Significant progress has been made in a study at Harper Adams University which aims to combat one of the most economically damaging crop pests to the agricultural industry globally.
The two-spotted spider mite (Tetranychus urticae) feeds on more than 3,800 different plant species worldwide, including fruit, vegetables and ornamental plants.
The damage caused by this pest is considered by the European Union to be a significant threat to food security. For this reason, more than $1 billion is spent annually in an effort to try and counter the damage caused by this pest to economically important crops.
Currently, there is a high reliance on chemical pesticides to try and combat it but it’s hoped that the use of its natural predator, another mite named Phytoseiulus persimilis, in biological control programmes could be increased. Use of this predatory mite is currently limited due to the expense and inefficiency of currently used rearing methods.
Joe Roberts’ PhD research project at Harper Adams aimed to create a chemical blend which could be used to encourage the predatory mite to feed on artificial diets and therefore make it easier to rear.
The 28 year-old from Hayle, Cornwall, said: “Phytoseiulus persimilis, the predatory mite, only feeds on the two-spotted spider mite. It has no visual organs so relies on its sense of smell to find its prey. It does this by detecting chemicals emitted by plants infested with two-spotted spider mites.
“By identifying the precise feeding stimuli of the predatory mite, it could be ‘tricked’ into feeding on an artificial diet, consisting of organic compounds similar to those produced by the two-spotted spider mite itself and its eggs. Mass-rearing of the predatory mite could enable them to be used more often in biological control programmes.
“For the study, I reared two-spotted spider mites on dwarf French bean plants. From there I conducted a series of experiments.
“One of the studies achieved in identifying and quantifying the volatile compounds emitted by the dwarf French bean plant when infested with the two-spotted spider mite which help the predatory mite identify its prey and eat.
“From these volatile compounds, I identified ten which are of ‘high interest’.
“To demonstrate the behavioural response of the predatory mite to infested and uninfested dwarf French bean plants I put a number of them into a Y-shaped choice arena that allowed them to choose between two different sections, or stay where they were. The greatest number of mites moved into the section that contained the chemicals from an infested dwarf French bean plant.
“Seeing that they had a preference to go to the section containing chemicals from an infested plant, I conducted more experiments to determine whether individual chemicals or two different blends of synthetic chemicals could also be used to attract the predatory mites. At the concentrations I used, the predatory mites showed no preference for either blend.
“The synthetic chemicals blends I’d created based on the ones I found to be released by an infested plant did not successfully lead to getting the predatory mite to eat an artificial diet. Different concentrations of chemical blends or non-volatile chemicals, rather than the volatile chemicals, need to be investigated in future research.
“A novel element to my investigation is that I found predatory mites follow trials made by their prey and that the greater the number of two-spotted spider mites walking the same trail, the stronger the trail they leave behind, so the more likely the predator mite is to follow it.
“I looked into seeing if this was because they were following the webbing the two-spotted spider mites create, hence where their name comes from, or if they were following a chemical composition. I found that they are following chemicals. However, identifying the chemicals has proven difficult as the analytical techniques required are not currently sensitive enough to detect the very small quantities of chemicals deposited by the two-spotted spider mite. When we can identify the chemical coming from the prey, it’ll supersede the chemical from the plant for use in encouraging the predatory mite onto an artificial diet. I’ve left the research in a way that someone can come back to it in the future when our knowledge and technology has advanced.”