SELECTIVE PESTICIDES AND BIOLOGICAL CONTROL IN WALNUT PEST MANAGEMENT

Transcription

1 SELECTIVE PESTICIDES AND BIOLOGICAL CONTROL IN WALNUT PEST MANAGEMENT N.J. Mills, C. Pickel, and J. Grant ABSTRACT In recent years we have also seen dramatic changes in the pest management tools in walnuts driven by Food Quality Protection Act -mandated reductions in organophosphate (OP) use, and the increasing costs of replacement pesticides. Codling moth as the key pest of walnuts has traditionally been controlled by multiple sprays of OPs applied per season. While the development of new approaches to the use of pheromone-based mating disruption shows promise as an alternative to reliance on insecticides, new reduced risk insecticides are replacing the OPs. Disruption of the biological control of secondary pests by natural enemies through use of these new reduced risk products can lead to reliance on pesticides for suppression of secondary pests such as the walnut aphid Chromaphis juglandicola. The goal of this project is to screen new pesticides to identify selective products that will enhance the level of biological control of walnut aphid in orchards as part of a larger western regional USDA- SCRI project ( From bioassays of currently registered and new pesticides in walnuts, it appears that applications of the more traditional OPs and the pyrethroids in will not only kill the parasitoid T. pallidus, but most of the walnut aphids also. While Warrior I is not as effective against walnut aphids as Warrior II it is also more compatible with T. pallidus and selective against the hyperparasitoid S. aphidivorus and thus aphid outbreaks are unlikely from the use of this product. Similarly, neither of the new diamides is likely to be disruptive, as Altacor is neither effective against the aphids nor disruptive to T. pallidus, while Cyazypyr kills both effectively. Of the other pest management products tested, both Success and Delegate have some potential to be disruptive as they are not particularly effective aphid materials but show strong acute toxicity to T. pallidus, with Success also being acutely toxic to the hyperparasitoid S. aphidivorus. Omite also has the potential to be disruptive to the biological control of walnut aphid due to its acute effect on T. pallidus and limited effect on the aphids. The two IGRs, Intrepid and Rimon, appear not to be disruptive from their acute effects, but the latter may prove to be disruptive through sublethal impacts on T. pallidus. Results from the USDA-SCRI project suggest similar acute toxicities for other orchard natural enemies, and suggest that plant-based volatiles identified so far can effectively be used to monitor the activity of a range of key natural enemies in orchards, such as parasitoids, syrphids and green lacewings. California Walnut Board 219 Walnut Research Reports 2009

2 INTRODUCTION In recent years we have also seen dramatic changes in the pest management tools in walnuts driven by Food Quality Protection Act -mandated reductions in organophosphate (OP) use, and the increasing costs of replacement pesticides. Codling moth as the key pest of walnuts has traditionally been controlled by multiple sprays of OPs applied per season. While the development of new approaches to the use of pheromone-based mating disruption shows promise as an alternative to reliance on insecticides, new reduced risk insecticides are replacing the OPs. The latter are likely to remain an important tool in the management of codling moth, but their impacts on natural enemies are poorly understood and often much greater than originally thought. Information developed by participants of a western regional USDA-SCRI grant suggests that we may be creating unstable management programs because of the negative effects of some of the newer insecticides on natural enemy populations. Disruption of the biological control of secondary pests by natural enemies through use of these new reduced risk products can lead to a reliance on pesticides for suppression of secondary pests that is costly both economically and ecologically, is detrimental to worker safety, and goes against the trend that consumers are becoming increasingly concerned about how food is grown. To remain competitive, stable IPM programs that effectively conserve natural enemies must be developed as even a small increase in natural enemy induced-mortality significantly lowers pest pressure. The walnut aphid, Chromaphis juglandicola (Kaltenbach) has been known in California for more than 100 years. When present in large number in the spring, aphid feeding reduces tree vigor, nut size and quality. During the summer, it induces a shriveling of the kernels before harvest. Extremely high populations of aphids may lead to leaf drop, exposing nuts to sunburn. The introduction of the parasitic wasp Trioxys pallidus (Halliday) from Iran in 1969 led to a dramatic success in the biological control of walnut aphid populations in California, and has provided sustained control of this devastating pest for the past 39 years. However, both growers and PCAs have noted that aphid outbreaks, though not consistent, are of increasing concern in the Central Valley requiring in season spray treatments required in some cases. As pest management practices in walnuts change to both reduce costs and make use of new pesticides, it is important to retain the benefits of the long-term biological control of walnut aphid provided by the introduction of T. pallidus. The goal of this project is to screen new pesticides to identify selective products that will enhance the level of biological control of walnut aphid in orchards as part of a larger western regional USDA-SCRI project ( Other collaborators in the larger project will be screening a broader range of natural enemy species to build a more comprehensive picture of the selectivity of new reduced risk pesticides that could disrupt the natural enemy control of other pests such as codling moth and spider mites. An additional benefit of the participation in the larger regional project is that the latter is to develop the use of herbivore-induced host plant volatiles as an effective tool for growers and PCAs to monitor the activity of natural enemies under different pest management practices. Pesticides that are currently or likely to be registered for use in walnuts in the near future will be bioassayed using both acute and sublethal bioassays to determine their impact on walnut aphid, T. pallidus and its dominant hyperparasitoid Syrphophagus aphidivorus, and Hippodamia convergens. OBJECTIVES California Walnut Board 220 Walnut Research Reports 2009

