Mites in plants – spider mites
Among the most common pests on plants are mites, whereby a basic distinction is made between two species, namely spider mites and gall mites. Gall mites are mainly found on woody plants and cause proliferations there. Normally the plant is not affected by the infestation, so that control is only necessary in the rarest of cases, but spider mites should be avoided at all costs so that the plant does not suffer major damage.
Gall mites have a maximum size of 0.2 mm and their saliva leads to furry coatings on the leaves, deformed buds, pox or the typical gall. Spider mites can grow up to 1 mm in size and, as the name suggests, they spin various parts of the plant. Since only spider mites have to be controlled, this article will deal exclusively with this mite species.
Detecting infestation by spider mites in plants
If spider mites have settled on a plant, they prefer to live on the underside of the leaves. Their mouthparts are bristles which they use to penetrate the plant tissue and suck out the sugar-enriched sap. Air collects in the tapped cells, so that the infestation can initially be recognized by bright spots on the upper side of the leaves. As the process progresses, they enlarge and, under certain circumstances, merge to form an area.
As the leaves are injured, more evaporation takes place and as a result they often turn yellow and dry out. Often the leaves fall off prematurely. If the infestation by spider mites is very strong, white webs on leaves and shoots can be seen at some point. These webs are supposed to protect the little animals, so you can see a clear relationship to the spiders. However, not every type of spider mite forms such webs; this is usually only the case with the common spider mite, but not at all with the fruit tree spider mite.
Spider mites reproduce at breakneck speed
In April and May, yellowish or reddish eggs are laid on the underside of the leaves, from which the mite larvae later emerge. Multiplication progresses extremely quickly when temperatures are high and humidity is low. As a new generation is produced every 14 days at the latest, which is then already sexually mature, masses of spider mites are quickly formed. If the climatic conditions are ideal, there are eight to nine generations a year. Female spider mites can lay up to 120 eggs, so there is no need to wonder about huge populations. The plant is virtually sucked out, growth stalls until it eventually dries up completely.
The so-called red winter females of the common spider mite group themselves on the dried leaves and can hibernate on them. Sometimes they also hide in holes in the ground or seek shelter under scaly tree bark. If plants are infested in the greenhouse, cracks are an excellent shelter. The fruit tree spider mite handles this somewhat differently, because in autumn it lays its eggs on the bark of the affected shoots. As the quantity of eggs is enormous, the clutch of eggs can often be recognised from afar by its red shimmer.
Prevent spider mites properly
It is best for the plants if there is no infestation by spider mites in the first place, so prevention is an important issue. First of all, it is important that the plants have enough moisture, so they need enough water regularly. A layer of mulch has proven to be a good way of ensuring that the moisture remains in the soil for longer. If it is extremely hot and dry in the summer months, the plants should be sprayed with water. This limits the activity of the arachnids somewhat, as they cannot tolerate high humidity. Especially in closed rooms, or even in the greenhouse, this is a measure that is easy to carry out.
Spider mites are particularly likely to attack weak plants if they lack nutrients. Good fertilization, which takes potassium into account, can strengthen the plants, because the tissue becomes firmer. Nitrogen, on the other hand, should not be administered, because it tends to soften the tissue. In an ideal location, plants have enough air and light, so frequent ventilation is necessary in the greenhouse.
How to effectively control spider mites
In order to take action against the spider mites, the infestation must of course first be reliably determined. It is important to start the control as early as possible, so that the little animals cannot multiply so much in the first place. As they are hardly visible to the naked eye, the underside of the leaves should be examined with a magnifying glass. The webs can be seen when
How to effectively control spider mites
In order to take action against the spider mites, the infestation must of course first be reliably determined. It is important to start the control as early as possible, so that the little animals cannot multiply so much in the first place. As they are hardly visible to the naked eye, the underside of the leaves should be examined with a magnifying glass. You can see the webs when you spray the plant with water.
If the plant is only minimally infested, it is often sufficient to rinse it with a sharp jet of water; indoor plants are placed in the shower or bathtub for this purpose. The spraying should now be done every few days, maybe everything is already over by then. Another possibility is to remove the infested parts of the plant, this is another way to contain the spread. All fallen leaves, such as weeds, should be removed from the area around the plant, as the mites could hibernate there. Please dispose of it with household waste and not with compost!
Predatory mites against spider mites
Spider mites can also be controlled biologically, for this purpose special predatory mites are needed, the effect is most effective in the greenhouse. Predatory mites eat eggs, larvae and the adult mites equally, so mass reproduction is reliably prevented. However, this measure requires the climate to be right for the predatory mites, i.e. an air humidity of 75 percent and a temperature of 25 degrees. In this case too, control should be started as early as possible; the use of predatory mites is also useful as a preventive measure.
Insecticide against spider mites
If none of these natural measures bear fruit, you have to buy appropriate plant protection products in specialist shops. In the case of chemical control, the underside of the leaves must also be treated, and several treatments may be necessary. Ideally, you should only use preparations that are gentle on beneficial organisms, otherwise valuable opponents will be destroyed. Good pesticides would be those based on rapeseed oil or potash soap, for houseplants pesticide sticks are the appropriate means. However, chemical preparations should not be used too often, as resistance often develops, so that the agents become ineffective over time.
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