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Biting back at the flies
Researchers find chemicals that explain why some animals, including people, get bitten more than others.
On holiday, does one of your family always get more mosquito bites than the rest? Researchers at Rothamsted are identifying the chemical reasons why certain individuals, amongst both cattle and humans, are consistently bitten more than others.
Scientists in Professor John Pickett's group at Rothamsted Research have helped to determine how plant-feeding insects use smell to locate and select an appropriate host, and they have utilised this knowledge to develop novel pest management strategies. Now they have turned their attention to animal pests and found that the same principles apply for biting and blood-sucking pests of vertebrates. This opens up the possibility of new veterinary and medical applications for pest control.
Early studies, funded by the EU and conducted largely with the Danish Institute of Agricultural Sciences, investigated control strategies for nuisance and disease-transmitting cattle flies. These flies cause reduction in milk yields, weight loss, hide damage and reproductive failure, causing substantial economic losses in animal husbandry systems. Evidence suggested an uneven distribution of flies within herds of Holstein-Friesian heifers, with some heifers having much higher fly-loads than others.
Starting with the horn fly, Haematobia irritans, a major cattle fly pest, the EU project revealed that fly loads on individual heifers could vary by as much as 64 fold within a herd, and that fly load rankings were consistent over time. What is more, researchers could manipulate overall numbers of flies in a herd by moving key heifers from one herd to another: exchanging four fly-susceptible heifers for four fly-resistant ones reduced the overall herd fly load and vice-versa. This demonstrated that the variation in fly distribution was determined by the presence or absence of crucial individuals. The big question was "how?"
"We are very familiar with semiochemicals, message-bearing chemicals that influence insect/plant relationships," explains researcher Dr Mike Birkett, "and we wondered whether they were at work here too. Unattractive individuals might lack attractive chemicals or, more likely, they might produce additional chemicals that mask the attractants." To test this, they collected and tested volatile extracts from heifers and from heifer urine (see box).
"We ended up with three compounds that acted as attractants and three that acted as repellents" says Birkett. "This is the first evidence that differential attractiveness to biting insects within the same host species is due to volatile chemicals from the host". Researchers are now looking at whether these compounds can be used to manipulate fly behaviour and help protect fly-bitten herds. One of the compounds, 6-methyl-5-hepten-2-one, has already proved successful at reducing fly loads when applied as a slow-release formulation from metal dispensers strapped to the cow's back. More tests are needed with the other repellents and attractants, as the ultimate aim of this work is to develop a full "push-pull" strategy. This involves the concurrent use of repellents and attractants to keep the flies away from the heifers whilst attracting them to traps or other sites for destruction.
Eventually, livestock breeding might offer another approach. "Insect resistance is a valuable trait for plant breeders," says Birkett. "If the natural resistance of some individual animals to flies is inherited, it could be selected for in animal breeding programmes".
Biting flies also find individual humans more or less attractive. Most of us have an idea of what our relative appeal might be by comparing mosquito bites on holiday. It turns out that such assessments are usually quite accurate. James Logan, a BBSRC-funded research student, working at Rothamsted (on a collaborative project with Professor Jenny Mordue at the University of Aberdeen) has questioned people about their susceptibility to mosquito bites and then tested their attractiveness to yellow fever mosquitoes in behavioural tests. The mosquitoes are placed in a Y-shaped tube and given the choice of moving upwind via either branch. Air flowing down one branch is laced with odour from the volunteer's hand, whilst the other branch is the control. "There was a strong match between people's perception of their attractiveness to mosquitoes and attractiveness in the Y-tube test" says James.
He is now investigating whole body odours by encasing volunteers in a foil sleeping bag for two hours and collecting the odours within. Using the coupled GC-EAG technique (see box), he has identified several compounds within human odour that arouse neural responses in the mosquito. Initial tests suggest that the ratio of these compounds varies between attractive and non-attractive individuals. "This could be the root cause of differential attractiveness" says Logan. Along with the rest of the Rothamsted team, he hopes to exploit these findings to develop a safe, odourless naturally-occurring insect repellent which, because it relates to natural host selection by mosquitoes, may prove more effective than conventional products.
The group envisage that these cattle fly and mosquito studies will form the basis for the development of novel and benign control systems for other arthropod pests of human beings and animals, including sandflies, mites and even the Scottish biting midge (with Professor Mordue), where differential attractiveness of the host species is expected.
To collect from heifers, the animals were housed individually in pre-washed stalls, and the volatiles emitted by the animals were collected using a porous polymer trap. A coupled gas chromatography-electroantennagram (GC-EAG) technique was then used to identify which odour components of these extracts are perceived by the horn fly, and also the face fly, Musca autumnalis. In this technique, high resolution gas chromatography (GC) is used to split the extracts into their component parts. These are then passed over the antenna of an insect. Micro-electrodes connected to the antenna detect electrophysiological responses, which indicate that the insect is sensing the compound. GC linked to mass spectrometry is then used to identify these compounds.
EAG active compounds were tested on flies in a wind-tunnel to see what effect, if any, they had on fly behaviour.
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