3 The general objective is to better understand the impact of new pesticides used for the management of primary pests in walnuts on the walnut aphid system, one of the best examples of an effective biological control in western orchards. The specific objectives are: 1. To compare the susceptibility of white and yellow walnut aphids to walnut pest management products using simple laboratory bioassays. 2. To assess the differential susceptibility of T. pallidus and S. aphidivorus to walnut pest management products using simple laboratory bioassays. 3. To use a demographic bioassay to test a subset of pest management products for their compatibility with and enhancement of biological control of walnut aphid. PROCEDURES Objective 1. To compare the susceptibility of white and yellow walnut aphids to walnut pest management products using simple laboratory bioassays The most effective way to compare the susceptibility of aphids to a variety of pest management products is to screen them through standard laboratory bioassays. From an earlier comparative study of bioassay techniques it was found that rearing 3 rd instars on treated leaf disks provided the most accurate and useful information on susceptibility of tree dwelling aphids to a variety of insecticides. Initial trails using walnut aphids on leaf disks showed that this particular aphid is too active and would not settle on the disks for a long enough period to be assayed in this way. As an alternative, we used whole walnut leaflets with clip cages to encourage settlement of the aphids. Using this approach, walnut leaflets were treated by being dipped for 3 secs into either a full strength solution (100% field rate) or a dilute solution (10% or 25% field rate) of each pesticide or into water as a control. To ensure effective coating of the leaf surfaces a surfactant (Agral 90) was added to both the insecticide solutions and the water control at a uniform concentration of 0.25ml/liter. Initial trails showed that this concentration was sufficient to give a uniform coating without having any impact of the survivorship of the walnut aphid. For each pesticide and aphid color morph a set of 8 treated leaflets were used for each of the three treatments (100%, 10/25%, and control). After drying, each leaflet was inserted into a vial of water with a cotton wool stopper to maintain the turgor of the leaflet, and inoculated with 10 3 rd instar aphids of one of the two color morphs. The aphids were then enclosed in a ventilated clip cage prepared from a 3cm diameter diet cup and the leaflet placed in a plastic sandwich box in an incubator at 22 C and 16h daylength. The survival of each color morph of the aphid after 48h was used as the endpoint for the assay. Isofemale lines of individual clones of both color morphs of the walnut aphid were collected in 2008 from a single orchard in Durham in the Sacramento Valley and were maintained in a glasshouse at UC Berkeley on potted walnut seedlings. Products tested included those currently registered in walnuts either for codling moth, husk fly or aphids including Guthion (as a historical baseline), Imidan and Lorsban (OPs), Asana, Brigade and Warrior (pyrethroids), Altacor and Cyazypyr (diamides), Agri-Mek, Delegate and Success (microbial metabolites), Intrepid and Rimon (IGRs), Omni oil (organic), a commonly used product for spider mites Omite California Walnut Board 221 Walnut Research Reports 2009

