Why a product that used to work no longer does — resistance and rotating product groups

A problem farmers often see is that a product that used to work well starts failing after spraying, so they keep raising the rate until in the end it does not work at all. This is resistance.

How does it happen?

In a large pest population, a few individuals happen to tolerate that substance naturally. When the same substance is sprayed again and again, the ones without tolerance all die and only the tolerant ones are left to breed. The next generation therefore has a higher proportion of tolerant individuals. Repeat this many times and the whole field's population becomes tolerant to that substance.

We do not make insects stronger. We just select the ones that were already strong to survive.

The most common misunderstanding

Switching brands is not switching substances. Two brands with the same common name are the same substance. And even with different common names, they may belong to a group with the same mode of action, so insects resistant to one will also be resistant to the other.

The group number on the label

Labels of insecticides, fungicides and herbicides carry a mode-of-action group code. This code tells you which system of the target organism the substance acts on.

Rotation that really works means rotating to a "different group", not a different brand. If you use one group this round, the next round should use a different group.

What helps besides rotating groups

Use products only when needed. Spraying by the calendar without checking whether pests are really present speeds up resistance with no benefit.

Use the rate stated on the label. Using less than the rate lets more moderately tolerant individuals survive, which directly speeds up resistance.

Spray thoroughly. Spots the droplets do not reach are refuges for pests, which become the breeding ground for the next generation. This is an advantage of drone spraying, because propeller downwash helps carry droplets down to the lower layers and under the leaves better than spraying that relies on pressure alone.

Use other methods as well. Field management, biologicals and conserving natural enemies all reduce the number of rounds that depend on chemicals, which is the most effective way to slow resistance in the long term. Important note This article explains general safety principles. It is not a recommendation to use any particular chemical. Mixing rates, the crops a product may be used on and the waiting periods required differ by product and change with government notifications. Always treat the product label as the authority, and consult an agricultural specialist or our staff before deciding to use it.