A Proactive Approach to Understanding Resistance to Novel OP alternatives as a Strategy for Sustainable Management of Obliquebanded Leafroller

View the project final report

Commodities

• Fruits: apples, cherries, general tree fruits

Practices

• Pest Management: chemical control, integrated pest management

Studies were conducted to assess the current levels of resistance and cross-resistance of obliquebanded leafroller (OBLR) populations and genetic potential of OBLR to develop resistance, and to characterize mechanisms responsible for conferring resistance to the recently registered reduced-risk OP alternatives, rynaxypyr and spinetoram. Bioassays of field-collected populations showed that low levels of resistance to the new chemicals, rynaxypyr and spinetoram, already exist in the field populations, and that resistance of field populations to spinetoram was correlated with their resistance to spinosad. Significant levels of resistance to rynaxypyr and spinetoram were recorded in a laboratory population as a result of selection for resistance in the laboratory. The evidence of resistance and cross-resistance in the field, as well as development of resistance in response to laboratory selection, indicate that the risk of resistance evolution against these chemicals exists. However, in the absence of selection pressure, the selected populations reverted to being susceptible, indicating that resistance to both rynaxypyr and spinetoram was unstable. Moreover, biochemical enzyme assays of the selected populations indicated that esterase activity was significantly increased in the rynaxypyr-selected population, whereas mixed-function oxidase levels were elevated in the spinetoram-selected population. The fact that resistance to both rynaxypyr and spinetoram was unstable and that these chemicals do not share detoxification mechanisms indicates that rynaxypyr and spinetoram could be effectively incorporated into resistance management programs through strategies of rotation. Implementation of such strategies at this point would be a proactive approach and would lead to management of OBLR and other major pests of tree fruits on a sustainable basis.

• Final Report

Introduction

Tree fruit are one of the biggest industries in Washington State, producing crops valued at nearly $1.5 billion, with an economic impact of over $6 billion on the state’s economy by providing for more than 140,000 jobs and service related activities (1). Washington’s tree fruit growers produce high quality fruit for the state, the nation and the world with over 241,986 acres dedicated to tree fruit crops (NASS-USDA). Insect pests are one of the major concerns of tree fruit growers and have the potential to reduce crop production substantially. Obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae), is one of the most destructive lepidopteran pests of tree fruits, causing severe damage by feeding on leaves and developing fruits (2).

Historically, broad-spectrum insecticides have been the primary tools to manage insect pests in tree fruits. Use of broad-spectrum insecticides such as organophosphates (OPs) for decades has resulted in the development of OP resistance and cross-resistance to other classes of insecticides in major pests, including OBLR, leading to control failure in some cases (3, 4). Additionally, regulatory actions such as the Food Quality Protection Act of 1996 (FQPA) have put restrictions on the use of broad-spectrum insecticides, leading to an OP phase-out (5). The development of insecticide resistance and implementation of FQPA, along with restrictions in international markets, have led to the development of new chemicals such as rynaxypyr and spinetoram which are safer to humans.

Rynaxypyr is an anthranilic diamide which belongs to insecticide resistance action committee (IRAC) mode of action class 28 (6). Anthranilic diamides selectively bind to ryanodine receptors (RyR) in insect muscles, resulting in an uncontrolled release of calcium from internal stores in the sarcoplasmic reticulum (7, 8), causing impaired regulation of muscle contraction which leads to feeding cessation, lethargy, paralysis and death of target organisms. Anthranilic diamides have very low vertebrate toxicity due to a 500-fold differential selectivity toward insect over mammalian RyR (8).

Spinetoram is a recently developed spinosyn belonging to IRAC mode of action class 5 (6). Spinosyns primarily activate the nicotinic acetylcholine receptors by acting on a unique and yet unknown binding site (9-11). Both rynaxypyr and spinetoram were highly effective against OBLR in both laboratory and field trials (12, Brunner unpublished data).

With the availability of these products, it was critical for tree fruit growers to incorporate the novel reduced-risk insecticides into IPM programs, but resistance remains a threat. Resistance management strategies are usually developed after it has occurred in the field, which is too late. In this situation, characterizing resistance in various field populations could be extremely valuable for OBLR management programs by detecting potential problem of resistance at an earlier stage, thereby allowing growers to change their OBLR control strategies and slow the spread of resistance. The successful management of insecticide resistance depends ultimately on a thorough understanding of the resistance mechanisms. The understanding of genetic basis of resistance and mechanisms conferring resistance, especially before its occurrence in the field, would be a proactive approach and could be extremely valuable in developing strategies to manage susceptibility, leading to delay the development of resistance. Therefore, this project was proposed to assess current levels of resistance and cross-resistance of OBLR populations and genetic potential of OBLR to develop resistance, and to characterize mechanisms involved in resistance of OBLR to rynaxypyr and spinetoram.

The primary goal of this project was to provide tree fruit growers with information that will enable them to incorporate the recently developed reduced-risk OP alternatives, rynaxypyr and spinetoram, into IPM programs to control OBLR and possibly other pests on a sustainable basis in an economical and environmentally friendly manner. This goal was achieved under the following objectives:

1) To determine the current levels of resistance and cross-resistance in field-collected populations of OBLR to the novel reduced-risk chemicals,

2) To assess the genetic potential of OBLR to develop resistance against the novel reduced-risk chemicals, and

3) To characterize mechanisms conferring resistance against these chemicals in OBLR.