Abstract

Resistance to spinosad insecticide in field populations of diamondback moth was discovered at Kunia, Hawaii, in November 2000, about two and a half years after its introduction. Leaf-dip assays of field populations from the islands of Oahu, Maui and Hawaii confirmed moderate to high levels of resistance compared with pre-introduction baselines. Resistance occurred despite label restrictions that were designed to prevent overuse. A major contributing factor was the lack of suitable alternatives and the unsynchronised use of pesticide classes that led to continuous population exposure. Region-focused resistance management plans were implemented by growers and University of Hawaii extension advisers in an IRAC sponsored program. The two goals were to mitigate resistance to spinosad and avoid resistance to emamectin benzoate and indoxacarb. The use of emamectin benzoate and indoxacarb was limited to month-long windows that were rotated. There was a ban on spinosad use in crucifer production until regional DBM LC₅₀ decreased to a 5 ppm level.

Keywords: Plutella xylostella, diamondback moth, IRM, resistance management

Introduction

The Hawaiian Islands have a wide range of climatic conditions that allow crucifers to be grown year round on most islands. The majority of cabbage, broccoli and cauliflower production occurs at elevations below 100 metres elevation on the islands of Hawaii, Maui and Oahu. Leafy crucifer varieties (choy sum, bok choy, gai lon, gai choy) are commonly grown in lowland farms below that elevation. The majority of farms are clustered in regions and range in size from 8 to 20 hectares. There are a few that are about 400 hectares in size. To meet annual fresh-market needs, more than 650 hectares of cruciferous crops are produced. Each grower routinely establishes plantings of 0.13 to 2 hectares every week in sequential, adjacent plots.

Diamondback moth (DBM) is the key pest of crucifers, but the crops can be affected by occasional caterpillar pests that include the imported cabbage webworm (Hellula undalis), imported cabbageworm (Pieris rapae), and cabbage looper (Trichoplusia ni). There are 12 to 17 DBM generations per year. DBM populations in the major production regions are resistant to organophosphate, carbamate, pyrethroid and organochlorine insecticides (Tabashnik et al. 1987). Although resistance to Bacillus thuringiensis (B.t.) has been documented, growers report that B.t. products are effective after periods of non-use.

Products from new insecticide classes (chlorfenapyr, emamectin benzoate, fipronil, indoxacarb and spinosad) were highly effective in laboratory and field tests against Hawaiian populations of diamondback moth (Mau et al. 1997, Mau and Gusukuma-Minuto 1999). Unfortunately, regulatory labelling of the products for crucifers has been slow and incremental. To date, only emamectin benzoate, indoxacarb and spinosad received national labels for use on cruciferous crops.

Emamectin benzoate (Proclaim, Syngenta Crop Protection, Inc.) was intensively used in Hawaii under emergency provisions from June 1996 to May 1998. Naturalyte Insect Control (spinosad, Dow AgroSciences) became available for use by Hawaii’s growers in April 1998 under the product name Success. During June 1998 through May 2000, spinosad was intensively used throughout the state. It was the only labelled insecticide that was highly effective for DBM management. The farm-focused resistance management restrictions for spinosad were included in the manufacturer’s product label. Use was limited to three times per 30 days followed by at least a 30-days of non-use. There was a maximum of 6 applications allowed per crop cycle. B. thuringiensis insecticides were only partially effective, but were used when spinosad could not be used. Emamectin benzoate was subsequently labelled for national use in June 1999. Indoxacarb (Avaunt, E.I. duPont de Nemours & Co., Inc) became labelled for use on some cruciferous crops in December 2000.

In September 2000, University of Hawaii extension entomology workers investigated complaints of non-performance of spinosad against DBM. At the time, spinosad was being rotated with emamectin benzoate. This paper discusses discovery and confirmation of resistance to spinosad at several locations in Hawaii. We outline the establishment of region-focused DBM resistance management programs that were created to conserve effective insecticides and mitigate resistance to spinosad.

Materials and Methods

Bioassay procedures were as follows. The leaf residue bioassay method described by Tabashnik and Cushing (1987) was used for the tests with the following modifications. Pesticide free cabbage seedlings were used. Discs (4.5 cm diameter) were cut from leaves using a cookie cutter. Each disc was attached to a paper clip, immersed in the appropriate pesticide solution and allowed to air dry for 1–2 hours. The assays were conducted in 50 x 9 mm sterile Petri dishes. III instar larvae were used in the laboratory bioassays. Where possible, F₁ larvae from each of the colonies were used in the assays. Eight to 12 larvae were placed on each disc. Larvae were allowed to feed for three days before final mortality data were taken. There were at least ten replicate cohorts for each treatment dose. Mean lethal concentrations (LC₅₀) for field populations were calculated using probit analysis (SAS).

The original assays for resistance to spinosad was first performed on a field-collected colony from a farm at Kunia, Oahu. Subsequent assays were performed on colonies established from other geographic regions throughout the State of Hawaii (Figure 1). Late stage DBM larvae and pupae were used in establishing the colonies. All colonies were reared on pesticide-free cabbage or rape. A susceptible colony that was collected in 1994 at Kamuela, Hawaii was used for the toxicity ratio (TR) calculations. Toxicity ratios were calculated by dividing the LC₅₀ of the assayed population by that for the susceptible laboratory population (Kamuela 94).

