Pesticide resistance is a growing problem for farmers all over the world, and also in Denmark. Pesticide resistance in weeds, insects and fungi is seen more and more frequently in the fields, where the usual remedies no longer apply. One of the reasons for the increasing problems with resistance worldwide is that at the same time as the increasing number of cases of pesticide resistance, there is a continuous reduction in the number of registered pesticides. This reduction is due to an increasingly restrictive approval procedure for herbicides, fungicides and insecticides, which means that companies do not apply for re-registration of products when data is lacking to meet current requirements. Where in the past it has been possible to eliminate resistance by switching to another pesticide, this is in many cases no longer possible. 

In a report "Update of note on status of and development in weeds, fungi and pest resistance to pesticides" to the Danish Environmental Protection Agency, senior researcher Solvejg K. Mathiassen, senior researcher Lise N. Jørgensen, and associate professor Michael Kristensen, all from the Department of Agroecology the development of resistance of the most common weed species, fungi, and pests in Denmark to the marketed herbicides, fungicides, and insecticides. 

"The note sheds light on how pesticide resistance has developed in recent years in weeds, fungi, and insects," explains Solvejg K. Mathiassen. She does not think the development is alarming, but it is important that agriculture focuses on reducing the risk of resistance development by integrating prevention, mechanical and chemical control in the plant protection strategy. In this way, the effect of the pesticides can be preserved for as long as possible.

2016 formed the baseline

The Danish Environmental Protection Agency's pesticide strategy states that the development of resistance must be monitored. Therefore, in 2016, a status was established for the spread of herbicide resistance, which describes the situation before the Danish pesticide tax was introduced. It also appears from the pesticide strategy that after a number of years, a new survey of the prevalence of resistance must be followed up and made. 

“In 2016, we made an overview of the prevalence of herbicide resistance in Denmark, before the reorganised pesticide tax possibly affected the choice of herbicides. It formed a basis that now enables us to assess the development over time,” says Solvejg K. Mathiassen. It is the plan to make a follow-up of the previous study in 2021.

Resistance in weeds

At the Department of Agroecology in Flakkebjerg (AU Flakkebjerg), the development of resistance in weeds has been monitored for a number of years. In the 2016 monitoring, the researchers screened the effect of two herbicides on 303 germinating seed samples from 2013-2015, and found resistance to the herbicides in eight percent of the samples and in six out of eight tested weed species - only common windgrass and cornflower were free from resistance at the time. Resistance was most common in slender meadow foxtail, where 30% of the samples collected were resistant.

In addition to monitoring, the development of resistance is monitored via an offer to farmers and consultants for resistance testing of seed samples collected in fields where resistance is suspected.

“Overall, we have found a total of resistance in 11 different weed species, however, with variation in how often and what types of herbicides are involved. We find the most widespread resistance to ALS inhibitors (minimum agents),” says Solvejg K. Mathiassen. The reason is that these agents have a very specific mechanism of action and that the agents are among the most frequently used.

Chick weed and slender meadow foxtail were the first species in which resistance was found, and the development in the number of resistance cases has not been steep. On the other hand, AU Flakkebjerg has received many samples of ryegrass in recent years, where resistance has unfortunately been found in a large part of the samples. Another major concern is that the first cases of resistance in annual rapeseed grass have been found.

Fungi

Researchers have also studied the development of resistance in fungi and pests.

“In relation to fungi, there are still only a few active groups of fungicides on the Danish market to combat the common diseases. This means that in practice it is difficult to implement an anti-resistance strategy where you mix or switch between agents,” says Lise N. Jørgensen. 

In relation to the control of septoria, in recent years there has been a decrease in the effect of the two most commonly used azoles, which according to the researchers gives cause for concern, as there are currently no viable alternatives to these. This means that the farmers' choice of varieties with good disease resistance has become even more important when attacking diseases must be kept at a low level with the least possible effort with fungicides.

“In addition, we have also seen increasing problems with resistance to SDHI agents. This is seen in, among other things, gray mold in strawberries, Alternaria in potatoes, barley leaf spot ramularia in barley, and septoria in wheat,” says Lise N. Jørgensen.

Pests

There is a small supply of insecticides in Denmark, and according to the researchers, this can potentially create a major resistance problem.

“In the control of most pests in open-air crops, pyrethroids are used, but we have seen many examples of resistance to them abroad, especially in peach aphids and grain aphids. And in rapeseed, the same means are used to combat common pollen beatle, rapeseed fleas, and a wide range of other pests, which can be problematic if, in the event of heavy attacks, the number of treatments aloud has been used and the possibility of solving the problem using alternative means are not an option,” Says Michael Kristensen. 

The resistance has not been reduced

The development in resistance shows a picture of more widespread pesticide resistance in several species combined with limited possibilities in relation to the use of pesticides.

"Therefore, it is very important that we try to prevent resistance, and the best way we can do that is by applying measures that reduce the pressure of pests in accordance with the principles of IPM (ed: Integrated Pest Management),” Says Solvejg K. Mathiassen, who along with her colleagues names a number of proposals for resistance prevention.