B9 – Spider mites

Introduction

Two-spotted spider mite, Tetranychus urticae, (formerly known as red spider mite) can damage some protected herbs, e.g. mints and tarragon. Although serious problems are uncommon on herb crops, the damage caused by their feeding can make fresh cut and potted herbs unmarketable.

Recognition and host plants

All stages of two-spotted spider mite (TSSM) are usually found on leaf undersides. The young mites and summer adults are up to 0.5 mm long, light green in colour, with two darker lateral marks on their backs (Fig. 1). In September or October, in response to shortening daylength and cooler temperatures, and also earlier in the year in heavy infestations when plants are senescing, adult females turn a brick-red colour (Fig. 1). The brick-red females that appear in the autumn find a sheltered place in the structure of the glasshouse or tunnel, or in plant debris, to hibernate. Over-wintered adults become active again in the spring, in response to increasing temperature and daylength. They move onto host plants , where they feed and lay small, clear, round eggs on leaf undersides (Fig. 2). The eggs hatch into 6-legged larval mites, that feed on the leaves and develop through two 8-legged nymphal stages into adults.

Herb host plants include mints, tarragon, lemon balm and lemon verbena. Many other protected edible and ornamental plants are also commonly damaged. Weed hosts include bindweed, black nightshade, mallow and willow herb.

Symptoms

Spider mites feed by extracting cell contents using their needle-like mouthparts. This damage causes fine yellow speckling to be visible on the leaves, which later develops into yellow or necrotic patches, making the plants unmarketable (Fig.3). In severe attacks, leaves or plants can senesce and the mites can produce extensive webbing.

Sources of infestation and favourable conditions

The source of the pest is usually overwintered females that hibernate in the glasshouse or tunnel structure and migrate onto susceptible plants in the spring. The pest can also be brought in on infested plant material. Spider mites have no wings so they cannot fly. However, they can walk from plant to plant, or along glasshouse or tunnel structures. They can also spin fine threads of silk which allow them to be carried on air currents. Spider mites can also be spread on people or clothing. Hot, dry conditions favour the pest and allow it to breed rapidly. Many generations per season can occur, and on nurseries growing all-year-round herbs with heat and light during the winter, the pest can continue to breed throughout the year.

Integrated Pest Management (protected herbs)

Cultural control

• Check all incoming plant material for damage symptoms, particularly highly susceptible herbs e.g. mints.
• Maintain strict weed control in and around glasshouses and tunnels.
• Use a thorough clean-up procedure at the end of each crop, particularly in September, to reduce numbers of overwintering mites. Dispose of unwanted and heavily-infested plants and plant debris promptly and carefully, and clean bench or floor coverings.
• Avoid moving people or equipment from infested plants to ‘clean’ plants on the nursery, and wash hands after handling infested plants.
• Maintain good ventilation and use shade screens during hot, sunny weather, to avoid very high temperatures which favour the pest.
• Misting infested plants with water, if practical, can reduce spider mite population growth during hot, dry weather.

Biological control

Several biological control agents are commercially available for the control of TSSM. Timings and rates of release within an Integrated Pest Management (IPM) programme should be planned carefully. If necessary, seek advice from the supplier or a consultant.

1. Phytoseiulus persimilis
This predatory mite feeds only on spider mites. It should be the first choice for biological control of spider mites if conditions are suitable. Phytoseiulus is effective if released in sufficient numbers at the first sign of the pest, and if temperatures reach 20°C for at least a few hours each day. Optimum temperatures for control of spider mites are 15-25°C, when the predator breeds faster than its prey. However, P. persimilis is less effective in the hot, dry conditions (above 30°C and below 60% relative humidity) that favour its spider mite prey. The adult predators are slightly larger than TSSM and are orange-red and shiny (Fig. 4). They have longer legs than spider mites and are very active, running around the leaves searching for prey. P. persimilis will eat both mobile stages of TSSM and their eggs. Younger predators are smaller and paler than the adults. The eggs are pale pink, oval and about twice the size of those of TSSM (Fig. 2). These can easily be seen with a hand lens when monitoring, on leaves with spider mite damage, and are a good sign that the predators are establishing. As the predators do not have wings they cannot fly, but they will move readily from plant to plant if these are touching. The predators are supplied in tubes with a bran or vermiculite carrier that is sprinkled on the plants. Products with larger numbers of predators in smaller amounts of carrier leave less carrier on the plants, deliver more accurate numbers of predators and allow more effective use of high doses in spider mite ‘hotspots’.

