Entomologists don’t often say things like, “It really is a hideous insect. I wouldn’t lose sleep if it were exterminated from the planet,” before admitting to having “horrible nightmares” when working with the creature in question. But the carnivorous screw-worm, Cochliomyia hominivorax, was never going to win any popularity contests.
“The following is gory,” warns Meg Allen, a Research Entomologist at the United States Department of Agriculture’s Biological Control of Pests Research Unit, before going on to describe the worm’s modus operandi. If you’re squeamish, you might want to look away now.
The adult fly lays its eggs in open wounds, on any warm-blooded animal – humans included. When the eggs hatch, the larvae burrow into the wound, attracting females to lay further eggs. “The maggots have wicked, sickle-shaped mouthparts to tear apart flesh, and hooks along their midsection to prevent easy removal,” adds Allen. They literally eat their prey alive.
The gruesome tenacity of the screw-worm means it has plagued humans and livestock for centuries. Luckily, a prolonged campaign aimed at wiping it out has been a great success so far: it has now been eradicated from the US, Central America, Puerto Rico and the British Virgin Islands. “The screw-worm eradication project is the most successful insect pest control story ever,” says Allen. 3 3 But how has this been achieved? Well, the screw-worm has been used as the agent of its own destruction.
This programme is a triumph of biological control, or biocontrol as it’s commonly known – the practice of curbing or eliminating pests, whether insects or plants, by biological rather than chemical means. This may involve manipulating their own behaviour, or using their own enemies to control them. For the screw-worm, it is the former.
The programme, which was based in Mexico but has recently moved to a new facility in Panama, centres on fly production. Hundreds of millions of screw-worm pupae are bred in a controlled environment and then sterilised, before being released from specially equipped planes in those countries where eradication programmes are in action.
The flies are sterilised by exposure to low doses of the radioactive material cobalt (which has to be protected round the clock by armed guards, because of the obvious security issues). When the sterilised males mate with existing populations, the female worms do not develop eggs and, over time, the insect will die out. This method of using sterile males to wipe out pests has also been successfully used with insects such as Mediterranean fruit flies, gypsy moths, boll weevils and tsetse flies.
Exotic natural enemies
Closer to home Here in the UK, a biocontrol programme is being launched, aimed at an invader that doesn’t have quite the widescreen horror of the screw-worm, but is no less pernicious a pest – Japanese knotweed.
The Department for Environment, Food and Rural Affairs (DEFRA) has approved plans for the controlled release of a tiny Japanese plant-eating insect, or psyllid, called Aphalara itadori, at two to three test sites in England with the aim of controlling knotweed. This will be the first official use of an exotic natural enemy against a weed in the UK. If this first phase is successful, the insect will be released at further sites. The attractive creamy flowers of knotweed fooled the Victorians into thinking it would make a good ornamental plant when they first introduced it to UK soil in the mid-19th century. But with no natural predators here, it wasn’t long before the plant was running amok in carefully tended flower beds and gardeners started to dig it up and throw it away, allowing it to rapidly proliferate in the wild.
Today, knotweed is a menace that can literally wreck buildings and transport systems. Its vigorously invasive roots can grow to seven metres in length and its stem can grow three metres in as many months. It’s a plant that has already cost the country millions of pounds to remove. It added considerably to the budget of the new Wembley Stadium and has taken several years to clear from two of the 2012 Olympic sites.
Along with structural damage, Japanese knotweed hampers natural diversity by stopping ecosystems from thriving – muscling out other species. In a laboratory setting, tests have showed that the psyllid can stunt the growth of knotweed. It’s hoped that in the wild, the insect will constantly debilitate the plant, allowing others to outcompete it. Progress will be slow, it’s expected to take 5-10 years for the psyllid to work its magic. In the long term, the aim is not to completely wipe knotweed off the face of Britain, more to make it manageable and less of a drain on our finances.
Nothing in the armoury
Dick Shaw, principal scientist on the knotweed project at CABI Europe (a not-for-profit research centre in Surrey) 3 3 says that while biocontrol has been around in essence for 100 years, this is an exciting time in general for the science. There are two reasons for this. One is necessity, partly because of the virtual ban on the use of chemicals in or near any body of water in Europe, as well as the Water Framework Directive, which requires all Member States’ water bodies to reach ‘good ecological status’ by 2015.
Shaw says without using biocontrol methods, “You might have a six-inch mat of floating weeds 20 miles long, killing everything underneath it, and nothing in the armoury apart from manually hauling the stuff out.” For this reason he believes there are huge opportunities for biocontrol as a method of keeping pests in check.
The other boost has come from improvements in technology. A technique known as molecular phylogeny uses DNA tools to characterise the relatedness between species. This is crucial to effective biocontrol. If you want to find the best and safest predators that are highly specific to a particular weed or animal, you need precision in understanding the relationships between species. “Before, this was down to human decisions,” explains Shaw. “Insects have always been better at distinguishing between plant species because they have spent thousands of years evolving with them.”
Anything that moved
By analysing their molecular building blocks, scientists are able to select possible biocontrol agents with incredible precision. It is when this precision is lacking that failures of biocontrol have slipped through the net – with disastrous consequences. The most famous example of a species introduction going badly wrong is the cane toad, transported from Hawaii to Australia in 1935 to control scarab beetles. It turned its attentions to anything that moved and could be swallowed.
Another is the African land snail, introduced to French Polynesia, which has indirectly led to the near extinction of the tiny Partula tree snail. Things also went wrong after the introduction of the harlequin ladybird. It was licenced for use in glasshouses in some areas of Europe. But against all rules and advice, some people released them into the wild, where they spread like wildfire.
Dick Shaw says that in the UK at least, rigorous risk analyses must be applied before any biocontrol agent can be released. Cane toads and African land snails were released against all expert advice at the time. “These were crazy decisions,” he says. “There have been 1000 releases of weed biological control agents around the world, and of those 1000, eight had non-target [unwanted] effects. Of those eight, seven of these results were predicted by scientists.” Carried out properly, biocontrol can be a very valuable weapon against pests.
After all, what knows the weaknesses of a species better than its natural born enemies? As Dick Shaw says, “We’re constantly surprised by how clever insects and fungi can be.”