Alternative Methods for Controlling Onion Thrips (Thrips tabaci) in Spanish Onions

Malheur Experiment Station

Information for Sustainable Agriculture

Alternative Methods for Controlling Onion Thrips (Thrips tabaci) in Spanish Onions

Lynn Jensen and Ben Simko

Malheur County Extension Service

Clint Shock and Lamont Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR, 2001

Introduction

Onions are a major economic crop in the Treasure Valley production region of eastern Oregon and western Idaho. Annually about 20,000 acres of onions are grown in the valley. Typically the onions are Spanish hybrids and are grown for their large size, high yield, and mild flavor. The value of the Treasure Valley onion industry for the 2000 production year was 94 million dollars. Over the past 10 years the value of the industry has ranged from a high of 140 million dollars to a low of 75 million depending upon market fluctuations.

The principle onion pest in this region is the onion thrips (Thrips tabaci, Lindeman) that causes yield reductions by feeding on the epidermal cells of the plant, thus reducing the photosynthetic ability of the plant. Onion thrips can reduce total yields from 4 to 27 percent, depending on variety, but may reduce yields of colossal sized bulbs from 28 percent 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 gone 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.

Mechanical straw mulching was introduced in 1985 as a means of improving irrigation water infiltration and reducing sediment loss. Some growers using this technique reported having decreased onion thrips pressure; a possible explanation for this may be enhanced habitat for predators.

New biological insecticides have been developed, including neem tree extracts (azadirachtin) and bacterial fermentation (spinosad). Both of these materials have previously been evaluated for thrips control and have performed poorly compared to the conventional insecticides. It was decided to test these products, along with Messenger, a harpin protein thought to enhance the plant's ability to withstand stress, in combination with straw mulch to provide predator habitat as an alternative program to the conventional insecticide program currently used by growers.

Materials and Methods

A 1.8-acre field was planted to onions (cv. 'Vaquero', Sunseeds, Brooks, OR) on March 23, 2001. The onions were planted as two double rows on a 44-inch bed. The double rows were spaced 2 inches apart. The seeding rate was 154,000 seeds per acre. Lorsban 15 G was applied in a 6-inch band over each row at planting at a rate of 3.7-oz/1,000 ft of row for onion maggot control. The field was divided into plots 40.3 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. The grower standard practice included Warrior (lambda-cyhalothrin) and Lannate (methomyl). The 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), Ecozin or Aza Direct (azadirachtin neem extract), and Messenger (harpin protein).

Insecticide treatments were applied weekly or biweekly during the first half of the growing season (Table 1). All insecticides were applied with water at 29.7 gal/acre. 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 23 at a rate of 952 lb/acre.

Thrips populations were sampled by two methods. The first was by visually counting the number of thrips on five plants. The second method was by cutting five plants at ground level and inserting the plants into a modified Berlese funnel designed to hold the plants. Turpentine was used to dislodge the thrips from the plant, where they would then fall into a jar containing 90 percent isopropyl alcohol. The collected thrips were then counted through a binocular microscope. Thrips populations were monitored weekly though the growing season.

The predator populations were monitored using pitfall traps that contained ethylene glycol. They were evaluated three times per week. The modified Berlese funnel was also used to monitor predators foraging on the plants.

Results and Discussion

The onions in the conventional treatment and the alternative control treatments looked similar throughout the growing season, with minimal thrips damage to the foliage. In contrast, the onions in the untreated check treatment had severe foliage damage due to thrips feeding. The thrips population as determined by the modified Berlese funnel is shown in Figure 1.

The visual plant counts (Fig. 2) did not correlate well with the funnel counts. Each is an average of five plants.

Some of the sample dates had statistical differences in thrips populations as shown in Tables 2 and 3.

Predator composition varied throughout the season but consisted mostly of spiders, big-eyed bugs, damsel bugs, and minute pirate bugs, with smaller populations of lacewings, ladybugs, assassin bugs, and rove beetles (Fig. 3). Spiders were initially more prevalent, followed by big-eyed bugs. Late in the season, minute pirate bugs were the dominant predator.

The highest populations of predators were in the alternative control plots, particularly early in the growing season (Table 4). Predator populations increased in the unsprayed and conventionally sprayed plots in August, but decreased slightly in the alternative control plots, although the population was still well above the conventionally sprayed plots.

The onions were harvested on September 13 and graded on September 14and 17 (Table 5). There was a significant increase in super colossal size bulbs in the alternative treatment compared to the untreated check. There was also a significant difference between the treatments and the untreated check in total yield. There was a trend towards more super colossals and greater total yield in the alternative control treatment compared to the standard control, but this was not statistically significant.

Conclusion

The alternative methods in this trial worked as well or better than standard grower practices. The test was not designed to determine the individual effects of straw, spinosad, azadirachtin, or harpin protein on yield and quality, but only to answer the question of whether these materials in combination might give thrips control, yield, and quality similar to the conventional spray program. While thrips control with the alternative program was not as good as the conventional program, yield and quality were excellent. The next challenges will be to determine if these results can be repeated, and to evaluate what each alternative product is contributing towards thrips control, yield, and quality.

Table 1. Application data for the alternative methods for controlling onion thrips trial. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.


