For Scottish potato growers, slug activity following periods of rain or irrigation and post-tuber formation, can be a difficult problem given the latest Metaldehyde Stewardship Group’s guidelines.

With a total of just 700g as/ha total of the active substance allowed in a calendar year and a maximum dose rate of 210g as/ha from August 1- December 31, significant pressure is mounting for growers to find an acceptable alternative to metaldehyde.

It is problematical, too, that the new guidelines require a 10m buffer zone free from pellets around every field boundary and watercourse.

With pressure on growers to use metaldehyde products in targeted and selective best practice, the availability of an alternative product in the form of ferric phosphate has been demonstrated to have significant and wide-ranging benefits in trials carried out the last two years by Dr Andy Evans, of SRUC’s Crop and Soil Systems Research Group.

Using funding from AHDB, trials using the slug-susceptible variety, Maris Piper, were carried out on two sites in Mid-Lothian and Fife, with a set further south in Lincolnshire.

The object was to prove that ferric phosphate was able to not only control slugs in the crop, reduce damage and preserve quality, but at a level where growers could be confident of its performance and efficacy, compared to metaldehyde alone, in sequence or in combination.

“We conducted the trials over three separate treatments to the crop at three identifiable timings,” explained Dr Evans. “The first application is crucial. This needed to be before the canopy closes over between the rows to enable good ground coverage of pellets.

“The second application needed to be after rain or irrigation, which would see the slugs come to the surface and activity increase. The third treatment would then involve an application post-desiccation to protect the tubers from ongoing slug risk prior to lifting.”

Two types of pellets were used in the trial – the metaldehyde was applied as De Sangosse TDS Major (4% metaldehyde as) and ferric phosphate (3% metaldehyde as).

Different combinations of product were trialled to compare efficacy with persistence. The main objective was to identify whether ferric phosphate could be substituted as a direct replacement for metaldehyde, without a drop-off in performance.

“In most situations, the ferric phosphate performed just as well as the metaldehyde,” reported Dr Evans. “Similar levels of control afforded by both products meant that the two were interchangeable. What we did notice, was that under wet conditions, the premium brand ferric phosphate degraded slightly quicker than the premium brand metaldehyde, but because of their similarity in performance, that could be managed by selecting the active ingredient according to weather conditions or irrigation applications.”