Abamectin

Research reveals how to combat insecticide resistance and manage leafminers in ornamental crops.

THE leafminer Liriomyza trifolii is a damaging pest of ornamentals throughout the world. It has been a serious pest in Florida for many years and was introduced into California some time in the late 1970s. Expanded traffic in flower crops of greenhouses appears to be the main cause for the expanding range of this species. Larvae damage the leaves by tunneling in the leaf tissue as they feed between the upper and lower leaf surfaces, creating the telltale mines. Due to the low damage threshold on some crops, such as chrysanthemums and gerbera daisy, the most widely used method of leafminer control is through the application of insecticides.

The widespread and intensive use of insecticides to control leaf miners has led to the development of insecticide resistance in L. trifolii to most of the chemicals registered for control, with some insecticides retaining their effectiveness for only two years after introduction. However, cyromazine, an insect growth regulator (Citation), and abamectin, a GABA agonist (Avid) have been used successfully for leafminer control since the mid-1980s on ornamental crops.

Prior to 2000, there was only one documented report of insecticide resistance in L. trifolii to cyromazine in the United States, which occurred in celery in south Florida in 1990. A widespread survey of crops in south Florida in 1990-1991 showed that the cyromazine resistance was limited to this particular crop and location. There were no published reports of abamectin resistance in leafminers prior to 2000.

In response to the development of cyromazine resistance in L. trifolii in Florida, the Florida Fruit and Vegetable Association organized a committee consisting of scientists from the University of Florida, the agricultural chemical industry, and commercial vegetable growers to address the problem and develop a leafminer resistance management program. The backbone of the insecticide portion of the program is a rotation of abamectin and cyromazine, never applying more than three consecutive applications of the same product. Since the introduction of the program, there have been no additional instances of cyromazine resistance in leafminers in Florida vegetables. In California, where leafminers are a major pest of ornamentals, rotation to a new class of insecticides for leafminer control is advised.

A recent study was conducted to further investigate the response of leafminer populations to insecticides from several commercial ornamental production greenhouses where less than satisfactory leafminer control was reported despite applications of Citation, Avid and a new leafminer control product, spinosad (Conserve).

Experimental Protocol

Three leafminer strains were obtained from commercial greenhouses and tested for susceptibility to the above-mentioned insecticides. Growers from all three locations reported using Citation, Avid and/or Conserve multiple times per crop. At the Syngenta Vero Beach Research Center, these strains were maintained under continuous culture without exposure to insecticides to determine the rate of reversion (i.e. a return to the susceptible condition) by conducting bioassays on the successive generations. Leafminer strain, CA-1, was obtained in July 2000 from gerbera daisy in Southern California; CA-2 was obtained in May 2001 from chrysanthemums in southern California; and GA-1 was obtained in October 2001 from chrysanthemums in Georgia. A leaf dip bioassay of leaves containing young larvae was used; the larval stage was selected for bioassay because Citation is only effective against larvae.

A susceptible reference culture was obtained from the University of CaliforniaRiverside that was started from leafminers collected from celery and mums in Southern California in 1980. This reference culture has been reared in the absence of insecticide selection pressure and without introduction of new adults. Sixty-four young (10 to 14 day-old) cow-pea plants were caged and exposed to several hundred three to four day-old flies for an oviposition access period (OAP) of two to four hours. The short time period for OAP allowed for a synchronous egg hatch and uniform larval development prior to treatment.

After OAP, plants were removed and held in a leafminer-free greenhouse for 72 hours to allow eggs to hatch and small mines to develop. The numbers of small mines (

Larval mortality, based on the number of larvae that failed to survive successfully to the puparial stage, was calculated for each dose. LC50 values -LC50 is the concentration of insecticide that resulted in fifty percent mortality of the sample population tested - were calculated based on the dose response. When discussing insecticide resistance, the term resistance ratio or RR50 is often used to describe the LC50 of the test strain divided by the LC50 of a reference strain. An RR50 greater than one indicates that the test strain is more difficult to control than the reference strain. Although not universally agreed upon by entomologists, the term "resistant" is generally used when the RR50 of the test strain is greater than 10.

Experimental Results

Based on the larval mortality comparison to the susceptible laboratory reference strain, results showed that the three strains varied in spectrum and level of resistance to the insecticides. At the time of collection, the CA-1 strain was moderately resistant to Citation and Avid but highly resistant to Conserve (Figure 1), whereas the CA-2 strain was susceptible to Avid and had a low level of resistance to Citation and a high resistance to Conserve. The GA-1 strain was susceptible to Citation and Conserve, but moderately resistant to Avid.

In relation to the reference strain, the RR50 of the CA-1 strain went from 18.1X, 22.0X and >188X with Citation, Avid and Conserve, respectively, at the fourth generation after collection from the commercial greenhouse, to 1.5X, 4.4X and 3.2X at the tenth generation.

The CA-2 strain was only 2.2X more tolerant of Citation than the reference strain by the third generation and normally sensitive (RR = 1.0) by the fifth. Despite the variability in response in Conserve data, this strain was evidently highly resistant to Conserve (RR = 1192 for the second generation), reverting to normal sensitivity by the fifth generation.

