Red Spider Mite
Two species of fruit pest have the common name Red Spider mite. One is the Fruit Tree Red Spider mite — Panonychus ulmi — which attacks mainly apples, plums, damsons and pears, and occasionally bush and cane fruits. The other red spider mite is also known as the Two-spotted Spider mite — Tetranychus urticae — and is more normally a pest of protected crops. Strawberries, hops, blackcurrants and cane fruits can all be attacked by Two-spotted Spider mites, particularly in hot, dry summers.
Damage by infestation with either species is caused by direct feeding of the mites on plant leaf tissue. The first symptom of attack is speckling of the upper leaf surface, where colonies of mites are feeding on the underside. As the population builds the foliage becomes dull, eventually turning a bronze colour followed by premature leaf fall. Severe attacks adversely affect both quality and yield of the fruit produced.
Fruit Tree Red Spider Mite (Panonychus ulmi)
Overwintering occurs as bright red winter eggs, normally laid from August to September on the underside of spurs and smaller branches. Egg hatch commences at the pink bud stage, normally late April to early May, and is 50% complete by petal fall. Egg hatch continues up to mid-June. Young mites move to the underside of leaves to feed immediately after hatching. Each mite moults three times before reaching the final adult stage.
Adult mites only reach about 0.4mm in length when fully grown. The female is oval with a convex back covered in several long bristles. The body is dark red with a white spot around the base of each bristle. Immature mites and male adults vary in colour from bright red to yellow-green. Newly hatched larvae have three pairs of legs and nymphs and adults have four pairs of legs.
Summer eggs are paler in colour than winter ones and are laid on the underside of leaves. Hatching of summer eggs normally takes place after 2 – 3 weeks. There are four or five generations annually with each cycle taking approximately four weeks from egg laying to adult maturity. Winter eggs are laid by females from September onwards but, if there is a shortage of food, winter eggs can be found on severely bronzed trees as early as August.
Two-Spotted Spider Mite (Tetranychus urticae)
Two-spotted Spider mites overwinter by hibernating in large numbers in the soil or in cracks in hop garden poles. Where straw is used as mulch for strawberry crops, they will also hibernate within the straw. Spring activity and feeding begins as soon as growth commences.
Extremely small, round, translucent eggs are laid on the underside of leaves and they hatch in around two weeks. Young mites moult after feeding and pass through two more immature stages before becoming adults. During the season up to seven generations can be produced.
Female adults only grow to about 0.5mm and the males are even smaller. Summer adults begin as yellow or green with dark green markings, getting darker as the season progresses. After autumn mating the females turn brick red in colour and search for suitable areas to hibernate.
Lorsban* WG (active ingredient chlorpyrifos) has good activity on non-organophosphorus resistant strains of spider mite. Timing of application varies with crop. Do not apply to crops in flower or to those in which bees are actively foraging. Do not make applications when flowering weeds are present.
Strains of spider mites resistant to one or more groups of acaricides are widespread. Where strains resistant to products containing chlorpyrifos occur, Lorsban WG is unlikely to give satisfactory control.