyl/drip trial, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.