A Two-Year Study on Varietal Response to an Alternative Approach for Controlling Onion Thrips (Thrips tabaci) in Spanish Onions

 

 

Lynn Jensen

Malheur County Extension Service

Clinton Shock and Lamont Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR, 2003-2004

 

 

Introduction

 

Onion (Allium cepa L.) is a major economic crop in the Treasure Valley of eastern Oregon and western Idaho. Annually about 20,000 acres of onion are grown in the valley. Typically Spanish hybrids are grown for their large size, high yield, and mild flavor.

 

The principal onion pest in this region is onion thrips (Thrips tabaci, Lindeman). Thrips cause yield reduction by feeding on the epidermal cells of the plant. Onion thrips can reduce total yields from 4 to 27 percent, depending on the onion variety, but can reduce yields of colossal-sized bulbs from 27 to 73 percent. The larger sized colossal bulbs are difficult to grow and demand a premium in the marketplace. Growers typically spray three to six times per season to control onion thrips. Treatments include the use of synthetic pyrethroid, organophosphate, and carbamate insecticides. The ability of these products to control thrips has decreased from over 90 percent control in 1995 to less than 70 percent control in 2000. Onion growers are applying insecticides more frequently in order to keep thrips populations low.

 

New biological insecticides with low toxicity to beneficial predators have been developed, including neem tree (Azadirachta indica A. Juss.) extracts (azadirachtin) and bacterial fermentation products (spinosad). Both of these materials have previously been evaluated for thrips control and have performed poorly compared to conventional insecticides. Studies during the past 2 years have shown that applications of spinosad and azadirachtin coupled with straw mulch are superior to conventional insecticide programs for controlling onion thrips in ‘Vaquero’ onions. Vaquero was used in the study because of its vigorous growth characteristics and resistance to thrips injury compared to slower growing varieties. The objective of this study was to test this program on varieties that are highly susceptible to thrips injury.

 

Materials and Methods

 

A 1.5-acre field was planted to the onion varieties Vaquero, ‘Flamenco’, and ‘Redwing’ (cv. Vaquero, Flamenco, Nunhems, Parma, ID; Redwing, Bejo Seeds, Oceano, CA) in a split plot design on March 14, 2003 and March 23, 2004. Vaquero is a yellow variety while Redwing and Flamenco are red varieties. Red varieties are generally assumed to be more attractive to thrips than yellow varieties. The onion varieties were planted as 2 double rows on a 44-inch bed. The double rows were spaced 2 inches apart. The seeding rate was 137,000 seeds/acre. Lorsban 15G^® was applied in a 6-inch band over each row at planting at a rate of 3.7 oz/1000 ft of row for onion maggot control. Water was applied by furrow irrigation. The field was divided into plots 37 ft wide by 100 ft long. There were three treatments with six replications.

 

The three treatments were a grower standard treatment, an untreated check, and the alternative treatment as described previously (Jensen et al. 2003a, 2003b). The grower standard treatment included Warrior^® (lambda-cyhalothrin), MSR^® (oxydemeton-methyl), and Lannate^® (methomyl). The untreated check did not receive any treatments for thrips control. The alternative treatment included straw mulch applied to the center of the bed plus Success^® (spinosad), and Aza-Direct^® (azadirachtin).

 

Insecticide treatments were applied 7-10 days apart during the growing season (Table 1). All insecticides were sprayed in water at 31 gal/acre in 2003 and 39 gal/acre in 2004. Straw was applied only between the irrigation furrows on top of the beds to avoid confounding irrigation effects with thrips effects. The straw was applied on May 1, 2003 at a rate of 1,080 lb/acre. Straw was not applied in 2004 because results in 2003 suggested it was not enhancing thrips control.

 

Thrips populations were sampled by two methods. The first was by visually counting the number of thrips on 20 plants. The second method was by cutting 10 plants at ground level and inserting the plants into a berlese funnel. Turpentine used in the berlese funnel dislodged the thrips from the plant into a jar containing 90 percent isopropyl alcohol. The collected thrips were then counted through a binocular microscope. Thrips populations were monitored weekly through the growing season.

 

The predator populations were monitored using pitfall traps that contained ethylene glycol. They were evaluated three times per week. The berlese funnel was also used to monitor predators foraging on the plants. The onions were harvested in September and graded in October of each year.

