July 1, 2002
Vol. 4 No. 6
High-Dose, Low-Dose, No-Hold-Em: Playing the Resistance Management Game
Dr. John Dunley, Associate Entomologist, WSU-TFREC, Wenatchee
Over the past few years, several questions have repeatedly come up regarding the evolution of insecticide resistance and certain resistance management practices used in tree fruit pest management. I would like to address a couple of these, perhaps clear them up, or perhaps raise even more questions that can be addressed in the future.
First, the low dose strategy for resistance management. Several people have asked if using low rates of insecticides actually increases the likelihood that resistance will evolve to that insecticide. The short answer is No. The rate of resistance evolution is dependent on the level of selection for that resistance. Put another way, the higher the ratio of resistant insects to susceptible insects after an insecticide application (a selection, in that the application 'selects' for resistant insects, allowing them to survive while removing the susceptible ones), the faster the increase in resistance. Using lower rates of insecticides tends to allow more susceptible insects to survive by chance, by avoiding the spray, by reducing residues, etc. The more susceptibles that survive, the less frequent the resistant trait in the next generation. Overall, this slows resistance development. Thus, resistance management recommends that the lowest effective dose be used. Other factors also come into play that help slow resistance. Lower rates tend to have less impact on beneficial insects (for much the same reasons as more susceptible pests survive), and the likelihood for biological control increases. Biological control is great for resistance management as the best way to prevent resistance is to not spray. So, should low doses be used in tree fruit pest management? Yes.
Another issue related to rates of insecticide applications and resistance management is the high dose strategy. In general, the high dose strategy attempts to use an insecticide rate that kills all of the susceptible insects, as much of the resistant insects as possible, and most importantly, all of the heterozygotes (those insects with both the susceptible and resistant genes, that tend to be more resistant than susceptibles, and more susceptible than resistant insects). Does the high dose strategy work in tree fruit pest management? No. The high dose strategy has been around for a long time, and has been shown in computer models to have potential as a management strategy. However, there are particular assumptions (conditions) that must be met for the strategy to work. If these conditions are not met, the high dose strategy actually selects for resistance very quickly.
The high dose strategy has recently been implemented successfully in transgenic Bt crops. The same reasons that it works in transgenic crops demonstrate its inappropriateness for tree fruit. Recall that the goal is to kill the susceptibles and the heterozygotes (they have one copy of the susceptible gene, and one copy of the resistant gene), leaving only a few resistant insects (these survivors will have two copies of the resistant gene). There must be very few resistant survivors, and their chances of mating with other resistant survivors must be very low. There must be a "susceptible reservoir" around, such that the susceptibles will move in from outside the treated area and mate with the resistant survivors. All of the offspring from these matings will be heterozygotes, and all of these offspring will be killed by the residue. In order to ensure that the heterozygotes will always die, there must be a constantly high insecticide residue (if the residue decays, some of the heterozygotes will survive, increasing the overall frequency of the resistant gene in the population).
Okay, I realize that my explanation of the high dose strategy is getting too complex, so I will cut to the chase - why it won't work in tree fruit. First, we do not have any insecticides that can maintain a uniformly high dose without decay. BT crops do this by having the plant produce the insecticide. Conventional insecticides decay (otherwise we would not have to reapply them), and you can get an idea of that rate of decay by considering how long we consider the residues "effective" (I would guess the longest are effective for about 14 days). So, we do not meet the requirement of having a constant residue.
The high dose also requires that each generation must have immigration from a susceptible reservoir, such that the few remaining resistants mate with these susceptible immigrants, producing heterozygotes that are all killed, and again leaving very, very few resistants for the next generation, when it will again be repeated. In Bt cotton, they require that at least 5% of the acreage be untreated conventional cotton (with no insecticides applied to control the target pest, essentially "farming" the pest insect, and not harvesting that 5%), or, if they want to control the pest in the susceptible reservoir, they must have at least 20% of the acreage conventional (and with this large proportion of non-Bt cotton, they are allowed to control the pest using insecticides other than Bt). In addition to the myriad of difficulties in managing susceptible reservoirs in tree fruit, you can imagine the difficulty of having to determine who can spray how much in order to maintain susceptible reservoirs, particularly when we have so many small orchards. The negatives outweigh any potential benefits: resistance is likely to move out of treated areas and contaminate susceptible reservoirs, and the high dose reduces the possibility for biocontrol, not only for the target pest, but for other pests as well. The bottom line is, transgenic Bt crops are a unique system where the high dose strategy can work: the plant provides constant residue, the residue does not effect biological control, and the system can be highly regulated through seed sales. I do not believe the high dose strategy can work in tree fruit pest management at this time. Again, the best strategy is to use only as much insecticide as necessary for effective control, and use insecticides only when necessary.
