Glasshouse or two-spotted spider mites (Tetranychus spp.) are one of the most significant pests for growers of protected tomatoes.
With a declining number of pesticides available to them, most growers have adopted an integrated and programmed approach to control using predatory mites (Phytoseiulus persimilis) in conjunction with specific acaricides.
Spider mites feed on the underside of leaves, with damage initially visible as small white speckles on the upper surface of the leaf.
As spider mite numbers increase, the affected tissue can take on a bleached appearance when the speckles merge together. This can reduce photosynthesis and result in plant death.
Adult female spider mites hibernate in glasshouse structures and on crop debris - emerging as soon as temperatures and day length increase and even earlier in heated protected crops.
With a rapid rate of spider mite reproduction and development, the potential for pesticide resistance therefore presents a significant and evolving challenge when establishing a successful control strategy.
With only a few toxic spider mites capable of significant crop damage, early intervention within a programmed approach to control is vital.
The key to spider mite control is ongoing and regular crop monitoring so curative measures can be taken as soon as possible. It is much easier to treat a few spider mites early on than to tackle populations after they have built up.
Growers should monitor and record when and where spider mites come out of their overwintering state (diapause), check areas of known previous infestations and keep an eye out around posts in the glasshouses - often they come down from the roof. Warmer areas can also provide important breeding and survival grounds on which to keep a closer eye.
At the first sign of spider mite presence, consider treating with Majestik/Eradicoat (Maltodextrin) as an initial response. Maltodextrin is a fast-acting contact bio-pesticide made of natural plant extracts. It has an important role in anti-resistance strategies and is approved for use on organic crops in the UK.
Both Majestik and Eradicoat have recently been classified by the Chemicals Regulation Directorate as approved pesticide products. They offer an immediate treatment, helping to prevent problems from escalating while biological controls are ordered and released.
They work by blocking the spiracles (breathing holes) of the pest - resulting in suffocation - while reducing the insect's mobility by coating its outer cuticle. Effective on a wide range of pests and to all development stages of spider mite, it can be used for hot spot or band treatments with no restriction on the number of applications.
But ensuring treatments reach the underside of the leaves is critical. If the spray does not come into contact with the pest, it will not kill it.
Once spider mites are seen, it is important to release Phytoseiulus persimilis into all affected areas and surroundings as early as possible. Phytoseiulus are very effective predatory mites that are used to control spider mites on a range of vegetable, fruit and ornamental crops.
Feeding only on spider mites, these natural predators achieve excellent results given the chance. But key factors in achieving good control with Phytoseiulus are to ensure early release and in sufficient numbers to achieve an even distribution - important because often they will sit and eat all spider mites found on a leaf before moving on elsewhere.
Treating surrounding areas and parallel crop rows is also important. Spider mites can easily spread up and down crop rows - transported on pickers' clothes. Growers should continue with weekly predator releases until the spider mites' presence has been brought fully under control.
Occasionally, mite hot spots can develop, in which case a further application of maltodextrin as a spot or a band treatment to the top third of the plants is needed to bring the predator to prey ratio back into balance. Biological controls can be released or re-released as soon as the crop is dry.
Oberon (spiromesifen) is a further acaricide option that can be used as part of an integrated approach. Belonging to a relatively new chemical group, it affects spider mite development by interfering with lipid biosynthesis. It also has a strong ovicidal effect on spider mite eggs with the added benefit that whitefly egg hatch is markedly reduced.
Oberon can be used even once biologicals have established. Although predator numbers are reduced for one or two weeks, the predator to prey ratio is not affected and the population of the beneficial insect recovers after treatment.
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