 

Treatment differences were compared using ANOVA and least significant differences at the 5 percent probability level, LSD (0.05).  Means were also compared using Duncan’s multiple range test.

 

Results and Discussion

 

Weekly thrips populations are compared in Figure 1. The alternative program had a significantly lower average thrips population than the untreated check in both years (Fig. 2). Visual damage to the foliage was observed with the variety Vaquero in 2004 but not in 2003. Flamenco showed severe foliage damage from thrips feeding. The visual thrips damage to Redwing appeared intermediate between Vaquero and Flamenco. Flamenco is less vigorous than Redwing and more thrips damage would be expected.

 

There were no yield differences between any of the treatments with Vaquero  in 2003 but the alternative treatment produced significantly more colossal- and super-colossal-sized bulbs in 2004 (Table 2).

 

Redwing significantly increased yield of colossal-sized bulbs with the alternative treatment both years compared to both the standard and untreated check and significantly increased in total yield in 2003 compared to the untreated check (Table 3).

 

Flamenco responded to the alternative treatments with significantly less medium-size yield and higher jumbo and colossal yield compared to the untreated check in 2003 (Table 4). There was a trend towards higher total yield and larger bulb size compared to the standard treatment but this was only significant in the colossal size class in 2003. The alternative plus standard treatments produced higher total yields than the untreated control in 2004.

 

Predator populations (Fig. 3) were significantly higher in the alternative and untreated check treatments than in the standard treatment. The predator population consisted mostly of spiders, big-eyed bugs, minute pirate bugs, damsel bugs, lacewings and lady bird beetles.

 

The 2004 season experienced an epidemic of iris yellow spot virus (IYSV) in the trial area and surrounding fields. The IYSV is a new disease currently spreading to most production areas of the United States and the world. Onion thrips are the vector, so this trial gave the opportunity to evaluate the alternative program for IYSV control (Table 5). The treatments grown under the alternative treatment were healthier and showed significantly less virus damage than the standard insecticide treatment or the untreated check.

     

Red onions often exhibit thrips scarring when placed in storage due to continued feeding by the insects. The alternative treatment produced significantly fewer damaged bulbs compared to the untreated check with the Redwing variety, and a similar though not significant trend with Flamenco (Table 6). Averaged over treatment, Redwing had less thrips injury than Flamenco.

 

Conclusion

 

The alternative treatments were equal to or in some cases significantly better than the standard insecticide program. There was a general trend towards higher yields in the larger bulb classes, which gives a higher return to the grower. The alternative program produced less thrips damage to red onions in storage and reduced the incidence of iris yellow spot virus.

 

 

References

 

Jensen, L., B. Simko, C. Shock, and L. Saunders 2002. Alternative methods for controlling onion thrips (Thrips tabaci) in Spanish onions. Pages 65-72 in Proceedings of the 2002 Allium Research National Conference.

 

Jensen, L., B. Simko, C. Shock and L. Saunders. 2003. Alternative methods for controlling thrips. Pages 895-900 in British Crop Protection Council: Crop Science and Technology 2003 Congress Proceedings. Vol. 2.

 

Jensen, L., B. Simko, C. Shock, and L. Saunders 2003a. Alternative methods for controlling onion thrips (Thrips tabaci) in Spanish onions. Proceedings of the 2003 Idaho-Malheur County Onion Growers Annual Meeting Feb. 2003. 7 pages.

 

Jensen, L., C. Shock, B. Simko, and L. Saunders 2003b. Straw mulch and insecticide to control onion thrips (Thrips tabaci) in dry bulb onions. Pages 19-30 in Proceedings of the Pacific Northwest Vegetable Association 17^th Annual Meeting.

Table 1.  Application dates for thrips control on two red and one yellow onion variety,  Malheur Experiment Station, Oregon State University, Ontario, OR, 2003-2004.

 

		Standard insecticide treatment		Alternative insecticide treatment
Application date		Insecticides applied	Rate/acre	Insecticides applied	Rate/acre
2003
June 7	Warrior		3.84 oz.	Aza-Direct Success	20.0 oz. 10.0 oz.
June 14	--------		-------	Aza-Direct Success	20.0 oz. 10.0 oz.
June 25	Warrior Lannate		3.84 oz. 3.0 oz.	--------	--------
July 3	-------		-------	Aza-Direct Success	20.0 oz. 10.0 oz.
July 7	Warrior MSR		3.84 oz. 2.0   pt.	--------	-------
July 11	------		------	Aza-Direct Success	20.0 oz. 10.0 oz
July 25	Warrior Lannate		3.84 oz. 3.0   pt.	------	-------
July 29	--------		-------	Aza-Direct Success	20.0 oz 10.0 oz.
2004
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 2.  Yield and grade of Vaquero onion with different strategies for controlling onion thrips, Malheur Experiment Station, Oregon State University, Ontario, OR.