4 (acaricide), and a widely used product for walnut blight Kocide-Manzate (fungicide). The full field rates of these products are shown in Table 1. Acute toxicity was estimated from the resulting survivorship after 48h. The survivorship data from the full field rate and dilute rate were corrected for control mortality using Abbotts correction and the 95% confidence intervals calculated from the 8 replicates of each. Objective 2. To assess the differential susceptibility of T. pallidus and S. aphidivorus to walnut pest management products using simple laboratory bioassays Laboratory assays were used to determine the effect of pest management products on the survivorship of adult Trioxys pallidus and Syrphophagus aphidivorus exposed orally and to surface residues. The same three treatments for each pesticide, as used in the walnut aphid assays, were used to assay the adult parasitoids (Table 1). Glass vials (8 x 2.5 cm) were filled with pesticide solutions and then drained and dried to provide a consistent residue on the inner surface. Similarly, small droplets of a honey-sugar-agar-water mixture ( ratio) were used as food for the adult parasitoids and treated by dipping in insecticide solutions to gain a surface coating of residue. A male and female pair of 1-2 day old adult parasitoids were placed into a treated glass vial, provided with treated food, and held at 22 C and 16h daylength. From replicate vials were used for each parasitoid species and each pesticide treatment, and adult survivorship was monitored after 48h. The acute mortality of parasitoid adults in each replicate vial was analyzed separately for the two parasitoid species and survivorship data from the full field rate and dilute rate were corrected for control mortality using Abbotts correction. Following Dorfman et al. (1980, Regulating Pesticides, NAS Press), 95% confidence intervals were calculated from a second order approximation of the maximum likelihood variance. Objective 3. To use a demographic bioassay to test a subset of pest management products for their compatibility with and enhancement of biological control of walnut aphid. This component of the project is currently underway and will be reported on in the future. RESULTS Objective 1. To compare the susceptibility of white and yellow walnut aphids to walnut pest management products using simple laboratory bioassays The acute toxicity of walnut aphids to the various products tested in the leaflet bioassays are presented in Fig. 1. The full field rate (100%) of the OP insecticides killed all 3 rd instar nymphs of both color morphs with the exception of Imidan which killed only 90% of the white color morph. Lorsban was more effective than the other two OPs in killing walnut aphids in the dilute treatment (25% field rate). All three pyrethroids also showed a high level of acute toxicity to both color morphs of the California Walnut Board 222 Walnut Research Reports 2009

5 walnut aphid in these bioassays, with the white color morph being particularly susceptible even at the 25% field rate. Of the two new diamides, Altacor showed little effectiveness against walnut aphid with both rates killing approximately 50% of both color morphs. In contrast, Cyazypyr proved more effective with both rates killing more than 97% of the aphids of both color morphs. Among the microbial metabolites, Agri-Mek killed 100% of both color morphs of the aphids at even the dilute (25%) rate, but the two spinosyns were less effective and their effect was independent of dilution rate. Delegate killed approximately 90% of the aphids while Success killed about 55%. The two IGRs, Intrepid and Rimon, had no acute effect on either color morph of the walnut aphid, as might be expected for a more selective class of insecticides. As an organic treatment, Omni oil killed approximately 90% of the aphids, but rather less of the yellow morph at the dilute (25%) rate. Surprisingly, at a full field rate, the acaricide Omite had an acute effect on the yellow morph of the aphids, as much as seen with Altacor. Then finally, walnut blight sprays of Kocide-Manzate were found not to have any impact on either color morph of the aphids as might be expected. Objective 2. To assess the differential susceptibility of T. pallidus and S. aphidivorus to walnut pest management products using simple laboratory bioassays The acute toxicity of these same products to adult T. pallidus exposed orally and to surface residues is presented in Fig. 2B. The two OPs, Guthion and Lorsban, both showed acute toxicity to T. pallidus with 100% of the adults killed within 48h even for the 25% field rate. In comparison to the OPs, the two pyrethroids insecticides did not show quite the same level of toxicity. In the case of Warrior, the original formulation of Warrior I showed little acute effect even at the full field rate, but the newer formulation of Warrior II showed a strong effect at the full field rate and killed almost 60% within 48h at a 10% dilution. The acute toxicity of Asana was very similar to that of Warrior II. This result for Warrior I is perhaps surprising for a pyrethroid, but is consistent with field observations made in 2004 that T. pallidus could be seen actively searching walnut foliage treated one week earlier with Warrior I. Of the two new diamides, Cyazypyr showed acute toxicity to T. pallidus killing 80-90% within 48h at both the full field rate and at a 10% dilution. In contrast, Altacor showed no acute toxicity even at the full field rate. The two spinosyns, Delegate and Success, also showed strong acute toxicity, killing 100% of the T. pallidus adults even at the dilute (10%) rate. Of particular note is the fact that Omite showed acute toxicity, killing 100% of the T. pallidus adults at both the full field rate and the 25% dilution. This is a surprising result but consistent with field observations of increasing aphid populations very late in the season after miticide applications. Neither of the IGRs, nor the walnut blight sprays had any significant acute effects on T. pallidus adults. California Walnut Board 223 Walnut Research Reports 2009

