Resistance to imidacloprid is widespread, though it still provides significant protection. Many growers in the Northeast rotate imidacloprid early in the year with spinosad later in the summer, an active ingredient with both conventional and organic formulations. Resistance to spinosad would threaten both conventional and organic growers. This study examined correlations between resistance to the two ingredients in six field populations on the South Fork of Long Island and one susceptible control population. It also involved selection in the lab on two different populations for imidacloprid and spinosad resistance, to ask how selection for resistance to imidacloprid affected resistance to spinosad, and vice versa. We found that resistance to spinosad was only weakly correlated with imidacloprid resistance in the field, though all the field populations were much more resistant to both spinosad and imidacloprid than the susceptible population. We found an asymmetrical affect of selection. Selection for resistance to imidacloprid did not affect spinosad resistance, but selection with spinosad also increased resistance to spinosad. The results indicate some linkage between the two treatments, and suggest including a third treatment (chemical, biological or cultural) to avoid using both imidacloprid and spinosad together in a single season.
The problem is the threat of cross resistance due to widespread use of two pesticides to control Colorado potato beetle, Leptintarsa decemlineata (Say) in Long Island and elsewhere in the Northeast. Many growers prefer in-furrow treatments because the protection precedes the insect, and imidacloprid is one of the few (the only one on Long Island) in-furrow systemic treatments, desired for aphid and leafhopper control as well as for CPB control. The rapid evolution of moderate to high levels of resistance on Long Island and elsewhere (Olson et al 2000, Mota-Sanchez et al. 2006), as well as extension efforts, has discouraged most growers from applying in-furrow and foliar neonicotinoids in the same season. However, most growers in the region apply imidacloprid in-furrow at planting, and spinosad at least twice later in the season on summer generations, and have done so for the last few years (Dale Moyer, personal comment). Spinosad is available in conventional and organic formulations. Resistance to spinosad will have a greater impact on organic growers, who have fewer other options for CPB control.
The presence of cross resistance maintains resistance at high levels when either treatment is used. It facilitates subsequent enhancement and modification of resistance, and leads to higher input rates for whatever treatment is considered most effective. The two currently alternated insecticides, imidacloprid and spinosad, both target the nicotinic acetylcholine receptor, although at different locations, and it has been claimed that they do not share modes of action which might facilitate cross resistance (Nauen and Bretschneider 2002). Some cross resistance between imidacloprid and spinosad has been suggested from the fact that a single Long Island population that is highly resistant (~300 fold) to imidacloprid is also more resistant (~10-fold) to spinosad than a reference susceptible population (Mota-Sanchez et al 2006). Cross resistance can arise because a single gene confers resistance to two or more insecticides, because two resistance genes are found close to each other on a single chromosome, or because two unlinked resistance genes both become fixed in a population. These different mechanisms have very different consequences for resistance management and evolution, but no studies have looked for genetic correlations within populations. This project combined resistance surveys in fields treated with or without imidacloprid at planting and with or without spinosad later in the season along with selection experiments in the lab to test for genetic linkage between imidacloprid and spinosad resistance.
Objective I. Carry our selection experiments in the lab to look for mechanistic linkage between spinosad and imidacloprid resistance.
Objective II. Look for correlations between spinosad and imidacloprid resistance among different fields treated with or without imidacloprid at planting and with or without spinosad later in the season.
Objective III. Assess the costs in time and inputs for fields with mixed treatments containing some in-furrow imidacloprid and some untreated portions, compared with uniformly treated fields.
Selection studies: In March 2007, 300 overwintering adults were collected from Riverhead, NY, subdivided into six colonies. 3,270 larvae were assayed for resistance to spinodad and imidacloprid. Two of the colonies were selected for resistance to imidacloprid, two were selected for resistance to spinosad, and two were not selected. Unfortunately, none of the selected colonies yielded enough larvae in the second generation to test for resistance, so this trial was ended and won’t be presented in results.
In June the survivors of bioassays of two fields in Bridgehampton, NY, belonging to Remi Wenofske and Sons Potato Farm, were used to create four colonies, one from each field selected for resistance to imidacloprid and one selected for resistance to spinosad. Selection was carried out on second-instar larvae by subjecting them to a 1 µl drop of either 5×10-5 g/ml imidacloprid dissolved in acetone or up to 2X10-5 g/ml spintor dissolved in acetone (exact dosage pending results of ELISA assay). The four colonies were selected positively for resistance to either spinosad or imidacloprid, and assayed for mortality at a single dose of each, for four generations, including the initial bioassay. At the end of the fourth generation, each colony will be reared for three generations without selection, to see whether resistance declines in the absence of selection. This drift phase is ongoing.
