Malheur Experiment Station Information for Sustainable Agriculture

Maximizing the economic value of insecticide applications

FOR THRIPS CONTROL - 2009

Lynn Jensen

Malheur County Extension Service

Clinton C. Shock and Lamont D. Saunders

Malheur Experiment Station

Oregon State University

Ontario, OR

Objective

Determine the economic value of insecticide applications to cultivars that are either sensitive or resistant to thrips feeding and iris yellow spot virus (IYSV) disease damage by determining yield and grade reductions from reduced applications.  Not all insecticides referred to in this report are registered for use on onions.  Always obtain and read the insecticide label to ensure that the product is registered for the crop for which it is intended.

Introduction

There is a need for growers to understand the economic impact of their decision about how often they should apply insecticides for thrips control. Since onion varieties vary in their ability to withstand thrips damage, and have different tolerances for IYSV, it is important to be able to make decisions with some idea about how it will impact the varieties they have planted.

Materials and Methods

Onion plots were 30 ft long by 14.6 ft wide.  The plot design was a split plot with one-half of each plot planted on March 20, 2009 to ‘Vaquero’ (Nunhems, Parma, ID) and the other half to ‘Charismatic’ (Seminis, Payette, ID).  Charismatic is generally believed to be more tolerant of thrips damage and injury from IYSV disease. The onions 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 at planting in a 6-inch band over each double row at a rate of 3.7 oz/1,000 feet of row for onion maggot control. 

Insecticide applications were made with a CO₂-pressurized bicycle sprayer. Materials were applied with water at 38 gpa. Each treatment was replicated four times. Thrips counts were made weekly by visually counting the total number of thrips on 15 plants in each subplot. Thrips and IYSV injury ratings were taken as a subjective measurement of foliage damage caused by thrips feeding.

A scale of 0-5 (0 = no injury, 5 = complete silvering of the leaves) was used. The onions were harvested September 17-18, 2009, and graded on October 14-15, 2009.

The experimental design was a randomized complete block split plot with four replications. Rates, application dates, and sequence are listed in Table 1.  Environmental conditions at each application date are listed in Table 2.

Results and Discussion

The thrips data are shown in Table 3 and the foliage damage evaluations in Table 4.  Thrips populations were significantly higher in Vaquero than in Charismatic.  This was especially true in the untreated check where the season average for Vaquero was 27.9 thrips/plant compared to 19.9 for Charismatic.  In Vaquero, the untreated check treatment had significantly higher thrips numbers compared to any other treatment.  The Charismatic untreated check treatment also had the highest thrips numbers, but there were three other treatments (4, 5, 6) whose thrips numbers were not significantly different from the untreated check.  Treatment 1, which included insecticide treatments every 7-10 days throughout the season, had the best thrips control in each variety.  Treatments 3 and 8 were next best in both varieties.  The combined thrips populations of both varieties are shown in Figure 1, where Treatments 1, 3, and 8 show the best thrips control.  The thrips and IYSV damage ratings were also significantly higher for Vaquero than for Charismatic, probably reflective of the higher thrips population in Vaquero.

Yield information is shown in Table 5.  The relationship between total yield, or yield of colossal or supercolossal bulbs and thrips populations was very weak for either variety (Fig. 2).  Treatments 7 and 10 gave the lowest yields for both varieties.  With Charismatic, Treatment 1 gave the best overall yield and grade, followed closely by Treatment 5.  With Vaquero, Treatment 4 gave the best overall yield and grade followed by Treatment 1, however, none of these treatments were statistically different.

Even though Charismatic had lower thrips populations and less thrips and IYSV damage, it was more likely to give a yield and grade response from insecticide applications compared to Vaquero.  When looking at the economic impact, Charismatic was likely to give a substantial net return from thrips control applications while Vaquero was likely to show a negative return, unless weekly insecticide applications were made during June and July (Table 6).  Net returns were greater when the price differential between jumbo and the larger colossal or supercolossal sizes was high, since controlling thrips had a greater impact on yield of the larger sized bulbs.  Insecticide applications were based on a cost of $50/application and net returns would vary, depending on whether a grower’s costs are more or less than the $50.

Most treatments varied in their impact on each variety, making it difficult to recommend which reduced treatment strategy might work best.  Growers should be aware of market trends and prices, or contract size parameters when deciding to reduce the number of pesticide applications.

Conclusions

Charismatic had significantly fewer thrips and less IYSV compared to Vaquero.  Weekly insecticide treatments to control thrips were cost effective for both varieties.

Reducing the number of insecticide applications influenced profitability, sometimes increasing net revenue and other times reducing net revenue.  Prices received for jumbo and larger classes influenced profitability, especially if the medium and jumbo prices are near those of the colossal and supercolossal classes.

More work is needed to identify which reduced treatments are best for each variety.

Table 1. Insecticide application dates and applied treatments. Malheur Experiment Station, Oregon State University, Ontario, OR, 2009.

Table 2.  Insecticide application information. Malheur Experiment Station, Oregon State University, Ontario, OR, 2009.

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Last updated  Tuesday July 19, 2011.