Rotation Helps Prevent Resistance

Why is everyone involved in crop protection, from extension personnel to consultants to chemical company representatives, so fixated on product rotation? Why do pesticide labels suggest you use other products instead of one repeatedly? The answer to those questions is simple ('it helps prevent resistance') but requires some explanation.

Creating a resistant population

If I wanted to develop a local population of insects (or a disease) that was resistant to a particular chemical, I would spray the same product over and over again. I might increase the application rate and frequency as well, just to increase the likelihood of only resistant individuals surviving. Hopefully, that hasn't been your regular strategy for pest control.

A certain portion of any pest population will be resistant to the chemical you spray. Since they survive the first application, they will form a slightly higher percentage of the genetic heritage of the next generation (not the whole population - don't forget about immigration of pests from elsewhere and that not every individual is exposed to a particular spray). The next spray will kill the susceptible individuals and the now more numerous resistant ones will live to reproduce. Keep spraying the same product and gradually less control will be achieved as more resistant individuals survive and breed to create the next pest generation. However, if your goal was to create a resistant pest population, you're well on your way! This isn't always as simple as it sounds - it can take many years and many generations for resistance to develop to the point where the product no longer provides control. Also, some pest populations develop resistance faster than others.

Chemical families

Here's where things get a bit more confusing. Pesticides can be grouped into 'families' based on their active ingredient. I could change to a different product but if the new product is of the same chemical 'family' or group, the results will likely be the same as spraying the original product again if the pests have developed 'cross-resistance'. Cross-resistance allows a pest to survive exposure not only to the chemical that originally produced the resistance but also to related compounds. For example, if a pest is resistant to Cymbush (a pyrethroid), it might be cross-resistant to Decis (also a pyrethroid). To break this cycle, you need to rotate to a product that is in a different chemical family. A good place to determine the chemical family of a product is a chart called 'Pesticide Tables' in OMAFRA Publication 360 - Fruit Production Recommendations. Sometimes switching to a different chemical family or group still does not provide control of a pest population. This situation is called 'multiple resistance' and is the worst case scenario for resistant populations. Multiple resistance can extend to two or more classes of pesticides with different modes of action.

Case Study

In Niagara peach orchards, oriental fruit moth (OFM) (Grapholita molesta) reached epidemic proportions in 1992-93 resulting in significant crop losses (some blocks were not even worth picking). A common pest control program for at least 25 years, primarily of Imidan (phosmet, an organophosphate), led to widespread organophosphate-resistant populations. In response, a pesticide rotation strategy was developed by Dr. David Pree and others at AAFC and was widely adopted in the industry. Lorsban was recommended for first generation control and pyrethroids were recommended for subsequent generations. This rotational strategy has served the industry well for 9 years without serious outbreaks of OFM. However, this is ultimately an interim strategy and the industry must set its sights on preventing pyrethroid resistance from developing. New strategies of control such as mating disruption will help to lengthen the useful life of effective control products by reducing the exposure of pest populations to insecticides.

Other steps to help prevent development of resistant populations

Use insecticides and fungicides only when necessary rather than on a routine, calendar-determined basis. Many diseases require preventative rather than corrective sprays; even so, paying close attention to weather conditions and biological factors can help reduce the frequency of sprays and fine-tune their timing. Field monitoring and regional agriphone messages are key components of this strategy.

Using pesticides based on monitoring instead of a calendar will reduce the selection pressure on both the target pests and on secondary pests that can become major problems if they develop resistance.

Thorough monitoring will also tell you where treatments are necessary for relatively non-mobile pests when immigration of the pests from elsewhere is not a major factor. For these cases you can relieve the selection pressure on your entire acreage and give your beneficial insects and mites a break by only targeting areas that have damaging levels of pests.

Alternate products from different families before you have a resistance problem, and replace materials if control failures occur. Avoid extremely persistent pesticides. Apply all pesticides at label rates - resistance can develop with repeated exposure to sublethal doses. Calibrate your sprayer and ensure you are getting good coverage, especially on large trees.

How do you know if resistance is developing?

To test if a fungal disease is resistant to a particular chemical, pathologists take samples of the disease from the field and expose them to fungicides in the lab or in controlled greenhouse experiments. If the rate required to kill the fungus is higher than the level needed in the past or higher than needed to kill a known non-resistant strain, then the pathogen is resistant.

Determining if a population of insects is resistant can also be time-consuming and expensive; insects must be collected from the field and exposed to pesticides in the lab. Determining the correct doses and comparing mortality with known susceptible insects requires the efforts of skilled researchers using specialized equipment.

The bottom line is that without some careful tests, you cannot know if a control failure is due to resistance or some other reason.

Failure of a particular product is not proof that resistance has developed. Failure of a product can happen for many reasons. Don't jump to conclusions if an insect or disease appears to be getting out of control, but if you have not been following the recommendations for preventing resistance, start immediately. Make sure your timing is appropriate, calibrate your sprayer, ensure good coverage, check your water quality, make sure that multiple spray products are compatible and that the product is registered for the target pest. If you think that you may have a resistant population, then that's all the more reason to begin best management practices to solve the problem.

In some cases, rotating to different products may be more expensive. New products and technologies, like mating disruption with pheromones, can also be more costly than existing insecticides. However, the costs of dealing with resistant populations of insects can be far greater. Finally, remember that it is easier and cheaper in the long run to work towards preventing resistance from developing rather than trying to manage it after it has developed.

Related Links

• Pesticide Tables, OMAFRA Publication 360, Fruit Production Recommendations