2. Neoseiulus (Amblyseius) californicus
This non-indigenous predatory mite is only licenced for release under full protection (i.e. not outdoors or in open-sided tunnels). The predator is very similar to Neoseiulus cucumeris, commonly used for thrips control. The adult mite is oval, transparent and about 0.4 mm long. When it has fed on spider mites, N. californicus develops pale orange markings on its back. It is a good predator of spider mites and their eggs, but will also feed on other mites, thrips larvae, fungi and pollen. Unlike Phytoseiulus, N. californicus can even survive for a while without food. It can tolerate both cooler and hotter temperatures (10-33°C) than Phytoseiulus, so should be considered for release on susceptible crops under protection, if temperatures are unsuitable for Phytoseiulus. N. californicus gives slower control of spider mites than Phytoseiulus, as it feeds on fewer mites per day and does not breed as quickly. However, N. californicus can give better long-term control of TSSM, as it can survive during periods when spider mite prey is scarce, and performs better in warm, dry conditions. However, on short-term herb crops, Phytoseiulus should be the first choice, as if conditions are suitable, it will control TSSM faster than N. californicus.

3. Amblyseius andersoni
This native predatory mite is available for release to both protected and outdoor crops. The predator is very similar in appearance to Neoseiulus cucumeris, commonly used for thrips control. A. andersoni is a good predator of spider mites but will also feed on other prey e.g. other mite species and thrips. A. andersoni will also feed on pollen and fungal spores, and can survive for a while without food or water, possibly by taking liquids from the host plant. Thus the predator can establish on some crops before infestation by spider mites. Most of the research to date with A. andersoni has been on fruit crops and on hardy nursery stock and there is no current experience with using it on herb crops. A. andersoni is unlikely to be suitable for use on short-term herb crops as it is slower to establish and to control spider mites than Phytoseiulus. However, A. andersoni could be considered for release on longer-term susceptible herb crops if temperatures are too cool or too hot for Phytoseiulus.

4. Feltiella acarisuga
The larvae of this predatory midge are voracious predators of all stages of TSSM. Feltiella can sometimes occur naturally where IPM is being used, but usually it will need to be purchased and released. The midge should be considered as a supplement to predatory mites if needed. Optimum conditions for Feltiella are 20-27°C and 80% relative humidity. The midges are supplied as pupae in plastic tubs or bottles, which should be placed in a shady place near to plants infested with TSSM. The adult midges emerge from the pupae and fly out of the release containers to look for spider mite colonies in which to lay their eggs. The adults’ ability to fly to find their prey means that they can disperse more easily than predatory mites in the glasshouse or tunnel, even where plants are not touching. Adult Feltiella have a reddish body, long legs and antennae and a gnat-like appearance. The eggs are oval, slightly sickle-shaped, translucent to yellowish and are visible with a hand lens. The larvae are yellow or reddish and about 2 mm long (Fig. 5). When fully developed, the larvae spin small white cocoons on the undersides of the leaves, in which they pupate.

Monitoring within IPM

• Check susceptible plants regularly for spider mite damage symptoms.
• Using a hand lens, check the undersides of damaged leaves for spider mites and eggs, predatory mites or Feltiella larvae.

Chemical control (protected herbs)

There are very few pesticides available for the control of spider mites on protected herbs. Effective chemical control of the pest is difficult due to the mites infesting the undersides of leaves. A pesticide should only be used if necessary and should be selected carefully, taking into account pesticide compatibility within IPM (see Section A – Principles of IPM) and harvest interval (see Table 2 on the homepage).

For specific information on safety of pesticides to individual biological control agents, consult your biological control supplier. General guidelines on the selection and compatibility of pesticides within IPM are given below:

‘Safe’ in IPM
The following products are safe to biological agents once spray deposits are dry, and should give some control of spider mites. However, repeated applications will have more impact on biological control agents than single applications. All act by contact only, so good coverage of the undersides of the leaves is necessary:

• Eradicoat or Majestik (maltodextrin). Acts by physical means. Approved for use on any protected edible or non-edible crop
• SB Plant Invigorator (a formulation of surfactants and nutrients). Acts by physical means. May currently be used on any crop.

‘Harmful’ in IPM

• Dynamec, Acaramik and Clayton Abba (abamectin) have Extension of Authorisation for Minor Use (EAMU) for use on protected herbs. Abamectin is harmful to many biological control agents. However, its persistence is short, thus it can be used within IPM if timed carefully. Repeated applications will be more harmful to biological control agents than single applications.
• Spruzit and Pyrethrum 5 EC (pyrethrins) are approved for use on protected and outdoor edible crops including herbs, and claims control of spider mites. Spider mite resistance to pyrethrins is likely, although the oil component of Spruzit should give some control. Pyrethrins are harmful to many biological control agents, but only for a few days after application, thus can usually be used with care within IPM programmes. N.B. Spruzit has been known to cause leaf damage to curly parsley. The product should be tested on a few plants first for any herb species, before using on a large scale, particularly if more than one application is made. See product label for full details.
• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A – Principles of IPM.
• Pesticide approval information in this guideline is current at 29 April 2013.
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/.  General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
• Always follow label recommendations or statutory conditions for use on Extension of Authorisation for Minor Use (EAMU) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
• Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with a EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
• Use pesticides safely.

Further information

HDC Factsheet 08/05. The biology and control of two-spotted spider mite in nursery stock.