Central Science Laboratory entomologist Helen Moran told the Tomato Growers' Association/HDC seminar at Warwick HRI last week that Tetranychus evansi, a species of red spider mite originating from South America that can attack tomatoes and peppers, is heading for the UK.
The mite has been found in Spain since 1995 and in southern France since 2003 - and could establish here under glass.
It was first intercepted in the UK three years ago on aubergines imported from Kenya. Moran said: "It has the potential to multiply rapidly, with 15 generations a year in heated crops."
Meanwhile, Warwick HRI's Dr Dez Barbara urged growers at the Brassica Growers' Biennial Conference to look out for signs of the fungal disease Verticillium wilt - which has already caused 100 per cent infection in cauliflower crops in California and 50 per cent yield loss in oil seed rape in Sweden.
He said: "We hope this does not ever become a real problem for you but I don't think we can afford to be complacent at the moment. Once you have got it in a field you will have trouble getting rid of it.
"On land where oil seed rape has been grown it persists for up to 20 years - it's not something that is going to go away from your fields quickly."
The main symptom of the disease - which can spread through infected soil from a previous crop - is a discolouration (or yellowing) of a plant's vascular system.
Barbara said: "Fungicide treatments tend not to work. By far the best route is to not get it in the first place."
Moran said the reason why biological controls were ineffective for T. evansi was because it out-competes its natural enemies. Tomato Growers' Association executive officer Gerry Hayman said tomato growers should be suspicious if they find red spider mite failing to react to the biological controls they normally use. "The danger is it could come in at low levels without you realising," he said.
T. evansi looks similar to the two-spotted spider mite commonly found on glasshouse crops in the UK and causes similar damage, with leaves yellowing and desiccating. It can be controlled by acaricide sprays.
Warwick HRI's Dr Rosemary Collier also warned growers at last week's brassica conference of another new pest - Pegohylemyia fugax - that is already common in Belgium and could spread to the UK. She said: "It is a fly larva that grazes on cauliflower curds, turning them brown. It resembles a cabbage root fly larva. There's already great concern among oil seed rape growers in Europe."