6 The bioassays of the hyperparasitoid are still in progress and currently are available only for Lorsban, Asana, Warrior I, Success and Intrepid (Fig. 2A). As might be expected adults of S. aphidivorus showed almost no tolerance to the pesticides tested. The only product to show no acute toxicity of those tested was the IGR Intrepid which showed no effect even at the 25% dilution. The other four products showed complete acute toxicity even though they represented three different classes of insecticide. DISCUSSION As the management of primary pests in walnuts moves away from the traditional broad spectrum OP insecticides to newer reduced risk products, the impacts of these newer products on the stability of integrated pest management becomes an important concern. Evidence suggests that the newer reduced risk pesicides are not always compatible with the natural enemies that contribute very effectively to the management of secondary pests, and in some cases can be equally disruptive as the more traditional OPs. For many of these secondary pests, the consequences of changing pesticide use can be complex, as there can be differential influences of these products at the different trophic levels (pest, primary natural enemy, secondary natural enemy). This is particularly evident in the case of the walnut aphid, a secondary pest which has been very successfully controlled through the importation of an effective primary parasitoid (Trioxys pallidus) from Iran in 1969 which has subsequently established throughout the Central Valley. This primary parasitoid is known to be heavily attacked by indigenous secondary or hyperparasitoids which have the potential to prevent it from providing effective biological control of walnut aphid. From our bioassays, it appears that applications of the more traditional OPs and the pyrethroids in walnuts will not only kill T. pallidus, but most of the walnut aphids also. While Warrior I is not as effective against walnut aphids as Warrior II it is also more compatible with T. pallidus and selective against the hyperparasitoid S. aphidivorus and thus aphid outbreaks are unlikely from the use of this product. Similarly, neither of the diamides are likely to be disruptive, as Altacor is neither effective against the aphids nor disruptive to T. pallidus, while Cyazypyr kills both effectively. Of the other pest management products tested, both Success and Delegate have some potential to be disruptive as they are not particularly effective aphid materials but show strong acute toxicity to T. pallidus, with Success also being acutely toxic to the hyperparasitoid S. aphidivorus. Omite also has the potential to be disruptive to the biological control of walnut aphid due to its acute effect on T. pallidus and limited effect on the aphids. The two IGRs, Intrepid and Rimon, appear not to be disruptive from their acute effects, but the latter may prove to be disruptive through sublethal impacts on T. pallidus. Some disruption has been observed from Rimon for natural enemies in apple orchards in the Pacific Northwest. Substantial progress has also been made through the first year of the western regional USDA- SCRI project on enhancing biological control in western orchards for which this walnut project forms a matching contribution ( Delegate and Warrior II were also acutely toxic to adult western predatory mite Galendromus occidentalis, the predatory bug Deraeocoris brevis and the woolly apple aphid parasitoid Aphelinus mali, while Rimon was of intermediate toxicity and Altacor had low toxicity. Two other important features of the selectivity of pesticides in walnut pest management are the timing of application in California Walnut Board 224 Walnut Research Reports 2009

