Set a Wasp to Catch a Fly: Nuisance Flies and Biological Control

With spring firmly settled in, and the grass finally starting to grow, fly season is fast approaching. While flies and midges are expected when dealing with livestock, too many can cause significant problems. Here we take a look at a few of the management option available, from chemical treatments to biological control.

The problems flies cause

Large populations of nuisance flies cause irritation.
Large populations of nuisance flies cause irritation.

In the UK, the main flies which bother livestock are house, stable, horn, and face and head flies. Many of these are biting flies which feed on blood. This causes pain and irritation to the cattle and can distract them from eating. This in turn decreases weight gain and milk yields. Added to that, welfare concerns can occur when fly populations are very large. This is sometimes a problem in enclosed barns, as cows can’t get away from the flies.

Similarly, in milking parlours, flies can cause problems. Constant tail flicking, kicking and irritable behaviour can make milking more difficult and more dangerous for workers, and make the experience more stressful for the cows.

Unlike ticks, flies move between multiple hosts, so can easily transmit diseases. Bacteria which cause Summer Mastitis and which cause New Forest Eye tend to spread this way. As a result, a herd can have sudden breakouts of these diseases when fly populations are particularly troublesome.

Standard fly control options

There are a number of different methods for nuisance fly control. Each method has its pros and cons, and can suit certain herds or farms. Management is usually different for grazing and housed herds, and can vary dramatically by location.

Environmental management

Flies will only breed in certain conditions. They are particularly fond of waste silage and muck heaps, and can breed readily in wet, low-lying ground. Keeping cattle away from these areas during peak season will help to reduce detrimental effects. However, this can be difficult to achieve if there is no other pasture for the cows to graze on.

Muck heaps provide the perfect breeding ground for flies.
Muck heaps provide the perfect breeding ground for flies.

In buildings, air movement systems such as fans can help to prevent flies getting in, but are not 100% effective. Additionally, the cost of installing such systems can be high. Another method can be to use pheromone traps for flies. These operate through use of bait scented with specific attractive pheromones. The flies enter the trap, fall into it, and die. These can be effective in enclosed spaces, but have limited usefulness in pasture.

Chemical management

Insecticides are a popular tool for fly control. These can be spot-on treatments or chemical sprays. These are applied directly to the cow, whereas other treatments are injected. Chemical ear tags are impregnated with pesticides and provide protection for the animal. Alternatively, insecticides can treat areas where flies might breed and prevent their population from growing.

While these treatments are often very effective at decreasing fly populations, as with treatments for internal parasites, there is evidence that resistance is beginning to develop. On top of that, some of these chemicals can be hazardous to workers and meat and milk withdrawal times must be considered.

Biological control for nuisance flies

What is biological control?

Biological control, simply put, is managing pests by using their natural enemies to keep population down. It is a technique that people have used for many centuries. One of the oldest records of this form of pest control is from 304 AD in China. Aggressive ant species were sold in nests to be placed among citrus trees. The ants would live on the trees and prevent insect damage from other species. Nowadays, biological control is widely used to keep pests down.

Biological control systems

Ladybirds and aphids are a classic example of biological control.
Ladybirds and aphids are a classic example of biological control.

Biological control works in a number of ways. One of these is the introduction of direct predators that will eat the pests and so remove them. An example of this would be in aphids. Aphids are very common pests on crop plants and can have devastating effects. To combat this, ladybirds are introduced which are natural predators of aphids.

Other methods involve using natural pathogens of the pest population which cause disease. Alternatively, direct competitors to the pest can be introduced to out-compete the pest species. Specific parasites and parasitoids can also help to reduce pest populations.

Controlling nuisance flies

For biological fly control, an emerging option is the parasitoid wasp species Spalangia cameroni. These tiny wasps are only around 3mm long but can have a huge effect on flies. The females of the species lay their eggs inside the fly pupae before they hatch into adults. The wasp larvae develop and feed on the fly larvae, eventually killing them in the process.

Pros and cons of biological control

There are a number of important benefits of using a method such as this. Firstly, the use of synthetic and chemical compounds is reduced. This decreases the speed at which resistance develops and helps provide more options for pest control. Secondly, biological control has a much reduced environmental impact than chemical methods. When done properly, biological control is sustainable and eco-friendly.

Parasitoid wasps prey on fly larvae, preventing them from reaching maturity.
Parasitoid wasps prey on fly larvae, preventing them from reaching maturity.

On the other hand, biological control systems are not perfect. Parasitoid wasps usually need redistribution every few weeks to help build up a large enough population to effect the flies. Equally, a system such as this will never completely wipe out a pest population, as the predator would die out too.

So what’s the solution to nuisance flies?

As with many situations, there is no cut-and-dry solution. Fly management is best assessed on a case-by-case basis and what works for one farm might not work for another. Biological control certainly has its place in managing these difficulties, but maximum efficiency might be obtained by using multiple control methods at the same time.