Figure 1. Major production regions for cruciferous crops in Hawaii and the specific localities where field-collections of diamondback moth were made (AP = Lower Kula, EW = Ewa, KN = Kunia, KU = Middle Kula, LM = Lalamilo, PA = Pahala, PC = Pearl City, PK = Puukapu, PO = Poamoho, WA = Waianae, VO = Volcano).

Results

Moderate to high levels of resistance to spinosad were found in several DBM populations (Mau & Gusukuma-Minuto 2001; Zhao et al. 2002). Toxicity ratios of resistant populations ranged from 204 at Puukapu, Hawaii to 1,340 at Ewa, Oahu (Table 1). Resistant field populations were found on all islands, but resistance was greatest on the island of Oahu where crops were grown at lower elevations (Table 1). Moderate resistance levels were found at Lalamilo and Puukapu, Hawaii. DBM populations at Volcano and Pahala, Hawaii, where spinosad had not been used, were very susceptible to spinosad.

Table 1. Toxicity Ratios (TR) for spinosad (Success 2 SC) for Hawaiian field-collected diamondback moth populations

¹Toxicity ratio = LC₅₀ of tested population divided by the LC₅₀ of the susceptible laboratory strain

Leaf dip assays were read at 72 hours.

There was adequate proof that susceptibility to spinosad had changed. A susceptibility baseline for the Ewa population had been performed just prior to registration of spinosad in 1998. That toxicity ratio was calculated as 1.0. Comparing the TR values, we found that there was a 1,340-fold decrease in susceptibility for the DBM population at Ewa.

Establishment of Regional Resistance Management Programs. Extension educators and Dow AgroSciences officials presented a series of regional seminars about development and management of resistance using information from the Insecticide Resistance Action Committee (IRAC), Dow AgroSciences and University of Hawaii. Alternative tactics for resistance management were discussed. Grower committees and extension advisers devised regional integrated resistance management (IRM) programs for DBM. Improved crop hygiene, conservation of naturally occurring biological controls, pest monitoring and rotation of insecticide classes were tactics that were included. A month-long crucifer-free fallow was considered, but it was not adopted because a majority of the farmers would lose income and market relationships for at least three months.

The voluntary program was based on education and peer pressure, not regulatory mandate. There were isolated cases of non-compliance with the insecticide rotation program during the first few months, but this was not widespread. Compliance and susceptibility monitoring helped to encourage growers to maintain the program. Random checks of grower records showed widespread compliance.

The regional programs were approved by growers and implemented in February 2001. There were similar plans for the three growing regions—Central Oahu, Kula, Maui, and Kamuela, Hawaii. Therapeutic insecticide use for DBM was based on grower determined larval thresholds and was subject to rotation of insecticide classes using month-long windows. Emamectin benzoate and indoxacarb were used in the programs. During the first rotation window, one could make a maximum of four applications of indoxacarb. A maximum of three applications of emamectin benzoate was allowed during the subsequent month-long window. If it was needed, a B. thuringiensis product was used for DBM control after two sequential weekly applications of emamectin benzoate. Use of organophosphate, carbamate and pyrethroid insecticides for control of occasional pests was greatly reduced to conserve DBM parasitoids.

Mitigation of resistance to spinosad. All of the regional groups voluntarily removed spinosad from their pest management programs to mitigate resistance levels. Many growers ceased using the insecticide after the first educational meeting. A monitoring plan was created to assure compliance. Random collections of crucifer leaves were taken at intervals during the year. Antibody assays were performed to detect the presence of spinosad residues. The assays of initial field samples were negative leading us to believe that growers fully participated in the mitigation program.

The susceptibility of DBM populations from three islands was measured at 3–6 month intervals after the start of the mitigation program. Dow AgroSciences and the University of Hawaii set a reintroduction LC₅₀ threshold for spinosad at 5 ppm. Susceptibility to spinosad increased more rapidly than we anticipated (Table 2). The spinosad thresholds for reintroduction of spinosad were met for Kula, Maui and Kamuela, Hawaii populations within 6–8 months after spinosad use was suspended. Growers in these regions will be allowed to use spinosad in 2002.

Table 2. Changes in field susceptibility of diamondback moth to spinosad during the resistance mitigation period

Discussion

How did resistance to spinosad evolve in such a short time (30 months)? A combination of factors created excessive selection pressure on DBM populations. Obviously, genes that conferred spinosad resistance were common throughout many regional populations. Spinosad was the only effective product available for managing DBM. It was used extensively because therapeutic treatments are usually necessary practically every week of the year. Emamectin benzoate became available 15 months after the introduction of spinosad and was used as a rotation partner. The greater cost per treated-acre of emamectin benzoate was an impediment to its use by some growers. It was impossible to do an effective farm-focused resistance management program given the small plot sizes and continuous weekly planting schedules. In essence, DBM populations were being exposed to spinosad every week of the year.

Can we prevent a similar fate for emamectin benzoate and indoxacarb? We now believe that adoption of the region-focused program will be helpful in prolonging the use of these products. Mitigation of resistance to spinosad has been demonstrated. This makes a three-insecticide class rotation program. Overall selection to each insecticide will be limited to four month-long windows.

References