How to deal with resistance?

Farmers who grow cabbage, cauliflower and broccoli (plants from the brassica family) are well aware of the diamondback moth, a light green caterpillar, smaller than 1 cm, that attacks these crops. In cabbage, the caterpillars pierce the leaves that form the heads, damaging sales; in broccoli and cauliflower, they lodge in the floral beginnings, causing consumers in fresh markets and processed/frozen products to reject the products. The greatest damage from the pest occurs in times of high temperatures and lack of rain, as these conditions are favorable for the growth of the pest population. In the Central-West region, the most problematic period runs from July to October.

chemical control

Insecticides are the most used products to control diamondback moths. Phosphorus and carbamate insecticides have been used since the 50s and 60s. Pyrethroids began to be used in the early 80s. In the beginning, they were sprayed once every fifteen days. Over the years, farmers noticed that such widely spaced applications were no longer having any effect; They then began spraying every seven days.

Today, pyrethroids are applied up to three times a week, often without success in controlling diamondback moths. As a result, many farmers have abandoned these products and are now using growth-regulating insecticides, which are generally more expensive than pyrethroids. When they were introduced in the Federal District in 1995, applications of growth regulators were fortnightly. Today they are weekly and some farmers think that the efficiency of these insecticides is no longer the same.

Why do insecticides become ineffective? We know that when applying an insecticide, we should not expect a mortality rate of more than 90% of the caterpillars in the crop. The caterpillars that survive are those that were not hit by the insecticide because they were hidden in places where the product did not settle (underside of leaves or inside heads of cabbage), or they are caterpillars that were hit, but due to their genetic characteristics they survived the insecticide. These are called "resistant insects".

If most of the surviving caterpillars are resistant, they will become adults that multiply and, as spraying continues, there will be a greater number of these resistant insects. Over time, the number of insects capable of surviving the insecticide increases so much that farmers conclude that the product is no longer efficient. In an attempt to solve this problem, they adopt one of the following measures: increase the dose of the insecticide; increase the frequency of application; mix products; replace one product with another.

These measures end up worsening the problem. An example of this is the diamondback moth's ability to survive high doses of insecticides, which was found in the Federal District. For deltamethrin, the recommended dose of the product is 0,24 l/ha. When caterpillars were treated with a dose of 0,75 l/ha (three times the recommended dose) none died. When treated with a dose of 30 l/ha (125 times the recommended dose) only 70% of the caterpillars died.

Resistance management

What can be done to delay the emergence of insecticide-resistant insects? Before using an insecticide, farmers must adopt practices that prevent the increase of the pest in crops. Therefore, crop residues must be eliminated and the planting of new crops next to old crops must be avoided.

When using insecticides, the recommended dose must be applied; mixing products should be avoided, to avoid the presence of insects resistant to all products used in the mixture, in a short period of time. To delay the emergence of resistant insects, farmers must use one insecticide at a time, in rotation.

Insecticide rotation

The most commonly used insecticide rotation scheme is to vary the products for each application. This scheme, in fact, ends up enabling the emergence of insects resistant to all the products used quickly. Why does this happen? The diamondback moth takes around 21 days to develop from egg to adult, that is, 21 days to complete a generation. For simplification purposes, this means that for 21 days the insects present in the crop will be the same.

Spraying alternated weekly, with at least two insecticides, means that at the end of 21 days we may have insects resistant to both insecticides applied. These resistant insects will multiply and their descendants will be subjected to the same insecticides again. Some of these insects will also survive and reproduce, further increasing the number of insects resistant to both products. To avoid the problem, where individuals resistant to the insecticides used appear in each generation, rotation must be used so that each product is applied during one generation of the pest (21 days), whether spraying is weekly or fortnightly.

In the next generation (next 21 days) another insecticide, from a different chemical group, must be used. Therefore, insects resistant to the first insecticide may die when in contact with the second. With this type of rotation, it is expected that the number of insects resistant to the insecticide used will remain low and, in the absence of applications, resistant insects will mate with susceptible ones, reducing resistance. The more insecticides from different chemical groups are introduced into the rotation, the longer it will take for resistant insects to emerge.

Insecticide selection

Normally, insecticides are well known by farmers by their trade name and, often, the products to be used in rotation are chosen by their trade name. However, products with different trade names, for example: Ambush, Arrivo, Baythroid, Bulldock, Decis, belong to the same chemical group. In the case of this example, to the chemical group of pyrethroids.

This means that, regardless of the commercial name, these insecticides, as they belong to the same group (pyrethroids), select the population of resistant insects through the same mechanism. Thus, insects resistant to an insecticide from this group will almost always be resistant to other insecticides from the same chemical group. Therefore, the insecticides to be used in rotation to control diamondback moths must be products that belong to different chemical groups (pyrethroids, phosphorus compounds, carbamates, Bacillus thuringiensis, growth regulators). The label shows the chemical group to which the insecticide belongs.

Synchronized use of insecticides

The diamondback moth is a pest capable of flying great distances. In the Federal District, pheromone traps were placed in a central pivot area cultivated with corn and wheat. The closest brassica crop was 12 km away. These traps captured males over a long period, indicating that, transported by the wind, these insects are capable of reaching great distances.

Taking into account the insect's ability to fly, insects resistant to an insecticide can leave their area and reach a new brassica cultivation area (these are immigrant insects). If the arrival location of these immigrants is an area where the farmer will use the insecticide to which the immigrant insects are resistant, low insecticide efficiency may occur in that area, even if the product has never been used by the farmer.

Studies on the actual flying capacity of the diamondback moth and the impact of immigrant insects on the efficiency of insecticides in areas that receive these immigrants are being carried out at Embrapa Hortaliças. Until more details are known about these issues, it is recommended that in the same brassica growing region, farmers seek to introduce a synchronized insecticide rotation scheme, that is, within a period of 21 days, all farmers in the region must use the same product.

Ineffective insecticides

For the insecticide rotation scheme to be successful, it is necessary to eliminate insecticides, which are known not to control diamondback moths, from the insecticide recommendation list. This helps to reduce production costs, as well as to make insecticide rotation programs viable, as there will be no risk of using an inefficient insecticide for 21 days. Ineffective insecticides can be identified in laboratory tests where cabbage leaves are treated with a commercial dose of the product and the caterpillars are fed with these leaves.

Products considered inefficient cause mortality rates below 80-90%, indicating that they should not be used or should be used in a restricted manner and in accordance with local conditions. Test carried out with diamondback moth populations collected in Barroso (MG), Tianguá (CE) and Taquara (DF) showed that the pyrethroid deltamethrin was ineffective in all regions evaluated (larval mortality less than 9%) and Bacillus thuringiensis it was inefficient only in Tianguá (larval mortality of 75%). Therefore, pyrethroids should not be used in the areas evaluated; and B. thuringiensis could be used in the Barroso and Taquara population area.

Currently, a complete mapping of the efficiency of insecticides for the control of diamondback moths is being carried out in the Federal District and inefficient products for each location are being identified.

Marina Castelo Branco

Embrapa Vegetables