British gardens have had a tough time of it this year with levels of ash dieback reaching epidemic proportions, and things are looking bleak with the emergence of 'killer slugs' in British gardens. According to the Royal Horticultural Society slugs have been the biggest pest problem of 2012, generating almost twice as many enquires to the RHS entomology department as the number two pest, cushion scale.
The 'killer slugs' are actually called Arion vulgaris, or the Spanish Slug, and have devastated crops in regions of Scandinavia. The Spanish Slug has earned the nickname of 'killer slug' as its eating habits include members of its own species, as well as anything else they come across such as dead mice and faeces.
The 'killer slugs' are brown, reddish brown, or bright orange; this colouration can vary but not within the same population, and can grow up to 5 inches long. They were originally thought to be especially large Arion flagellus, or the Spanish Stealth Slug. It is only recent analysis of the male genitalia, which is distinctively different in Arion vulgaris, that has confirmed them as a new established species within the UK.
The invasiveness of the species is down to several factors, one of the major reasons is Arion vulgaris' willingness to colonise environments that are inhabited by humans. The slugs are most frequently found in agricultural and horticultural habitats with permanent, dense vegetation; such as grasslands and gardens, they are especially abundant in compost heaps. The synanthropic nature of the 'killer slugs' means the possibility of the slugs being dispersed through trade is especially high. The wide variety of food sources available to the slugs, as they appear to eat most biological material, is another contributing factor to their invasiveness.
The 'killer slugs' are thought to have arrived in the UK via imported goods, possibly salad leaves, and have quickly established themselves in the south-east. One of the main reasons the 'killer slugs' have adapted so well to living in the UK is that they are used to living in dry climates where eggs are more likely to dry out before hatching. Because of this the slugs evolved to lay up to 400 eggs in a single summer, but when the eggs are laid in moist climates more of the offspring are surviving. The eggs are laid around September and November before the adults die off, the eggs begin to hatch in March or April and rapidly grow, maturing in June and July. This means come spring the UK can expect to see an explosion of the 'killer slug' population, with further breeding taking place in summer.
Dr Ian Bedford, head of the John Innes Centre's Entomology facility, found hundreds of the slugs in his own garden and believes the 'killer slugs' are going to be a major problem: "We want to look at the environmental impact because we know that where this slug appeared in northern Europe, other indigenous species disappeared. I haven't seen any other species here since probably May time."
One problem scientists are facing is determining the true extent of the slug's invasiveness. Dr Bedford has urged people to contact him if they find an unusually large number of slugs in their garden along with their location, and preferably postcode. Dr Bedford can be contacted by emailing email@example.com or by tweeting @drianbedford.
If you do find an unusually large number of slugs in your garden and do want to manage them, after contacting Dr Beford, using chemical control such as methaldehyde or a carbamate containing bait pellet is one of the best solutions. But it is always good to remember to use the bare minimum of slug pellets as they can be toxic to other wildlife. The nematode Phasmarhabditis hermaphrodita is a popular biological method of controlling slug populations; this has a high effectiveness against juvenile Arion vulgaris but larger members of the population seem to be resistant, so applying the nematodes to your garden as soon as the temperature increases may be a good idea. However the best method of managing the slug population is still going outside at night with a torch and removing the slugs by hand.
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