Slug attacks are relentless on one heavy land farm in the Vale of Glamorgan. Sarah Henly asked slug expert Prof David Glen to offer a solution
ITS difficult to remember a recent autumn when slugs havent been headline news. And this one will be no exception, according to Prof David Glen of IACR Long Ashton.
The wet and mostly mild winter followed by a fairly damp and warm summer will have favoured the build-up of slug numbers and created perfect conditions for attack. High pest activity is evident in many areas.
Someone who is acutely aware of that fact is Mike Price, tenant of Boverton Place Farm in South Wales. He has spent thousands of pounds over recent years on molluscicide pellets to protect winter wheat and oilseed rape seedlings, and his cheque book is now open again.
"We rarely get away with anything less than applying two lots of pellets, one at drilling and another about three weeks later when damage typically re-appears. And even that doesnt do the whole job in some fields so we go in with a different active ingredient in November. The bill can come to as much as £50/ha."
Mr Price needs to come up with some new strategies, not least because he has just taken on a new unit in Herefordshire, also owned by the Duchy of Cornwall, with heavy soils that could also be slug-prone. Can Prof Glen offer solutions to counteract the existing problem and prevent a similar occurrence on the new unit?
Field B10, in to Gemini oilseed rape, was harvested last month without a desiccant and it yielded 4.3t/ha (1.75t/acre). There remains a large amount of stubble and trash, which is due to be ploughed under before drilling winter wheat.
Prof Glen suggested prompt action to remove the stubble and get the trash decomposing as soon as possible. Volunteer rape was now satisfying the field slugs and round-backed slugs voracious appetite, and allowing populations to increase.
"Slugs can lay a batch of up to 50 eggs every few days in August if the weather is favourable, and oilseed rape is their favourite meal. So its not surprising that they stay and feed on emerging volunteers after harvest. It pays to remove the stubble as soon as possible after harvest," said Prof Glen.
Baling behind the combine and selling the straw could be a profitable option this summer in particular because there is a large demand following the foot-and-mouth crisis, which has dented long-term supplies. Mr Price has already sold some barley and wheat straw by public auction for about £175/ha (£71/acre) and £153/ha (£62/acre) respectively. He is concerned, though, about removing valuable supplies of phosphorus and potassium from the rotation in straw.
Prof Glen acknowledged this, but stressed the importance of trash burial to remove the food source. He also extolled the virtues of the stale seedbed technique – cultivating then leaving the soil undisturbed for a while before final seedbed preparation. That would not only deal with the trash but would also benefit weed control, he added. Volunteers and weeds that emerge can be killed using glyphosate just before drilling.
"Start with a light cultivation to incorporate most of the trash. You neednt work the seedbed so much as to make it hard to prepare it later on. And remember that cultivating itself helps to kill slugs – the more you do, the better."
Ploughing has a greater effect on slug numbers than minimally cultivating or direct drilling, but the technique that leads to the best seedbed in a given situation will give the crop the greatest chance to outgrow slug damage, said Prof Glen.
Mr Price has tried discing and pressing after subsoiling, though he usually ploughs field B10. Whichever technique he uses, he struggles to create a fine tilth on the heavy soil. Rolling helps, but only if it is not excessively wet or dry.
Mr Harrington said he had heard of an organic farmer who rolls at night to squash as many slugs as possible in the process. Prof Glen suggested it could be useful, if practicable, to roll at the time of peak soil activity, which in the autumn is just after dusk, say 6-7pm.
"Rolling is a good strategy whenever it will result in the breakdown or compression of clods without smearing. In a fine tilth, slugs often cant find seed among the soil particles whereas in a cloddy seedbed with fewer air pockets, the seed is easier to come by."
The grey field slug can travel several metres in a night. Its posterior tentacles can smell food and pellets from a distance of 10-20mm.
Turning to molluscicide pellets, Prof Glen suggested applying them to the field after cultivating and before drilling, to give control a kick-start. Mr Price hasnt tried applying them before the crop is in the ground. His occasional adviser, Brecon-based agronomist Jon Harrington, has considered it but is concerned that it would cost a lot and do only half a job.
"I have seen some work carried out at Newcastle University which looked at the feasibility of treating the previous crop with pellets to attempt to reduce the adult populations, but I could see little benefit from doing so. My advice to Mr Price has been to carry out preventative measures as far as possible and to apply good quality metaldehyde pellets to the prepared seedbed, if necessary treating again as soon as damage appears."
The key objective has to be to establish the crop as quickly as possible so that the plants can grow away from the pests. Most crops will tolerate a substantial level of grazing once they have become established, but would be wiped out by that same slug population feeding at the time of germination.
Prof Glen agreed, but suggested the Rolls-Royce of treatments for field B10 – pellets three to four days before drilling onto a ready prepared seedbed, followed by further applications when necessary. He recognised the danger of preparing the seedbed and leaving it ready, but thought it may be worth the risk early in the autumn, provided that the weather window would remain suitable for drilling.
Mr Price asked which type of pellets slugs preferred, and which formulation he should use. He has tried a wide range of products (see table) and tends to think that, of the three available active ingredients, methiocarb is the best for serious slug problems.
Though Prof Glen believes the different products and formulations must taste different, he hasnt noticed any preference for a particular type of active ingredient. Slugs will happily eat any pellet, although they sometimes stop feeding before they have taken a lethal dose. This used to be a problem with metaldehyde pellets because the active ingredient paralyses their mouthparts. That is one reason why the stomach poison methiocarb is seen as superior.
"When methiocarb first came out, it offered advantages over the existing metaldehyde pellets, but improvements in formulation chemistry have turned metaldehyde round. You can get formulations such as Optimol that include a feeding stimulant to encourage greater intake before the mouthparts become affected. You can buy pasta-based formulations like Metarex which offer extended durability in wet conditions. Nowadays, it is horses for courses," he said.
The main advantage to using metaldehyde as opposed to methiocarb or thiodicarb is its safety to non-target organisms. It doesnt kill naturally occurring slug predators, such as the carabid beetle Pterostichus melanarius, explained Prof Glen. This large beetle preys on slugs from June to September, so using metaldehyde in oilseed rape situations could have an impact on control. By the time wheat is in the ground, beetle activity is waning, so safety to carabid beetles may not be such an issue, he added.
Mr Harrington admitted to preferring metaldehyde for this reason, and because it offers more baiting points per sq m (see table), enabling slugs to find the pellets more readily. He believes an abundance of natural enemies could be the reason why some of the fields on Mr Prices new farm – Monkhall in Herefordshire – are relatively free from slugs.
"Weve had a look-see and can find very few slugs in a soil and situation not unlike the one in field B10 at Boverton. Some of the fields, with standing oilseed rape stubble and groundsel weed, are prime candidates for attack, so we can only suppose that beetles are helping to keep slug numbers in check."
Prof Glen advised Mr Price to keep a close watch on fields at high risk, and treat with pellets as necessary rather than as a routine. He stressed that slug traps can be valuable in assessing the risk of damage to wheat, provided that trapping is done before cultivations when the soil surface is moist.
"Traps can be deceptive in dry conditions because slugs will not be active enough on the soil surface to be recorded. For oilseed rape, there is not yet a reliable threshold trap catch for slug damage. Looking under surface trash and digging can be a useful way to check for the slimy pest," he concluded.