Spider Mite Control

Felti-line a

Feltiella acarisuga

 

How does it work?

Feltiella acarisuga is a midge in the family Cecidomyidae, the larvae of which are predatory upon mites in the family Tetranychidae. This includes the common Red Spider Mite or Two-Spot Mite Tetranychus urticae, and the Carmine Mite Tetranychus cinnabarinus. Feltiella acarisuga will not normally feed on other arthropod prey or pollen, and so cannot become established in the absence of spider mites. It will eat eggs of predatory mites if these are present in spider mite colonies.

Adult midges are very small and fragile, and pinkish-brown in colour, with characteristically curled antennae. They are very short lived and are not predatory , feeding mainly on honeydew from aphids. They can sometimes be seen sitting on spider mite webbing beneath leaves. They are much smaller, paler and more fragile than adult Sciarid Flies, which can sometimes also be seen under leaves.

Approximately 60% of the adults are female, and each female F. acarisuga lays up to 25 sausage-shaped eggs amongst colonies of spider mites. Each egg hatches to produce a small larva, which begins to feed on eggs of spider mites. The maggot- like larvae are yellowish-orange in colour, with distinct white organs visible along the sides of the translucent body. The colour is most obvious in older larvae; the smallest larvae are almost transparent, and are difficult to see. Larger larvae feed on all stages of the spider mite, leaving empty ‘husks’ on the plant where they have been feeding. There are four larval stages, lasting just over 1 week in total, and a pupal stage lasting about 12 days at 22° C.

Once fully fed, the larvae spin a silken cocoon, usually adjacent to a leaf vein. Cocoons can be on either the upper or lower leaf surface. Initially these cocoons are white, but as the pupa develops within, they show a darker colour. Immediately before emergence of the adult, the pupa wriggles out of the cocoon. The empty pupal case can often be seen as a translucent white ‘finger’ projecting from, or immediately next to, hatched cocoons.

When and where should you use it?

Feltiella acarisuga may be used on any crop where spider mites are a pest, including tomatoes, cucumbers, peppers, aubergines, and protected ornamental crops such as roses. It sometimes occurs spontaneously in glasshouse crops in summer, but usually when spider mite populations are high and the crop is suffering damage. . Feltiella is sometimes attacked by a native parasitoid, Aphanogmus fulmeckii, (A. parvulus) once it is established in the crop. This parasitoid initially attacks the larva of Feltiella, but completes development in the pupa, and emerges from the cocoon through a round hole in the side.

Feltiella acarisuga is not suitable for preventive release, because it will not persist in the crop in the absence of spider mites. It should therefore be released when spider mites are known to be present, as an addition to other control measures such as Phytoseiulus persimilis and Amblyseius californicus.

Adult F. acarisuga are able to fly and colonise areas of the crop away from the release site, but may not do so if there is plenty of spider mite nearby. It is therefore advisable to make releases throughout the area of the glasshouse infested with spider mite.

Feltiella acarisuga will produce diapausing pupae in short day lengths, and is not normally suitable for use during winter. Even the use of supplementary lighting to extend day-length does not always permit use in winter.

How should you use it?

Place tubs of Feltiella cocoons immediately next to identified colonies of spider mites in the crop. Make a hole in the filter paper disc in the lid, and ensure that this does not close again. Always place the tubs in the shade close to spider mite colonies, and ensure that irrigation systems do not fill the tub with liquid.

Rates of release will be variable depending on the number and size of spider mite infestations. For localised outbreaks, use one tub (250 adults) per spider mite colony and repeat weekly for four weeks. For a more general level of spider mite infestation, use 1 tub/1000m², and repeat weekly for a minimum of four weeks.

Repeat releases over at least four weeks, or until Feltiella larvae and cocoons are seen on the leaves of the crop (See, ‘How does it Work’ for a description of the larva). Note that empty, hatched cocoons persist on the leaf for long periods. These can usually be identified because the hatched pupa appears as a translucent ‘finger’ projecting from one end of the cocoon.

When should you not use it, and what are the alternatives?

Feltiella does not perform well in winter,, even when there is supplementary lighting to extend the day-length. Neither should it be used preventively, before spider mite is seen.

In either of these circumstances, it is better to release Phytoseiulus persimilis or Amblyseius californicus. The former is a specialist predator that which can consume large numbers of spider mites, but does not persist when they are not present. It is relatively cheap, and can be used as a preventive treatment when spider mite is expected to arrive in the crop from adjacent plants, or during the spring when spider mites are emerging from diapause. Amblyseius californicus is less specialised, and can persist on other species of mite, thrips and pollen.

Do not release Feltiella if Macrolophus caliginosus is present in large numbers, or is being used for spider mite control. Macrolophus is a generalist predator, and will readily consume the Feltiella larvae in preference to spider mites.

Do not use Feltiella on crops where sulphur is being vaporised routinely for control of Powdery Mildew.

What will it do?

Feltiella acarisuga will provide additional control of spider mite, particularly at high density. It is capable of cleaning severe infestations of spider mite, but it will not do so quickly, and crop damage may well exceed acceptable levels.

What will it not do?

Feltiella acarisuga will not control Broad Mites (Tarsonemidae) or Rust Mites (Eriophyidae).

Existing damage to leaves will remain even when all spider mites have been cleared from the area, so on crops where damage thresholds are low it should be used with caution.

Chemical compatibility

Fumigation with sulphur on a regular basis can prevent establishment. One possible mechanism for this is a prevention of mate finding or host finding by adults.

The fungicides chlorthalonil and fenarimol are reported to be compatible, as is the acaricide fenbutatin oxide.