Field trials: Two approximately four-acre sections of larger fields planted by the Wesnofskes were left untreated with imidacloprid at planting, and two other four-acre sections were treated with imidacloprid at planting but not treated with spintor later in the season. Eggs from the first summer generation were collected from those four fields, and from two other fields nearby. The fields untreated with spintor were treated with Kryocide later in the season. The summer generation was only large enough on two fields to assay, one field that was treated with Admire at planting and with Kryocide later, and one field untreated at planting and treated with spinosad later. In addition, a laboratory susceptible colony, originating from field collected clutches in Maryland, was assayed for resistance to both spinosad and imidacloprid. All assays were conducted on late (6-8.5 mg) second instar larvae. Approximately 30 larvae were treated with a 1 µl drop at each of seven doses for each treatment, and an acetone control and scored 24 hours later. Data were assayed using Polo-PC (LeOra Software 1987).
Selection: Both populations responded positively to selection; mortality to the selected ingredient, imidacloprid or spinosad, declined over the course of selection. However, cross effects were not consistent between the two treatments (Figure 1). After four generations of selection, populations selected for resistance to imidacloprid were unchanged or less tolerant of spinosad, while both populations selected for resistance to spinosad were also more resistant to imidacloprid. This is especially striking because resistance to imidacloprid is costly (Baker et al. in review), and resistant populations left untreated usually decline in resistance (unpublished data). This experiment is ongoing with all four colonies being maintained without selection for up to three generations to record any declines in resistance in the absence of selection.
Figure 1: Change in mortality at selective dose from the first through fourth generation of selection.
Figure 2: Resistance to spinosad as a function of resistance to imidacloprid. Black line is a regression including the Maryland susceptible laboratory population (open circle, P=0.04). Grey line is a regression omitting Maryland susceptible population (ns.).
Field trials: There was more variation for resistance to imidacloprid than to spinosad. When examining only the fields on long island, there was no significant trend in resistance (df=5, P = 0.19; grey line, Figure 2). When the Maryland susceptible population was included the regression was significant (df=6, P = 0.04, R2=0.52; black line, Figure 2). Unfortunately populations in the field were only high enough in two locations to assay the second summer generation for resistance, one field was treated with imidacloprid at planting but not spinosad, and the other was left untreated with imidacloprid at planting, but treated twice with spinosad later. In the Admire® treated field resistance to Admire declined from the first to second generation, a typical result (Baker unpublished) as plants grow and newer growth has lower imidacloprid concentrations than initial plant growth (Olson et al. 2004) with second summer generation resistance 0.4 times that of the first, and spinosad resistance was slightly lower as well, 0.9 times that of the first. In the initially untreated field resistance to spinosad rose significantly from the first to second summer generation, with an LD50 twice that of the first summer generation, but imidacloprid resistance rose as well, with resistance 1.6 times as high in the second summer generation.
The results of the first generation field trials was similar to that of Mota-Sanchez et al. (2005). In that study, two populations, one resistant to imidacloprid, and one susceptible, were compared in tolerance to both imidacloprid and spinosad. The imidacloprid resistant population was significantly more resistant to spinosad as well, though the resistance ratio was almost 41 times greater for imidacloprid than for spinosad. In the present study, a regression of spinosad resistance on imidacloprid resistance was significant only when a reference susceptible population was included. If the Long Island populations are pooled the resistance ratio for imidacloprid is 10 times that of spinosad. On average the six Long Island populations were 30 times as resistant to imidacloprid as the susceptible colony and 3 times as resistant to spinosad. There are limits, however to what these kinds of correlations can explain. Fields or populations that are more resistant to Admire may have motivated growers to start using spinosad in earlier years, or more frequently due to larger population sizes after imidacloprid treatment, leading to a correlation in tolerances without the need of any mechanistic link or pleiotropic effect of resistance genes. The selection experiment was intended to tease out any mechanistic link between imidacloprid and spinosad resistance.
The results of the selection experiment are more difficult to interpret because they were not symmetrical. Selection with imidacloprid appeared not to have any effect on spinosad resistance, while selection with spinosad appeared to increase resistance to imidacloprid. The asymmetric response to artificial selection is consistent with the two fields monitored over two generations, whether the field experiencing spinosad selection saw an increase in imidacloprid resistance, while the field that did not receive spinosad observed a decline in imidacloprid resistance. This asymmetric pattern has been observed before in response to artificial selection for parasite resistance in drosophila (Fellowes et al. 1999). One possible explanation raised in that study is that there might be two resistance genes, one with a general, and the other a specialized affect. If that were the case here, selection for imidacloprid resistance favors the specific resistance allele, while selection for spinosad resistance favors only a generalized resistance gene. Resistance to imidacloprid is polygenic (unpublished data, Zhao et al. 2000), which is consistent with both general and specific resistance genes to imidacloprid.
