With the introduction of numerous new insecticide/miticide active ingredients as chemical tools for pest management, fruit growers need to adopt an approach to prevent pests from quickly developing resistance to new products.
Resistance Management for Secondary Pests
Often the best resistance management tool for secondary pests that do not feed directly on the fruit (mites, aphids, leafminers, scales, etc.) is to encourage biological control by predators, parasitoids, and pathogens. This not only reduces selection pressure by reducing the number of sprays for that pest, but biocontrol agents can reduce resistance levels by indiscriminately feeding on both resistant and susceptible prey.
Resistance Management for Fruit-feeding Pests
For pests feeding directly on the fruit and for which there is almost no tolerance for fruit injury, pesticides will likely remain the best means of control.
Fruit growers throughout Pennsylvania now have at their disposal some highly effective and selective chemical tools to assist them in their battle against codling moth (CM), Oriental fruit moth (OFM), and the leafroller complex, which includes the tufted apple bud moth (TABM) and obliquebanded leafroller (OBLR). Both CM and OFM have caused serious problems for many growers over the past 10 to 12 years, resulting in the rejection of many loads of fruit destined for both the fresh and processing markets. The leafrollers have been serious pests since the early 1970s. All four of these pests have developed varying levels of resistance to a number of insecticidal chemistries over the years, including the organophosphates, carbamates, and pyrethroids. Additionally, spider mites and pear psylla have outstripped all other fruit pests in their ability to develop resistance to pesticides of multiple classes in only a few short years and deserve special consideration for resistance management. With the introduction of more than thirty new insecticide/miticide active ingredients as chemical tools for pest management, fruit growers need to adopt an approach to prevent this group of pests from developing resistance quickly to these new products.
Mechanisms of Insecticide/Miticide Resistance in Tree Fruit Pests
Target site change
This has really only been shown in mites and pear psylla in our fruit crops, but it is more common in other pests, e.g., on cotton. It involves a modification of the action site in the pest to make it less sensitive to the pesticide as in the classical studies with the neurotoxic organophosphate insecticides and changes in the neurotransmitter acetylcholine. Many of our older miticides worked by inhibiting the mitochondrial electron transport (METI) system that governs cellular respiration. Changes to this system led to widespread resistance to products like Pyramite, Nexter, Portal, and Fujimite within only 3 to 5 years. Alternating products with differing modes of action is usually an effective strategy with this type of resistance.
This consists of several mechanisms in tree fruit pests where mixed-function oxidases (MFOs) are the most important, e.g., in leafminers. (Resistance in most lepidopteran pests like codling moth and leafrollers is thought to be mostly based on a group of esterase enzymes.) These enzymatic systems are thought to have evolved in insects and mites to deal with toxic compounds in plants while feeding and have only relatively recently been utilized to also chemically break down pesticides. Predators and parasitoids who eat pests that have already broken down these plant poisons do not have extensive enzymatic detoxification systems in general and are much less likely to develop resistance to pesticides for this reason. Since enzymatic systems efficiency is based on the type of chemical bond in the pesticide, mode of action of the pesticide has little to do with its susceptiblity to breakdown.
Resistance to pesticides of different classes has been shown in our leafrollers where the esterase enzymes that broke down azinphos-methyl also broke down the insect growth regulator Dimilin before it had ever been used because of similar chemical bonds between the two compounds, despite completely different modes of action. Alternatively, leafrollers that were resistant to the OP insecticide azinphos-methyl were not resistant to chlorpyrifos, which is also an OP with the same mode of action, because its molecular bonds were not susceptible to the same enzymes that broke down azinphos-methyl.
To a lesser extent glutathione-S transferase enzymes are also important in breaking down pesticides in some fruit pests. Pear psylla is such a potent adversary for pesticide resistance because it not only has target site resistance, but also all the types of enzymatic resistance. Synergism of pesticides with products like piperonyl butoxide (PBO) or some sterol-inhibitor fungicides like Rally occur when these nontoxic compounds bind up the enzymatic systems of resistant pests to allow the pesticide to work uninhibited.
Use of the New Chemical Tools in a Resistance Management Approach
How should growers use these new products within the same season? Much is made of Insecticide Resistance Action Committee (IRAC) codes for rotating pesticides of differing modes of action, but this is really most effective on pests where target site modifications are the main source of resistance. As outlined above, rotations of pesticides with differing modes of action is less effective when enzymatic detoxification is the main source of resistance. It is, however, the best place to start as pesticides of differing classes often have greatly differing chemical structures. To be truly sure of a good resistance management rotation, however, routine bioassays of field populations of pests should be conducted to determine if their efficacy is holding.
According to the IRAC mode of action classification, Altacor, Besiege, Exirel, Minecto, and Voliam Flexi have a similar mode of action (i.e., ryanodine receptor modulators), thus they belong to Group 28. Delegate has a totally different mode of action (i.e., nicotinic acetylcholine receptor allosteric activators), thus it belongs to Group 5.
Second, all of these products are highly active against CM, OFM, and the two major leafrollers, TABM and OBLR. Since five of the products have a similar mode of activity (i.e., they are chemically related) and we want to delay the onset of resistance to either group for many years, we highly suggest that they be used only against a specific generation of a pest(s) and that the grower rotate to other compound(s) or to other chemical classes (e.g., Group 1--organophosphates and carbamates, Group 4--Nicotinic acetylchlorine receptor agonists [Assail, etc.]) of compounds to control the next generation of the targeted pest(s). For example, if a grower decides to use Delegate (Group 5) during the first generation of CM flight and egg hatch, then he/she should rotate to products in Group 28 or some other nonrelated group for the second-generation flight of CM. The same approach applies to OFM and the leafrollers.
A new trend in pesticide companies is the use of prepackaged mixtures to make a selective product more broad spectrum--in a sense, making "a new azinphos-methyl." While this may at first give some level of synergism (not always proved), it can lead to the creation of a "super bug," which develops enzymes that break down both types of pesticides. For resistance management, the most current thought is to rotate pesticides rather than to mix them in each spray for this reason. The use of premixes can also mean loss of biocontrol of mites, aphids, and scales, and they are not always appropriate for each application since only one type of pest (e.g., codling moth) might be present. While premixes may be convenient when applying a spray for both codling moth and brown marmorated stink bug, growers may lose some flexibility in what they want to apply.
All the products listed above are excellent and highly effective insecticides, and some have activity against other pests as well. In order for these chemicals to be used most effectively, it is extremely important that growers achieve thorough coverage since the pests must ingest many of these products for them to be effective. If thorough coverage is not achieved, the desired level of control will not be attained. Many of these products are fairly residual in their activity, but growers should not stretch the period between sprays too long. And, finally, if your pest management plans call for using both of these chemical groups, use one chemical group for a single generation of a pest and then rotate to the other chemical group or some other unrelated group for the next generation. Do not use any of these products for consecutive generations of a targeted pest within the same growing season.
We highly encourage growers to use these products carefully and sparingly so we do not lose them to pest resistance in the future.