Suppressing infestation

How to combat red spider mites in orchards

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The common red spider mite is the most important mite pest of apples and pears in certain production areas. Particularly severe infestations were noted in certain areas of the Western Cape in the 2017 to 2018 season, due to the heat and drought conditions experienced.

So why do red spider mite problems vary from year to year? The answer has a lot to do with the environment.

Environmental conditions that result in high populations include hot and dry conditions, the destruction of natural enemies by spraying pyrethroids and hard chemistry (a lack of predators stimulate females to lay more eggs), highly infested crops and weeds nearby, as well as insufficient water supply to the crop.

Mites are often less in numbers after rain that can wash their populations from leaves, while humid conditions favour dew and create conditions favourable for predators that control red spider mite populations.

Know your enemy

The more farmers know about mites, the easier it is to control them.

The red spider mite is a small yellow/olive-coloured mite which have dark (red or crimson) patches on either side of the body, and is less than 1mm long. They are not insects or spiders, however they are related to ticks, spiders and scorpions and they have eight legs.

Mites can normally be found on the underside of leaves and tend to have patchy distribution in orchards. As the population increases, the mites will aggregate at certain sites such as the growing points of the plant. Even though the mites are found in high numbers on the underside of the leaves, a magnifying glass might still be needed to see them.

Damage is caused by the mites sucking sap from the cells on the underside of the plant leaves. The mites have piercing-sucking mouthparts and they feed on the chlorophyll thereby reducing photosynthesis and destroying the leaf epidermis with a consequent loss of moisture.

This damage is similar to that of severe drought conditions and could cause substantial weakening and possible virus transmission in orchards.

White speckles are quite noticeable in the early stages and will occur on the upper leaf surface. As the mite population increases, the leaves take on a bleached appearance and start dying. Other symptoms include stippling on the leaves, chlorotic spots, or leaves can change colour to bronze, rusty, purple or yellow-brown. Severe attacks from red spider mites will cause the leaves to drop, causing a setback in tree crops.

When there is a high infestation, the plants will be covered with a fine webbing. This webbing does a few things. It protects the mites from natural enemies. It also protects the mites from insecticide sprays, especially the contact insecticides. It anchors the mites and their eggs to the plant and act as walkways between the leaves and other plants for the mites. It is also therefore a method of dispersal. Females will disperse from plants on threads of webbing and they will drift or be blown onto other plants. This is also called ballooning. Wind is an important method of dispersal, but humans, animals and machinery also play a role.

Temperature is critical to the life cycle of the red spider mite. Normally population reproduce over 14 days at 21°C, but at 30°C this lifecycle can be brought down to less than a week. There are five developmental stage and overlapping generations do occur.

The adult females lay eggs after one to two days and they will lay between 70 and 150 eggs in their lifetime of three to four weeks. Females don’t need to mate as the unfertilised eggs will produce male mites and the fertilised eggs will produce female mites. Distinguishing characteristics between males and females are that males have more pointed abdomens and are smaller than the female mites.

The eggs are laid on the underside of the leaves which is also where the adults occur. During high infestations they can also occur on the sides of leaves, on stems, as well as on the fruit. When the humidity is high the egg laying rate of the mites are less.

As the summer comes to an end the mites will turn deep red and migrate from the plants to hibernate in cryptic habitats such as cracks in the ground, under tree bark, crevices or in crop debris. Red spider mite can get through winter without feeding and re-emerge in the spring and summer to re-infest the apple and pear trees. During moderate winters, the mites will continue to feed when plants are available.

Take control of the orchard

To control red spider mite, farmers need to keep a close watch and spray in accordance with infestation levels.

Regular scouting is essential to identify the problem early. If more than five adult mites occur per leaf, a chemical spray should be considered. A well-timed application should limit the number of applications. Most products such as systemic products will not kill the eggs. Regular inspections are required and a repeat application may be necessary at a 10-14 day interval.

If the population is very high, an ovicide should be considered in combination with a contact or translaminar acaricide.

The following spray tips are important:

  • Ensure good coverage of the lower leaf surfaces as the mites hide on lower surfaces and spin webs which can protect them from the contact sprays.
  • High spraying pressure is important to ensure smaller droplets for deeper penetration or use systemic/translaminar products that penetrate leaves also from above.
  • Spot treat clusters of mite infestations before applying a general application over the rest of the orchard.
  • It is essential to alternate between different groups of pesticides with different modes of action. Mites develop resistance to pesticides quickly, and have developed resistance towards most acaricides, making control difficult. Resistance develops quickly as mites produce large numbers of offspring and their lifecycles are very short, resulting in successive generations within a short amount of time.

Few insecticides are effective and some may aggravate the problem by killing natural enemies such as predatory mites (Phytoseiulus spp.). When mite populations are high, these predators are ineffective.

Given the relative ineffectiveness of insecticides, farmers have to combat mites by also addressing the factors that can lead to plants being more attractive to red spider mites.

Examples are excessive application of nitrogen fertilisers, trees that grow in dusty places such as adjacent to dusty farm roads, and trees that are stressed by low moisture and high ambient temperatures. Scouting of trees right next to dirt roads is essential because it is from here that infestations spread to the rest of the orchard.

Weeds are important alternative hosts for mites, especially solanaceous weeds and Oxalis spp. Therefore, weed control in orchards is important to reduce pest reservoirs.

Humans and animals are unfortunately important sources of infestation. Mites are brushed off onto moveable objects in infested orchards and may drop off at any time, infesting new areas. Movement in infested orchards should be limited and infested orchards should never be visited before moving to a clean orchard.

Syngenta’s solution

Syngenta’s Agrimec Gold® and Voliam Targo® offer good protection against mites.

The mode of action of Agrimec Gold® is translaminar and has stomach and contact action. Translaminar means that it penetrates the leaf tissue and forms a reservoir of active ingredient within the leaf.

Abamectin, the active ingredient, targets the nerve cells. The red spider mite does not die instantly, but feeding is immediately inhibited, followed by paralysis and eventual death.

For fruit growers that want to export their pome fruit, Voliam Targo® represent a unique one-can formulation that can address numerous problems. Voliam Targo® is IPM compatible ensuring the beneficial insects in the pome orchard can contribute to controlling the mites.

The combination of a miticide to suppress spider mite populations along with a lepidoptera active ingredient in Voliam Targo, ensures that both codling moth and mites can be targeted. These two pests can occur in the orchard at the same time.

Unlike pyrethroids, Voliam Targo® will not induce the females to reproduce more. When mites are only moderately abundant and a broad-range pyrethroid is applied, a dramatic increase in mites can result. Pyrethroids kill all the natural enemies, and stimulate mite populations, as more female mites are produced.

Tia Ferreira (Insect Control Lead, Syngenta) and Christian Giesel (Campaign Lead Cereals and Deciduous Fruit, Syngenta)

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