False Widow Spiders: Overhyped or Overlooked?

In late summer 2013 the false widow spider suddenly jumped onto centre stage. Within days the media and the public became very agitated about spiders, and reports of swollen limbs and terrified families were on the front page of national newspapers. We go into detail of the False Widow Spider and provide some much needed clarification on the matter.

Until the summer of 2013, most UK pest controllers were seldom involved in spider control. Spiders were a minor pest that the householder usually dealt with themselves without professional help, catching most pest controllers in the industry off balance when they suddenly reached prominence. Pest control training courses and publications do not cover UK spiders in any detail, and while a few pest control companies claimed to be busy on spider work, the rest were busy frantically researching on the internet or calling their trade association.

Tricky Identification

Spiders are of course arachnids, not insects, and are therefore more closely related to mites, ticks and scorpions. In the UK, we have around 650 native species of spiders, the vast majority of which live outdoors on vegetation or on the ground, although a few are regularly found in and around buildings. They are all predatory, feeding on other invertebrates. Some species use a silken web to entangle their prey, while others actively hunt down their food. Most spiders use venom to subdue their victim, but only a few spiders in the UK have sufficiently powerful mandibles to penetrate human skin. The spider causing the excitement in recent months was the false widow spider, Steatoda nobilis. This species is believed to have been introduced to Devon from the Canary Islands in the 1880s, and has gradually spread northward and eastwards, with most confirmed records lying south of a line from the Wash to Pembrokeshire. Although S. nobilis was introduced, there are also five other closely related false widow spider species that occur in the UK. S. grossa and S. bipunctata in particular are widespread and commonly found in sheds, outbuildings and porches as well as within homes. “For us, it has been tricky to identify the various false widow spiders with complete confidence, even using the online images. It would be helpful to have a proper spider identification guide” suggests Ian Miller of South London based Cleankill Environmental Services. Typically, the false widow spiders spin a loose tangle of webbing, with a tube of silk in one corner that leads into a crevice where the spider hides in the day. This web is often at least 1.5-2m above ground level, and sometimes higher. “The typical locations for us have been around window and door frames, and eaves,” adds Ian Miller, but they have been reported from a wide range of other locations, including walls, downpipes and gutters, waste pipes, porches, and outbuildings. The spiders of course increase in numbers during warmer months, but in late summer and autumn they may become particularly conspicuous as the males wander more extensively in search of a mate. “Certainly our requests for false widow work always peaks in October” confirms Brian Duffin of Hampshire-based Rokill Pest Control Services.

Not Aggressive

False widow spiders are not aggressive, and will not launch an unprovoked attack on people. When bites do occur, it seems to be a result of accidental contact with a spider. The venom of S. nobilis can cause a short-lived reaction, described as similar to a wasp sting, but occasionally it is longer-lasting and extends beyond the site of the bite. In the UK, the recently publicised cases of serious reactions to so-called spider bites are unlikely to be a reaction to the venom itself, but are more likely to be the result of secondary infection of an initial skin lesion such as MRSA from hospital visits. Given that S. nobilis has been present in a large area of southern England for over a century, living in and around homes, the rarity of bites (around 10/year reported to the Natural History Museum, London) is an indication of the very low threat from this species. The threat is much less than that presented by wasp stings, or tick-borne Lyme disease, for example.

Customers and Spiders

We had a trickle of requests for control of false widows over the last few years, but in 2013 it became a flood. Most callers would report finding a spider that looked exactly like the one on our website, and were worried their family was at risk.. However in most cases, once we reassured our domestic callers that their lives were not in immediate danger, they were satisfied. However institutions such as schools clearly have a responsibility to their pupils, so with these customers we would often become more closely involved.

Prevention and Control

The CoSHH Regulations of course require us to consider non-chemical control options for pest problems. For customers that wanted something done, but were cautious about pesticide use, we would offer an inspection and a thorough vacuum clean of the affected area. Normally that seemed to be sufficient. In the longer term, then sealing potential harbourage points in high risk areas, such as in porches and conservatories, may further reduce the chance of contact with the spiders. However numbers of our domestic customers wanted to err on the side of caution, and have a pesticide applied, despite reassurances about the low risk from spiders. In su ch cases, where safe to do so, we would treat crevices and other likely harbourage areas with a residual pyrethroid or carbamate insecticide. In general, we find that spiders are even more susceptible than insects. However as the spiders are widespread across the areas where they do occur, we advise customers that insecticide treatment of a porch or garage is unlikely to give lasting and complete elimination. The spiders will gradually recolonise the treated area. In terms of choice of pesticide, remember that spiders are of course arachnids, so the ‘other crawling insects’ phrase often used on pesticide labels does not cover use against spiders. Pesticides which explicitly mention spiders on the label are relatively few, but include Pelgar’s Cimetrol and Bayer’s Ficam W. Spiders are likely to be found in situations where surfaces are porous, e.g. concrete, brick and wood, so formulations such as wettable powders and suspension concentrates are likely to give a longer-lasting effect. Regarding outdoor treatments, residual insecticides used in pest control are not approved for the treatment of foliage or garden plants and in the unlikely event that vegetation needs to be treated, this should be carried out with a pesticide explicitly approved for that use, typically a horticultural pesticide. In addition, users of such products must hold the PA1 and PA6 Certificates of Competence, as the RSPH exams which most pest control technicians undertake do not cover the use of horticultural pesticides.

Respect Beneficials

In the areas where false widow spiders occur, then there will almost certainly be a range of beneficial species present too, such as other spiders, hibernating ladybirds and lacewings. Before any treatment is carried out, whether it is chemical or non-chemical, then the potential impact on non-target species needs to be considered, and discussed with the customer. There is a balance between controlling what is actually a very low risk pest, and causing potential damage to organisms that are beneficial in the garden.

This Summer?

Fortunately, thanks to the internet, most pest control organisations are now much better informed about spiders than they were the last few summers. Hopefully those providing training, and responsible for training manuals, will also be able to update their material before long. So, looking ahead, how much spider work can we expect in 2016? Our view is that although spiders have definitely increased their range in recent years and the work has therefore increased too, the 2013 surge was largely driven by media coverage. Who knows what the media might latch onto this summer, it might even be Asian hornets!