The last question is one that has been asked frequently over the past year: can resistance develop to the insect growth regulators (IGRs)? The short answer, unfortunately, is yes. The development of IGRs for tree fruit pest management is been a big step forward in our IPM programs. These are typically soft on biological control agents, and as effective replacements for more broad-spectrum insecticides, they have greatly increased opportunities for biological control. However, just like any other insecticide, they select for resistance. There are a multitude of examples of tree fruit pests (mostly codling moth and leafrollers) developing resistance to IGRs in Europe, where there are more IGRs available, and they have been used for a longer period of time. In addition, there is evidence here that cross-resistance can occur between Guthion resistant codling moth and leafrollers and some of the IGRs. So, while IGRs are great for IPM, they still must be used wisely to preserve their efficacy, to that they can contribute to pest management for a long time. I repeat: use them only when necessary.
Resistance evolution can be difficult to understand, and resistance management strategies and tactics can be very difficult to implement. Nevertheless, to maintain chemical control tactics in IPM, the near inevitability of resistance must be dealt with. Resistance management, as a component of IPM, attempts to prevent or at least slow the development of resistance. With the recent addition of several new insecticides to the IPM toolbox, please consider resistance management in your IPM decisions.
(Wal)nuts to You, Codling Moth! Mating Disruption in California Walnuts
As a reader of this newsletter you probably know that the Areawide II program is researching many aspects of pheromone-based IPM for apples and pears. What you may not know is that Areawide II is also funding IPM research on walnuts in California and that much of this research is relevant to some of the challenges that Northwest pome fruit growers face.
California leads the nation in the production of many crops, and walnuts are one of them; close to 98% of US production, and 40% of world walnut production, comes from California. Over 200,000 acres are planted to walnuts in the Golden State, producing a crop valued at nearly $280 million in 2000. Most of the acreage is in the Central Valley, with the bulk of production coming from the northern San Joaquin and Sacramento Valleys. Mature walnut trees are large, often maintained at 35' to 50' high. Harvest occurs in September and October, depending on variety and region.
Several pests infest walnuts, of which the codling moth (CM) is the most economically harmful; up to 40% of the harvested crop can be damaged with severe infestations. The acreage in the warmer districts of the San Joaquin and Sacramento Valleys is especially at risk from CM. California walnuts face three full generations of codling moth, and a partial fourth can develop in some regions. First generation CM larvae bore into the blossom end of the nutlets and can cause them to drop. The larvae of later generations can enter the nut anywhere on its surface, but prefer to enter where two nuts touch. The larvae will feed on the meat of the nut, causing contamination and fungal growth. Codling moth damage facilitates the entry of another lepidopteran pest, the navel orangeworm.
Walnut pest management has relied upon broad-spectrum insecticides, especially organophosphates (OPs). Codling moth management involves one to three sprays per year. Chlorpyriphos (Lorsban) is the most widely used OP, although phosmet (Imidan), methyl parathion (Penncap-M) and azinphosmethyl (Guthion) are also used. Insecticide costs are typically $60-100/acre per season.
OPs have been a central part of walnut pest management, as they have been in Northwest apples, but there are increasing problems associated with their use. CM resistance to OPs has been well documented. The use of OPs has led to outbreaks of secondary pests in walnuts by disrupting biological control. Spider mites, aphids and scale insects are often well controlled by natural enemies, but OP use can result in extra insecticide applications for these pests. OPs have been detected in waterways in the Central Valley, which is a concern with drinking water contamination and may lead to further use restrictions. Finally, the Food Quality Protection Act and California state regulations are leading to the further restriction and elimination of many OPs, so growers of walnuts (and apples and pears, among other crops) need to be prepared with alternatives.
What are the alternatives? Other pesticides are registered for CM control, and all have advantages and limitations. Pyrethroids, such as Asana and Pounce, may provide adequate CM control but are even more disruptive of biological control than OPs. Three insect growth regulators are registered on walnuts: tebufenozide (Confirm), pyriproxifen (Esteem) and diflubenzuron (Dimilin). All are minimally disruptive of biological control but are relatively weak CM insecticides and are suitable only in low to moderate pressure situations. Methoxyfenozide (Intrepid) may soon be registered on walnuts, and would provide a more active version of Confirm. Several neonicotinoid insecticides with CM activity, acetamiprid (Assail) and thiacloprid (Calypso), may also soon be available to walnut growers. Experience with these materials will show whether they, too, may disrupt biological control.
Mating disruption of codling moth (CMMD) may be a good option for many walnut growers. Walnut orchards are grown on level ground and have thick canopies, two factors that help to maintain the pheromone within the orchard. Low levels of CM infestation can be economically tolerated in walnuts, so control doesn't need to be quite as close to 100% as in apples. CMMD can supplement the less disruptive and less robust CM insecticides like the IGRs and neonicotinoids. Research by University of California scientists, particularly Drs. Bob Van Steenwyk and Steve Welter, has repeatedly shown that using CMMD dispensers can contribute to effective CM control.
