Growers are increasingly using insecticide mixtures of at least two active ingredients to enhance pest control. These include both on-farm tank mixtures and commercial co-formulated products.

Mixtures can be effective in several ways:

• by adding a low toxicity compound to boost the efficacy of the main active ingredient
• by combining insecticides that target different life stages of a pest
• or by targeting a complex of crop pests that vary in their sensitivity to different active ingredients.

If the chemicals within a mixture affect different target sites, the likelihood of two (or more) mutations being present simultaneously is extremely low and so should reduce the rate at which resistance evolves.

But mixtures can lead to overuse of insecticides. They are less flexible than single insecticide applications, making it difficult to target the right dose for the right pest at the right stage in the life cycle and specifically when that pest exceeds an appropriate economic threshold.

Genetics at play

In some cases, insecticide resistance will be suppressed by the use of mixtures. For instance, when resistance to insecticide A is based on a single recessive gene, the rare resistant homozygote (individuals carrying two resistant alleles) that are not controlled can be targeted by mixing with insecticide B, provided it has a different Mode of Action (MoA).

However, these assumptions are often too simplistic and do not consider changes in genetic dominance as insecticides decay over time. For example, one dose of insecticide may kill a susceptible pest (either a homozygote with two dominant susceptible allele and one recessive resistant allele), whereas a low dose may allow the heterozygote to survive.

The mixture strategy also relies on having the background information on the frequency of resistance alleles in a population – something that is generally not available until resistance is first identified.

Increased risk

There are several ways insecticide mixtures can increase the rate at which resistance evolves. Mixtures may more rapidly select for mechanisms of resistance that are common to both insecticides, such as various mechanisms of detoxification that are common in insects and mites.

Spray applications of insecticide mixtures containing two products with quite different decay times (for example, chlorpyrifos and lambda-cyhalothrin) are likely to increase the risk of resistance evolution, particularly when resistance alleles tend to be dominant and when there is resistance to multiple MoA groups.

In gran crops, this seems likely for green peach aphid and cotton bollworm populations, and to some extent for redlegged earth mite. The situation for diamondback moth remains unclear.

When mixtures are used to target more than one pest, they are more likely to lead to unnecessary and inappropriate applications because economic thresholds of one pest might not have been reached or the timing of the application might not be optimal.

Mixtures can also be harmful to beneficial insects, which then provide less subsequent control than they would normally. For instance, the fungicide mancozeb is particularly harmful to predatory mites when combined with chlorpyrifos.

Resistance-management strategies generally aim to reduce the number of different active ingredients used in a particular spray window. This means that mixtures can reduce options in later windows because multiple actives have already been used.

The most effective approach to managing insecticide resistance is rotation, not mixtures. Mixtures need to be justified carefully alongside single active products. Mixtures may be warranted when multiple pests are present at high levels and at particular times of the year, but this information should be established prior to a decision being made.

More information: Professor Ary Hoffmann, 0408 342 834.