7 relation to the phenology of natural enemy activity and the development of new tools for monitoring natural enemy activity. In general, we have very little information on patterns of activity of natural enemies through the season due to the difficulty of monitoring natural enemies. It is well known that in response to herbivore attack plants release volatile odors to attract natural enemies and these herbivore-induced plant volatiles (HIPVs) can be developed for monitoring purposes. Results this season from the USDA-SCRI project have demonstrated that a combination of HIPVs can be used to monitor parasitoid, syrphid and green lacewing activity in walnut orchards (Fig. 3). In addition, the seasonal pattern of activity of key natural enemies such as the green lacewing Chrysopa nigricornis can readily be monitored (Fig. 4). These results show four flight periods starting around mid April in California and continuing through to the end of September and that the phenology is very similar on a day-degree basis among sites in both California and Washington. The patterns arising from this work point to important differences between pest management products in terms of their selectivity with regard to walnut aphid, its primary parasitoid and hyperparasitoids. Further testing will be necessary to complete the sublethal bioassyas and field tests will be used to verify these initial observations. However, initial results highlight the complexity of the effects of these pesticides and indicate the importance of understanding these effects for stabilizing IPM in walnuts. Table 1. Commercial name, active ingredient and field rate of products tested in Objectives 1 and 2 Product Class Active ingredient Field rate (100%) Guthion OP Azinphos-methyl 0.75 lb/acre Imidan OP Lorsban OP Chlorpyrifos 16 fl oz/acre Asana Pyrethroid Esfenvalerate 8 fl oz/acre Brigade Pyrethroid Bifenthrin Warrior II Pyrethroid Lambda-cyhalothrin 2.56 fl oz/acre Altacor Diamide Rynaxypyr 4.5 oz/acre Cyazypyr Diamide Cyazypyr 20.5 fl oz/acre Agri-Mek Avermectin Abamectin Delegate Spinosyn Spinetoram 7 oz/acre Success Spinosyn Spinosad 3.2 fl oz/acre Intrepid IGR Methoxyfenozide 18 fl oz/acre Rimon IGR Novaluron 50 fl oz/acre Omni oil Organic Omite Acaricide Propargite 10 lb/acre Kocide-Manzate Fungicide Kocide oz/acre Manzate Prostick 28.8 oz/acre California Walnut Board 225 Walnut Research Reports 2009

8 Fig 1. Acute mortality of 3 rd instar walnut aphids after 48h enclosed in clip cages on walnut leaflets that were dipped in pesticide solutions at 10/25% and 100% of the field rates (see Table 1) Proportion dying in 48h (Abbott corrected + 95% CI) % Yellow 100% Yellow 25% White 100% White Guthion Imidan Lorsban Asana Brigade Warrior II Altacor CyazypyrAgri-Mek Delegate Success Intrepid Rimon Omni Omite Kocide- Manzate Pesticides California Walnut Board 226 Walnut Research Reports 2009

9 Fig. 2. Acute mortality of (A) adult Syrphophagus aphidivorus and (B) adult Trioxys pallidus after 48 h of exposure both orally and to surface residues of pesticides at 10/25% and 100% of the field rates (see Table 1). Proportion dying within 48h (Abbott corrected +95% CI) B Pesticide -0.2 Guthion Lorsban Asana Warrior IWarrior II Altacor CyazypyrDelegate Success Intrepid Rimon Omite Kocide- Manzate /25% 100% Pesticide Lorsban Asana Warrior I Intrepid Success Proportion dying within 48 (Abbott corrected +95%C % % 1.0 A California Walnut Board 227 Walnut Research Reports 2009

10 Fig. 3. Mean response of three key groups of natural enemies to combinations of HIPVs (MS methyl salicylate, Ger Geraniol, PE 2 phenylethanol) in three walnut orchards from July through September Mean per trap per wee Chrysopids Syrphids Parasitoids Control MS Ger Ger+PE MS+Ger+PE Fig. 4. Monitoring the seasonal flight pattern of the green lacewing Chrysopa nigricornis in two walnut orchards in California and three apple orchards in Washington in 2009 using HIPVbaited traps. California Walnut Board 228 Walnut Research Reports 2009