Standard treatments				Alternative treatments
Date applied	Product	Formulation	Rate/acre	Date applied	Product	Formulation	Rate/acre
6/6	Warrior	1.0 lb/gal E.C.	4.0 oz	5/21	Messenger	Harpin protein 3%	2.8 oz
	MSR	2.0 lb/gal E.C.	1.0 qt	5/24	Messenger	Harpin protein 3%	3.5 oz
6/14	Warrior	1.0 lb/gal E.C.	4.0 oz	6/6	Success	2.0 lb/gal a.i.	10 oz
	Lorsban	4.0 lb/gal E	1.0 qt	6/13	Success	2.0 lb/gal a.i.	10 oz
6/30	Warrior	1.0 lb/gal E.C.	4.0 oz		Messenger	Harpin protein 3%	4.5 oz
7/9	Warrior	1.0 lb/gal E.C.	4.0 oz	6/21	Ecozin	3% E.C.	10 oz
	Lannate LV	2.4 lb/gal WSP	3.0 pt		Success	2.0 lb/gal a.i.	10 oz
7/16	Warrior	1.0 lb/gal E.C.	4.0 oz	6/29	Ecozin	3% E.C.	10 oz
	Lannate LV	2.4 lb/gal WSP	3.0 pt		Success	2.0 lb/gal a.i.	10 oz
					Messenger	Harpin protein 3%	4.5 oz
				7/9	Ecozin	3% E.C.	10 oz
					Success	2.0 lb/gal a.i.	10 oz
					Messenger	Harpin protein 3%	4.5 oz
				7/16	Ecozin	3% E.C.	10 oz
					Success	2.0 lb/gal a.i.	10 oz
				7/31	Success	2.0 lb/gal a.i.	10 oz
					Aza Direct	1.2% E.C.	10 oz

Figure 1. Thrips populations during the growing season from modified Berlese funnel traps. Malheur Experiment Station, Ontario, OR, 2001.

Figure 2. Thrips populations during the growing season from visual plant counts. Malheur Experiment Station, Ontario, OR, 2001.

Table 2. Weekly thrips population as counted visually on five plants. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

Date: 6/06 6/13 6/20 6/27 7/03 7/11 7/17 7/26 8/01

Check 0.5 4.3 1.9 9.3 7 18.5 14.1 36 3.4

Standard 0.4 1.3 2 7.4 6.5 10.2 7.4 30.1 4.5

Alternative 1 1.5 2.2 5.7 7.8 15.3 14.8 21.6 4.3

LSD (0.05) NS NS NS NS NS 6.2 5.9 NS NS

Date:	6/06	6/13	6/20	6/27	7/03	7/11	7/17	7/26	8/01
Check	0.5	4.3	1.9	9.3	7	18.5	14.1	36	3.4
Standard	0.4	1.3	2	7.4	6.5	10.2	7.4	30.1	4.5
Alternative	1	1.5	2.2	5.7	7.8	15.3	14.8	21.6	4.3
LSD (0.05)	NS	NS	NS	NS	NS	6.2	5.9	NS	NS

Table 3. Weekly thrips population from Berlese funnel counts on five plants. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

Date: 6/06 6/13 6/20 6/27 7/03 7/11 7/17 7/26 8/01 8/08

Check 2.1 6.4 15.2 3.3 19.1 43 22.1 11.5 11.7 21.3

Standard 1.0 0.6 2.4 0.9 12.7 6.7 11.5 19.9 27 30.7

Alternative 2.7 2.6 10.5 1.3 10.2 18.6 32 10.4 10 22.7

LSD (0.05) NS 4.1 NS 1.5 NS 18.8 NS NS 12.0 NS

Date:	6/06	6/13	6/20	6/27	7/03	7/11	7/17	7/26	8/01	8/08
Check	2.1	6.4	15.2	3.3	19.1	43	22.1	11.5	11.7	21.3
Standard	1.0	0.6	2.4	0.9	12.7	6.7	11.5	19.9	27	30.7
Alternative	2.7	2.6	10.5	1.3	10.2	18.6	32	10.4	10	22.7
LSD (0.05)	NS	4.1	NS	1.5	NS	18.8	NS	NS	12.0	NS

Figure 3. Predator composition by month. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

Figure 3. (continued) Predator makeup by month. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

Table 4. Comparison of predator population by month and by treatment as measured by pitfall traps and Berlese funnel. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

June
July August Total

Check 26 25 43 94

Standard 5 6 13 24

Alternative 64 57 33 154

LSD (0.05) NS NS 6.2 44.5

	June	July	August	Total
Check	26	25	43	94
Standard	5	6	13	24
Alternative	64	57	33	154
LSD (0.05)	NS	NS	6.2	44.5

Table 5. Onion grade and yield as influenced by commercial and alternative insecticide controls. Malheur Experiment Station, Oregon State University, Ontario, OR, 2001.

Yield

Treatment Super colossal
>4¼ in
Colossal
4-4¼ in
Jumbo
3-4 in
Total
Super col.
colossal
jumbos
Medium
2¼-3 in
Total marketable
yield
No. 2

------------------------------------------------cwt/acre-------------------------------------------

Untreated check 32.1 193.1 612.7 837.9 49.4 887.3 23.5

Standard 46.9 254.5 628.8 930.2 36.4 966.6 27.2

Alternative
control
63.7 305.1 609.6 978.4 30.2 1,008.6 24.5

LSD (0.05) 23.8 NS NS 94.8 NS 64.0 NS

	Yield
Treatment	Super colossal >4¼ in	Colossal 4-4¼ in	Jumbo 3-4 in	Total Super col. colossal jumbos	Medium 2¼-3 in	Total marketable yield	No. 2
------------------------------------------------cwt/acre-------------------------------------------
Untreated check	32.1	193.1	612.7	837.9	49.4	887.3	23.5

Standard	46.9	254.5	628.8	930.2	36.4	966.6	27.2

Alternative control	63.7	305.1	609.6	978.4	30.2	1,008.6	24.5
LSD (0.05)	23.8	NS	NS	94.8	NS	64.0	NS