Given the plethora of commercial pesticide products available for ant control on the domestic market one might be forgiven for mistaking the ant as the most virulent UK pest. This article explores the efficacy and safety of the pesticides aimed at controlling a pest that is also a beneficial indoor insect.
Although ants become a nuisance when they enter homes in large numbers they are also useful housemates. Whilst foraging, worker ants are likely to kill and eat any insect they come across including flea and fly larvae, bedbugs, young silverfish and clothes moths. They will also clean up organic debris in cracks and crevices. A control strategy should aim to keep ants from becoming an indoor nuisance but should not eliminate them from other environments(1). The most common UK ant encountered by pest control officers is the Black Garden Ant (Lasius niger)(2).
Ants are social insects, living in colonies usually with three distinct castes: workers, the queen and males. Castes carry out different functions and usually look different from each other. Workers generally enlarge and repair nests, forage for food, care for the young and queen and defend the colony. Nurse workers feed larvae with regurgitated liquid or partially masticated flesh and carry them about. If the nest becomes flooded adults will transport pupae to higher ground(3).
Ants are likely to forage for most available foods in the home. Uncovered food, including pet food, and packaging in waste bins are the most vulnerable. Although a nuisance ants are unlikely to cause the deterioration of food and there is little evidence to suggest that they are vectors of human pathogens. It is however advisable to be cautious, as it is not possible to be sure where they last visited. Sought after foods should be stored in tightly sealed containers.
In order to decide upon the most suitable control method it is necessary to ascertain what ants are feeding on, how they are entering the house, and whether they are nesting indoors or outside.
Indirect control should aid to reduce or eliminate access to food, water and shelter. The process of reducing ant invasion is also likely to reduce other kitchen pests.
Care should be taken to store food in appropriate containers. Glass containers should be fitted with rubber seals or rubber or plastic gaskets fitted to lids. Plastic containers should have tightly fitting lids. It is worth noting that ants are capable of entering screw top jars via connecting threads. Newly bought foods should be stored as soon as possible.
As major decomposers of organic matter ants will forage for any scraps of food in the home. Kitchen surfaces and floors should be cleaned frequently and thoroughly to minimise sources of food. Food wrappers and containers should be cleaned carefully before discarding or recycling. It is advisable to keep food waste in bucket type plastic containers with tight fitting lids until collected for disposal or composted. Keeping a spare for rotational use to enable containers to be aired will prevent the unnecessary build up of smells. Whilst composting is a highly effective method of disposing of organic waste care should be taken not to incur a different pest problem.
Permanent sticky barriers are an effective way of excluding ants from houseplants. Barriers can be home made or may be purchased commercially. Ants will not cross any form of sticky barrier.
Access points into food preparation areas should be sealed. A good quality silicone caulk will minimise the risk of cracks re-opening as a response to normal structural movements. Silicone seal may also be bought as a paint-on sealer for larger scale applications. A smear of petroleum jelly or a strip of duct tape will work as a temporary closure.
Direct Physical Controls
Foraging ants may be mopped up using a cloth or sponge dipped in water and detergent. This will only act as a temporary measure as ants are likely to be replaced quickly by others from the nest. However this is a useful measure to take once access points have been sealed.
Moats of water are effective to protect food sources such as dishes of pet food. It is necessary to add a small amount of detergent to prevent ants floating across water using surface tension. Care must be taken not to inadvertently provide ants with a bridge across the moat.
If ants establish a nest inside a plant pot it is possible to encourage them to move to another home by flooding the pot. An alternative nest site can be provided in the form of a similar pot filled with compost or loose, dry soil. A length of dowling rod or wooden stick can provide a bridge between the two pots. The affected pot should then be flooded repeatedly until ants are observed carrying pupae to the surface and crossing into the second pot. If some ants take refuge in the foliage of the plant they should be left for an hour or so to descend into the drying soil after which flooding can be repeated. Flooding with boiling water or soapy water will effectively kill ants nesting beneath outdoor surfaces such as patios and paved areas.
Direct chemical controls
There are currently at least 100 different commercially available amateur insecticides directly marketed as ant controls. Table 1 provides a summary of pesticides available and their environmental effects.
Sorptive dusts have a desiccating effect on insects and are particularly effective when blown into cracks and wall voids before sealing. Boric acid dust is poisonous to ants and is the least toxic to mammals. Care should be taken not to inhale dusts and where possible they should be applied by professionals. Anyone applying dusts should wear protective clothing including goggles and a dust mask to protect lungs and eyes.
Silica aerogel combined with pyrethrum is faster acting and also presents a minimal risk to mammals. The product dries to a thin film of white dust on application. It will be effective in the short term to knock down an infestation but should be combined with physical controls for long term control.
A drench of insecticidal soap or a household soap and pyrethrum mixture will kill some ants and cause the remainder to move the nest. This is particularly effective to cause ants to move nest away from house.
Boric acid baits have the advantage of being directly applied to the insect pest. Ants collect the bait on foraging trips partially ingesting the poison and passing the rest on the other members of the colony on their return to the nest. A small amount of the poison may be added to food in a container with a pierced lid to provide access. A pharaoh ant infestation in a 22nd floor flat in Plaistow, London was aggravated as ants were prevented from taking bait back to the nest due to the inhabitants killing every ant they saw.
Ants rarely invade homes in great numbers and are relatively easy to control in such circumstances. A range of direct physical controls will ensure that infestations are not repeated. They are beneficial insects and should not be eliminated all together. Good housekeeping is usually enough to prevent ants becoming a nuisance. Pesticide treatments are often harmful to humans, domestic pets and the wider environment. If chemical treatment is required, boric acid baits housed in bait stations present the least risk.