30 August 1996

PSDs resistance policy

Blackgrass resistance to herbicides is on the increase. Products which could once be relied upon to leave fields weed-free sometimes no longer do so. Andrew Blake asked the Pesticides Safety Directorate to outline its approval policy against the background of shifting efficacy

RESISTANCE is nothing new, and the approvals system has to take it into account. That is the underlying message from PSD, which points out that resistance to insecticides has long affected product labels.

Before any pesticide product, including a herbicide, can be PSD approved for marketing its efficacy has to be tested. "We assess whether the data submitted by the applicant demonstrates that the claims made on the product label are supported," says Tim Godson, specialist in herbicide efficacy at PSD.

The assessment includes positive aspects such as control of a pest, weed or disease and negative ones such as levels of phytotoxicity to the crop treated and to following crops, and the development of resistance, he explains.

Range of conditions

Information to help do this comes from the applicant and other organisations, with trials usually being carried out for at least two years under a range of conditions.

Efficacy testing is required under both UK and EC pesticides legislation. Although the EC system places more emphasis on certain aspects, the overall requirements are similar. "The impact of the new European authorisation process on efficacy is likely to be minimal," says Mr Godson.

So how does PSD respond to growers concern at the spread of weeds, particularly blackgrass, which show signs of resistance to herbicides, especially isoproturon and the so-called fops and dims?

"Products will only be ineffective where resistant weeds are present and where the level of resistance is high," maintains Mr Godson. "Although blackgrass resistance is an increasingly important problem it does not occur on all farms."

In many cases resistance is not complete and isoproturon can still play an important role in managing resistant blackgrass, he adds.

"PSD is not complacent. Over the coming months there are to be meetings where the situation will be reviewed. We will take an active part in them."

Experience with insecticide resistance indicates the way ahead, he suggests. Where problems occurred statements were added to product labels to warn of the existence of resistance. Where available, information on combating or delaying the development of resistance was also added.

During the course of the PSDs review programme, resistance problems are continually examined. "In some cases the problem had become so widespread that much of the population was not controlled. Where this happened PSD evaluated the available data on how well resistant populations were controlled with certain products and the data on the spread of resistance. Where the problem warranted it, very strong warnings were placed on labels or uses were revoked where products were no longer effective.

"With resistant blackgrass, we are in a situation where much of the population is still susceptible," says Mr Godson. PSD is encouraging companies to explore strategies for the control of resistant blackgrass and to prevent its spread."

Where information can be placed on labels this is encouraged. But because resistance is dynamic with cross-resistances varying, much of the information is best conveyed in leaflets or bulletins that can be regularly updated, he explains.

"As a number of types of resistance are present in blackgrass populations, with differing cross-resistance to various herbicides, it is best to ensure that as many types of herbicides with different modes of action remain available to users."

If PSD were to revoke the approval of certain pesticides to which resistance had developed but remained effective in many cases, selection pressure on other pesticides would increase, he argues.

The Directorate will continue to require submission of data on this aspect and consider its position as the resistance situation develops, he stresses.

On the subject of label phrases, in Oct 1992, after on-farm problems began, PSD required a warning to be added to labels of all products that claim to control blackgrass. It stated: "Strains of blackgrass have developed resistance to many blackgrass herbicides, this may lead to poor control." This was done to alert users to the growing problem.

In Jan 1994, after guidelines on dealing with resistant blackgrass had been drawn up, the Directorate issued a statement in The Pesticide Register saying that the following wording could be added to labels: "The Weed Resistance Action Group has produced guidelines on avoiding and coping with resistant blackgrass. Copies of the guidelines may be obtained from your distributor, crop adviser or product manufacturer."

After the recent review of isoproturon, products containing this active ingredient must include reference to these guidelines on their labels, says Mr Godson. &#42

&#8226 Resistance not novel – long known in insecticides.

&#8226 Always taken into account in approval system.

&#8226 Efficacy tests must involve resistant populations.

&#8226 Withdrawal of products could increase selection pressure.

&#8226 Label warning notices needed.

&#8226 Varying cross-resistance patterns merit advisory leaflets.

&#8226 PSD constantly monitoring resistance developments.