The two-spotted spider mite (Tetranychus urticae) multiplies particularly fast in warm and dry surroundings.
Drip-feed fertiliser/irrigation systems and computer climate control produce a dry environment ideal for its development.
They make a network of webs beneath which they prick the plant and lay their eggs.
Chemical control for thrips is suitable for spider mites also, but if it is used intensively then resistance may appear.
Nowadays biological methods of control appear to be an alternative to chemicals that is well worth considering.
Spider mites belong to the Arachnid class (4 pairs of legs). They are of the Tetranychidae family in the order Acarina.
Under the general term ‘spider mites’ come various small mites including the two-spotted spider mite (Tetranychus urticae). This is a fearsome destroyer of cultivated plants, especially under glass where conditions favour its development, which is very rapid and can destroy whole crops.
The body features of the adult are these:
They are visible to the naked eye. The males are 0.3 mm long with a narrow body fairly slender at the front. The females are somewhat larger (0.5 mm).
The body of the adult is globular and oval. Its epidermis is hairy and it varies from yellow to green in colour: the females can take on an orange-red tint in dry periods and in autumn. Both males and females bear a pair of black patches on each side of their body.
This spider mite has 5 development stages:
The larva and both nymphs each include an active and a resting period.
Spider mite populations have a 3:1 ratio of females to males. They mate as soon as they emerge as adults, and a single mating is enough to fertilise all a female’s eggs. Fertilised females lay eggs that will produce both males and females, while unfertilised females can also lay, but their eggs will produce females only.
Under optimal conditions a single female can lay over 100 eggs, the amount depending on temperature and humidity. The eggs are round, between 0.1 and 0.2 mm in diameter. They are transparent when laid and go opaque afterwards. They are often laid on the underside of leaves.
The larvae have three pairs of legs and when they hatch are translucent with red eyes. From when they begin feeding they turn to a light yellowish green or a darker green. When they have fed enough they immobilise themselves on the leaves for the change into the first nymph stage.
These first nymphs are a little bigger than the larvae; the main difference is the presence of a fourth pair of legs. The nymphs develop from light to dark green, and patches on their body become easier to see. After feeding, the first nymph enters a resting state and turns into the second nymph.
At this stage the difference in size between the two stages is greater, but the colour stays the same. Males and females are now distinguishable, the males being small and thin while the females are larger and rounder. Once fed, the second nymph also rests, and then turns into an adult.
The time taken for this development cycle depends on temperature, humidity, the plants eaten and the age of the leaf. At 20ºC (68ºF) the whole cycle from egg to egg takes 17 days, compared with 7 at 30ºC (86ºF). Conversely, below 12ºC (54ºF) development stops, while temperatures over 40ºC (122ºF) are lethal.
The population usually builds up more slowly in winter, when some females may become dormant, which means they no longer feed or lay.
The two-spotted spider mite stays on young leaves or in buds for choice, on the underside of leaves and in the folds of the shoot: it avoids the light. Larvae, nymphs and adults can cause damage to cyclamen plants. They have a feeding tube with which they puncture epidermal leaf cells and suck out the contents; the plant’s organ reacts by showing yellow spots followed by general yellowing, then a more leaden colour and, in the case of severe attacks, complete wilting. The yellowing (chlorosis) is due to loss of chlorophyll which leads to a serious decrease in photosynthetic activity.
In any case the yellow patches themselves spoil the ornamental value of the cyclamen.
What is more, the injected saliva tends to poison the plant tissue.
Another observable feature of spider mite infestation is the mite’s ability to weave webs, which when the colonies are large may cover whole plants. The webs start mainly on the underside of leaves, and also contribute to the reduction in photosynthesis. They function as protection for the mites against wind and weather and also against control measures. Under the webs, the mites move around and lay.
Because females make up so high a proportion of the population, each male is always near to an adult female. Whether mating takes place or not, there are many eggs laid. Populations therefore grow rapidly and the reproduction rate is high.
The mites spread from place to place by falling on the ground, when they move towards new plants. The webs and threads which they weave also enable them to spread above ground level. They may be carried when pots or other things are moved. They prefer a warm dry climate, though, and will therefore only establish themselves in the appropriate parts of the glasshouse.
Biological control is the best answer to this mite. In fact the two-spotted spider mite was the object of the first biological control agent to be put on the market.
Control of Tetranychus urticae is made possible by the use of a specific predator mite, Phytoseiulus persimilis. This predator mite belongs also to the Acarina order, but to the family Phytoseiidae. It is of a size comparable with its prey, round in shape and orange in colour, with well-developed back bristles.
Its development stages are the same as those of its prey, only its larval and nymph stages do not have a rest phase. The development cycle takes around 10 days at 25ºC (77ºF) with a relative humidity of 75%. It thus has the potential to out-breed its prey.
Only a fertilised female lays eggs, near to a spider mite colony. When the temperature is between 17ºC (63ºF) and 28ºC (82ºF) a female can lay as many as 60 eggs. The eggs are about twice the size of those of Tetranychus urticae. The larvae that hatch do not feed; but from the first nymph stage, and throughout adult life, feeding is continuous, and the predator eats all stages of the spider mite.
Spider mites are the sole food of the predator. It appears that it hunts by following the smell of damaged plants and of the spider mite’s webs.
It can easily move around in the search for new prey when the population density is low compared with its own. At 20ºC (68ºF) the female predator lays more eggs and the population contains more females; what is more, the generation time is shorter and so numbers build up more swiftly. The optimum temperature range for predation is 15ºC (59ºF) to 25ºC (77ºF). Above 30ºC (86ºF) or 35ºC (95ºF) the predator stops feeding.
This predator mite is sold under the names Spidex and Spidex plus, Phytoseiulus-system, Phyto-line p. These commercial preparations contain adults.
The mite Amblyseius californicus is also used as a predator, marketed as California-system, Ambly-line cal and Spiral.
Because chemical treatments have been intensively used, Tetranychus urticae has become resistant to many plant hygiene products. Moreover this intensive application harms the natural enemies of this mite.
The constant development of the regulations and homologations of phytosanitary treatment products, and the differences in regulations according to each country make it impossible for us to include updated information on homologations. Each producer will have to contact his local plant protection bureau to obtain the latest updates concerning the regulations and use of phytosanitary products. We strongly advise testing beforehand on a plant sample in order to measure the chemical’s activity (establishing the dose) and any effect on the plant (plant poisoning).
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