Tomato growers were advised by a leading tomato pest expert that employing the latest chemistry, as part of an IPM strategy, could help overcome problems relating to variations between tomato pests, and combat resistance.
The advice was handed out at a recent tomato seminar, held at HRI Wellesbourne. Hosted by Certis, the event marked the launch of Oberon, the new insecticide for the dual control of spider mite and whitefly in tomato crops.
Dr Rob Jacobson of Stockbridge Technology Centre pointed to three forms of spider mite that display different growth rates and mobility that could impact on control.
He said the red form of Tetranychus cinnabarinus reproduces more quickly than either the green form of Tetranychus cinnabarinus, or the Tetranychus urticae two spotted spider mite. The Tetranychus urticae is also normally green, but the overwintering resting phase is red, so identification is difficult with similar colour forms virtually indistinguishable unless examined closely under a microscope by an expert.
Jacobson noted the green Tetranychus cinnabarinus form is less mobile than the other two forms, so placing predators in the midst of the colony delivers best results. Conversely, the more mobile red Tetranychus cinnabarinus and green Tetranychus urticae will be controlled most effectively where predators are also placed on surrounding plants.
With identification of different green mites impossible for growers to ascertain in a glasshouse situation, the damage threshold may be reached before the predator population gains control. “This means that a second mode of defence providing a rapid knockdown will be necessary earlier and more often,” he warned.
Jacobson added that the problem may be compounded according to how rapidly the spider mite population increases on the tomato cultivar grown. A higher density of hair-like structures known as trichomes will slow population growth, and that’s dependent on the cultivar and the growth stage of the plant. “It’s an opportunity for plant breeders,” he said.
“Rather than the familiar white speckling on leaves we are also seeing a different type of more intense damage caused by spider mites. Total leaf necrosis known as ‘hyper-necrosis’ can be caused by relatively small populations of both red and green forms of Tetranychus cinnabarinus.” He added that more research is required, but speculated that hyper-necrosis may be induced by feeding affecting stomatal activity, resulting in heat stress damage and the collapse of whole leaves.
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