Regarding the GA-1 strain, the LC50 value at the third generation with Avid decreased to just over three-fold resistance. At the fourth generation, Avid resulted in an LC50 of 0.41 ppm, or 2.4-fold resistance. With Citation, a RR50 of 5.0 remained at the third generation, but decreased to 0.4 by the fifth. At the final bioassay (fifth generation), the LC50 values with all three materials were similar to that of the reference strain.

Discussion

Based on significant and dramatic increased LC50 values compared to a reference culture, these results indicated that L. trifolii from three commercial ornamental greenhouse production operations had developed resistance to Citation, Avid and Conserve. This represents the second documented case of Citation resistance in the USA (the first from ornamentals) and the first documented cases of resistance to Avid and Conserve in leafminers. In the four months prior to collection of the CA-1 strain, the greenhouse received nine applications of Avid at 8 oz/100 gal, four applications of Citation at 2.66 oz/100 gal and four applications of Conserve at 22 oz/100 gal, all at labeled rates. Longer-term use of these compounds on CA-1 and the history of the other two strains was not well defined. The only performance data were derived from grower comments, with all reporting less than satisfactory control. However, since no actual counts were made following application of a specific material, it is impossible to correlate leafminer control failure with the laboratory assays reported here.

With the reports from growers and the large increases in LC50 values, it is very likely, however, that performance was impaired. In the month preceding the collection of the strain, CA-2 received two applications of Avid at 8 oz/100 gal and several applications of Conserve at 8 oz/100 gal, but no Citation application. With GA-1 strain, the grower reported less than acceptable control with a rotation of the three compounds. In our study, only the CA-1 strain possessed a moderate level of Citation resistance, 18-fold at the fourth generation, while CA-2 and GA-1 possessed very low levels of resistance - 8.2 and 5.4-fold, respectively, at the second generation. Reversion to a susceptible condition occurred in eight to nine generations in CA-1 and four to five generations in CA-2 and GA-1.

These results suggest that strains possessing a lower level of Citation resistance will revert more rapidly to a susceptible level than those possessing higher levels of resistance. This is supported by the findings of G.L. Leibee and J.S. Ferguson (unpublished data), who reported that a cyromazine resistant strain of L trifolii, which possessed 88-fold resistance at the third generation, reverted to four-fold resistance by the 16th. In the study, the resistant strain did not fully revert, with four-fold resistance remaining at the 28th generation. With the three strains in the present study, the response to Citation reverted to a level of approximately that of the reference strain.

With Avid, strains CA-1 and GA-1 possessed moderate levels of resistance, 22.0- and 30.6-fold, respectively. The CA-2 strain had only a 1.9-fold increase in LC50. With the GA-1 strain, there was a dramatic decrease in resistance between the second and third generations - from 30.6-fold to 3.2-fold. Within five generations, this strain reverted to only a 1.4-fold increase in LC50 over the reference strain. With the CA-1 strain, there was 22-fold resistance at the fourth generation. It is probable that the level of Avid resistance was higher in earlier generations. Reversion in this strain to 3.1-fold resistance occurred by eighth generation. A resistance level of over four-fold remained through the 10th generation, indicating that this strain would maintain a low level of resistance. This suggests that strains possessing a high level of Avid resistance may not revert to a completely susceptible level. However, Avid may still provide leafminer control at these low levels of resistance when used at labeled rates with proper insecticide rotation.

Spinosad was registered in 1997 for ornamentals as Conserve by Dow AgroSciences and was rapidly adopted in commercial greenhouses, due to leafminer resistance concerns. Both CA-1 and CA-2 possessed extremely high resistance to Conserve; however, GA-1 was susceptible. CA-1 possessed greater than 188-fold at the fourth generation, therefore, based on reversion rates observed in this study, it is probable that the resistance was higher in earlier generations. CA-2 possessed 1192-fold resistance to Conserve at the second generation and the reversion in both CA-I and CA-2 occurred very rapidly, with a precipitous drop between the sixth and ninth generations in CA-1 and between the third and fourth generations in CA-2. Compared to the rate of reversion observed with Citation and Avid, reversion to Conserve occurred very rapidly over one to three generations.

No cross-resistance to Avid was detected in a cyromazine-resistant strain of L. trifolii (G.L. Leibee and J.S. Ferguson, unpublished data). The present study supports these findings. Based on data from bioassays in the present study, there is no cross-resistance between Avid and Conserve. In addition, the CA-2 strain possessed a very high level of resistance to Conserve (1192-fold) but a very low level to Citation (8.2-fold), suggesting that cross-resistance does not exist between these two insecticides. The very different modes of action between these two compounds (an insect growth regulator [Citation] and a nerve toxicant [Conserve]) would also indicate that cross-resistance would not be expected. Given the different modes of action and no cross-resistance among these three insecticides, they should be considered in rotation for leafminer control in an insecticide resistance management strategy.

In our study, in the absence of insecticide selection pressure, all three resistant strains of L. trifolii reverted to a level of susceptibility to Avid, Citation and Conserve approximating that of the reference strain. Ideally, similar to the leafminer resistance management strategy implemented in Florida vegetables in 1991, by using insecticides only when necessary and employing a rotation of effective materials (e.g. Citation, Avid and Conserve), with no more than three applications of the same material in succession, the development of insecticide resistance in leafminers in ornamental crops can be managed.

About the author: J. Scott Ferguson is with Syngenta Crop Protection, 7145 58th Avenue, Vero Beach, FL 32967.

Copyright Meister Media Worldwide Feb 2005
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