However, CMMD has been only slowly adopted in California walnuts. In 2000, it is estimated that 263 acres were treated with CMMD out of the close to 120,000 acres at risk from CM in the state. Walnuts pose a particular problem to the use of hand-applied CMMD dispensers. Dispensers need to be placed in the upper reaches of the tree canopy and the height of the trees (30' or more) requires the use of man-lifts instead of application poles or even ladders, a slow and costly procedure. The large walnut canopies require the use of high rates of pheromone dispensers (400/acre or more). The hot summers and long growing season may result in the need for a second application of dispensers, making this approach less economical. The logistical problem with hand-applied dispensers in walnuts has been a key reason for the push to develop newer mating disruption technologies for walnuts.
Research in California, funded in part by the Areawide II program since 2000, is focusing on evaluating alternative pheromone delivery technologies for mating disruption. Large scale testing and implementation efforts using pheromone mating disruption have been led for the past 4 years by the Pest Management Alliance, which includes a team of cooperative extension advisors Carolyn Pickel, Terry Pritchard, Walt Bentley, Bill Olsen, Janine Hasey, Joe Grant, and Rick Buchner), USDA (Doug Light), UC Berkeley (Welter, Cave, and Van Steenwyk), CAFF (Community Alliance with Family Farmers), the Walnut Advisory Board, and the California Department of Pesticide Regulation. Replicated treatments were established from Tehama County down to Fresno County in a range of testing conditions.
The long-term strategy is to help develop the newer technologies, establish their efficacy and optimal usage, and then transition into an implementation phase in years 3-5. Dr. Welter, along with Frances Cave and Matt Singleton, are evaluating two pheromone delivery approaches: aerosol emitters ("puffers") as single high-dose pheromone sources, and sprayable formulations of microencapsulated codling moth pheromone.
The puffers are made by Suterra (formerly Consep) and are still being tested on an experimental basis at densities of one puffer per 1-3 acres, with one can of pheromone lasting an entire season. Current implementation designs are based on the estimated effective plume sizes developed from mark-release-recapture studies using sterile codling moths. The 2001 plots were selected to represent a range of codling moth pressures, from the low levels typically found in commercial orchards to very high populations in marginally managed or abandoned situations. Puffers were placed in a line or "curtain" perpendicular to the wind to suppress downwind CM populations in several walnut orchards as part of an effort to define effective pheromone plume size. Results varied between orchard depending on pest pressure and the size of the orchard and the distance between the control and test plots. Excellent control was achieved in some locations, but orchards with high pressures were quickly overwhelmed. Studies in 2001 were not designed to provide commercial control, but rather to test the effective areas of suppression from the puffers.
Both 3M and Suterra have provided the sprayable pheromones. Conventional spray equipment is used to apply these materials. After establishing in walnuts the efficacy of available mating disruption products in 1999 and 2000, good results were obtained in 2001 using monthly applications of a Suterra formulation of the codling moth pheromone. Plots of sprayable pheromone were established in a number of commercial walnut orchards, with grower standard and control plots nearby. Both sprayable pheromone formulations in UC-Berkeley and Pest Management Alliance Trials provided adequate damage suppression in low-pressure situations. However, these programs also failed in high-pressure situations, similar to other mating disruption approaches. Research using coordinated, replicated trials in five different locations in California walnuts in 2002 is focusing primarily on defining adequate rates as well as some comparison between the 3M and Suterra products. Use of the new kairomone lure (DA) has provided enhanced trap capture and better tracking of flights in pheromone treated orchards compared to standard pheromone lures in 2000 and 2001.
Coinciding with the University of California research is a large-scale walnut IPM implementation project, centered in the San Joaquin Valley. The Center for Agricultural Partnerships (CAP) has initiated an effort to increase the use of CMMD by the state's walnut growers, with a goal of having close to 25,000 acres using the method by 2004, the third field year of the project. 2002 is the first year of field implementation of CMMD in this project, with about 1000 acres involved. Cooperating in this effort are growers and their organizations, UC researchers and farm advisors, crop consultants and the Walnut Pest Management Alliance program. This project, led by Pat Weddle, is encouraging the adoption of not only CMMD but also several of the new, more selective insecticides. It is also facilitating the sharing of pest monitoring data and working with the DA lure to validate the effectiveness of CMMD in commercial walnut production.
Walnuts and apples are dissimilar crops but share a potentially serious codling moth problem. The research and implementation projects in California complement the efforts underway in Northwest apples and pears. Much of what is learned in the work in the Golden State's walnuts will be relevant to the changing IPM picture in Western pome fruits. The coordinated efforts of tree fruit and nut entomologists in three states under the Areawide II umbrella are providing timely help for growers adapting to pest management challenges.