Fellowes M. D. E., A. R. Kraaijeveld & H. C. J. Godfray. 1999. Cross-resistance following artificial selection for increased defence against parasitoids in Drosophila melanogaster. Evolution 53: 966-972.
Insecticide Resistance Action Committee (IRAC). 2005. IRAC mode of action classification, version 5.1. http://www.iraconline.org/documents/moa/MoAv5_1.pdf
LeOra Software. 1994. POLO-PC: probit and logit analysis. LeOra Software, Berkeley, CA.
Mota-Sanchez, D. Hollingworth, R. M., E. J. Grafius, and D. Moyer 2006. Resistance and cross-resistance to neonicotinoid insecticides and spinosad in the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae). Pest Management Science 62: 30-37.
Nauen R., Bretschneider T. New modes of action of insecticides. Pesticide Outlook. 2002. 12:241–245.
Olson, E. R., G. P. Dively, and J. O. Nelson. 2004. Bioassay determination of the distribution of imidacloprid in potato plants: Implications to resistance development. Journal of Economic Entomology 97: 614-620.
Zhao, J.-Z., Bishop, B. A. and Grafius, E. J. (2000) Inheritance and synergism of resistance to imidacloprid in the colorado potato beetle (Coleoptera: Chrysomelidae). Journal of Economic Entomology, 93, 1508-1514.
Though spinosad and imidacloprid are not in the identical IRAC pesticide classification there appears to be some cross resistance between the two treatments. Continuous use of imidacloprid early in the season and spinosad on later generations is not a long term resistance management technique, despite high costs of resistance to imidacloprid. A more sustainable rotation would be to omit imidacloprid in the first season following rest, and to avoid spinosad, using pesticides that do not target the nicotinic acetylcholine receptor, such as growth inhibitors (novaluron) or antifeedants (cryolite) in the second season of potato, rather than spinosad. For 2006, the Wesnofskes will leave one rotated field (20 acres) untreated with imidacloprid, and we will monitor resistance both this season and next to observe the long term effects of rotation of insecticides away from imidacloprid.
Education & Outreach Activities and Participation Summary
Although the field trials and laboratory selection experiments are completed, the economic analysis and outreach has just begun. The results up to this point were presented at the Long Island Agriculture Forum on January 19, 2007, an audience that included most of the potato growers on Long Island (about 12 farms), and about 20 others. A paper for Pest Management Science is in preparation, as well as articles for UMass Vegetable Notes and UMEXT Spudlines newsletters.
Avoiding use of both imidacloprid and Spintor should save money, before accounting for future savings due to resistance management. Yield did not vary according to the growers on the fields treated with or without Admire®. Pesticide costs vary. Imidacloprid prices have declined, to the point where a label rate of Admire Pro® is under $60 per acre in 2006. This is roughly comparable to the cost of two treatments of Kryocide® at high label rates (15 lb/ acre) or 2-3 treatments of Spintor at high label rates (4.5-6 oz. / acre). Skipping Admire will require some treatment for leafhoppers and in some years aphids, typically about $15/ acre (e.g. Acephate, Carbaryl) or higher for organic treatments (e.g. Pyganic). Skipping Admire® in the first year of a potato rotation, controlling leafhoppers, then treating with spinosad twice, costs from $65-$95 per acre, depending on number and rate of Spintor® application. The following year, Admire® followed by Kryocide® would cost $120 per acre. This total of $185-215 per acre compares with current costs of $240 per acre for Admire® followed by Spintor®, although with increasing levels of resistance to both Amire® and Spintor®, more growers will start to apply a third treatment, such as a single spray of Kryocide®, increasing costs.
Areas needing additional study
The question of which genes contribute to resistance to neonicotinoids and spinosad is an interesting one. However, the practical longer term effects of treatment rotation need validation. Skipping imidacloprid lowers resistance in that year compared to treated fields, but does that effect carry over to the following season? The fields left untreated with imidacloprid last summer will be assayed this season as well, but they were smaller fractions of much larger fields. Larger scale, entire-field treatments are needed to test the efficacy of treatment rotations. Similarly, does skipping spinosad lead to lower resistance both to imidacloprid or spinosad the following year?