 

2003
Treatment		Medium	Jumbo	Colossal	Super- colossal		Total yield
		cwt/acre
Untreated check		9.7	459.7	464.1	124.0		1057.5
Standard		9.8	451.0	489.6	140.9		1091.3
Alternative		10.9	446.1	484.2	145.2		1086.4
LSD (0.05)		ns	ns	ns	ns		ns

 

 

2004
Treatment		Medium	Jumbo	Colossal	Super- colossal		Total yield
		cwt/acre
Untreated check		17.6	586.1	254.5	29.8		888.0
Standard		11.9	511.3	306.9	52.3		882.4
Alternative		14.8	409.3	377.4	126.9		928.4
LSD (0.05)		ns	ns	76.9	71.9		ns

 

 

Table 3.  Yield and grade of Redwing onion with different strategies for controlling onion
thrips, Malheur Experiment Station, Oregon State University, Ontario, OR.
2003
Treatment		Medium	Jumbo	Colossal	Super- colossal		Total yield
		cwt/acre
Untreated check		12.0	726.4	107.4	4.0		849.8
Standard		14.2	724.2	174.3	2.2		914.9
Alternative		11.6	701.2	240.2	6.9		959.9
LSD (0.05)		ns	ns	62.2	ns		56.3

 

 

2004
Treatment		Medium	Jumbo	Colossal	Super- colossal		Total yield
		cwt/acre
Untreated check		57.6	395.1	9.1	0		461.8
Standard		50.8	509.0	15.4	0		575.2
Alternative		52.1	445.6	36.9	0		534.6
LSD (0.05)		ns	ns	16.5	ns		ns

 

Table 4. Yield and grade of Flamenco onions with different strategies for controlling onion thrips, Malheur Experiment Station, Oregon State University, Ontario, OR.

 

2003

Treatments	Medium	Jumbo	Colossal	Total yield
	cwt/acre
Untreated check	121.5	380.5	1.0	512.4
Standard	107.1	442.3	9.2	565.5
Alternative	94.0	486.1	19.1	606.9
LSD (0.05)	16.9	55.5	7.8	51.8

 

2004

Treatments	Medium	Jumbo	Colossal	Total yield
	cwt/acre
Untreated check	128.1	175.1	0.3	512.4
Standard	101.0	275.3	1.0	565.5
Alternative	82.2	305.9	10.7	606.9
LSD (0.05)	ns	ns	ns	51.8

 

 

 

 

 

Table 5. Average iris yellow spot virus injury for insecticide treatments, Malheur Experiment Station, Oregon State University, Ontario, OR, 2004.

 

Treatment	IYSV*
Untreated	1.5
Standard	1.7
Alternative	2.2
LSD (0.05)	0.4
*Scale: 0 = dead, 5 = healthy, no lesions.

 

 

 

 

Table 6. Thrips injury on two stored red onion varieties, Malheur Experiment Station, Oregon State University, Ontario, OR,  2003.

 

	Thrips injury
Treatment	Redwing	Flamenco
	(0 = no injury, 10 = severe injury)
Alternative	1	1.3
Standard	1.3	1.6
Untreated check	1.5	2.1
LSD (0.05)	0.3	ns
Varietal differences
Redwing	1.27
Flamenco	1.68
LSD (0.05)	0.39

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2003

 

 

 

2004

 

Figure 1. Thrips populations with different treatments in an alternative thrips control program, Malheur Experiment Station, Oregon State University, Ontario, OR.

 

 

 

 

 

 

 

 

 

2003

 

 

 

 

2004

	a
			ab
					b

 

 

Figure 2. Average season-long thrips populations in an alternative thrips control program, Malheur Experiment Station, Oregon State University, Ontario, OR.

 

 

 

a

                      

Figure 3. Predator populations in the alternative thrips trial, Malheur Experiment Station, Oregon State University, Ontario, OR,  2003.