Spider mites

Damage

Fruit damaged by spider mites.

© A. M. Varela, icipe

Generally, spider mites prefer the undersides of leaves, but in severe infestation will occur on both leaf surfaces as well as on the stems and fruits. They suck the sap of plant tissues. Infestations are most serious in hot and dry conditions. Because they multiply very fast they are able to destroy plants within a short period of time. Spider mites spin silk threads that anchor them and their eggs to the plant. The fine web produced by spider mites protects them from some of their enemies and even from pesticide applications. 

The most destructive spider mite species in East Africa is the tobacco or tomato red spider mite (Tetranychus evansi). This mite is a very serious pest in tomato crops and other members of the Solanaceae family (tomato, potato, eggplant, tobacco and wild plants and weeds like black nightshade, bitter apple and wild gooseberry). This species originates from Brazil, South America and was accidentally introduced into Southern Africa during the 80's. 

Since then this spider mite has slowly been moving northwards. Nowadays it is one of the major constraints into tomato production in Kenya, Mozambique, Malawi, Namibia, Zimbabwe and Zambia. When left uncontrolled the farmer can loose his or her production within a week time. 

The two spotted spider mite (Tetranychus urticae) and the carmine spider mite (Tetranychus cinnabarinus) cause yield loss on tomatoes only in exceptional cases such as: very hot and dry conditions, destruction of natural enemies, the presence of other highly infested crops in the near vicinity and insufficient water supply to the crop. For more information on this species refer to datasheet on tomato (click here). 

Damage by spider mites on beans is most severe when mite feeding occurs early in the vegetative period. For more information refer to section on spider mites on datasheet of beans (click here). 

Another important species is the cassava green mite (Mononychellus tanajoa), an important pest of cassava. This mite is green in colour at a young age turning yellowish as adult. It was accidentally introduced from South America and its rapid spread becoming one of the most important pests of cassava in Africa. For more information refer to datasheet on cassava (click here) 

The cotton red mite (Oligonychus gossypii) is a widely distributed mite in Africa. It is commonly found on cassava, mainly during the dry season, but it is much less economically important than the cassava green mite. It also attacks cotton, citrus, peach, papaya, beans, okra, peanut, and ornamentals.

The coffee red mite (Oligonychus coffeae) may be a pest of unshaded coffee and tea in localised attacks during the dry season. They attack the upper surface of mature leaves. As a result the upper surface of fully hardened leaves turn rusty, purple or yellow brown colour. Under drought stress young leaves may also be attacked.
 

Host range

Spider mites have been recorded from a wide range of wild and cultivated plants - including beans, cassava, cotton, citrus, okra, tomato, papaya, potato, tobacco, strawberry various cucurbits and legumes. 
 

Symptoms

First symptoms are usually clusters of yellow spots on the upper surface of leaves, which may also appear chlorotic. This gives the leaf a speckled or mottled appearance. Feeding by spider mites may lead to a change of leaf colour in some plants such as okra, cotton, coffee, tea and some ornamentals. Attacked leaves turn bronze, or rusty, purple or yellow brown colour. Spider mites and webbing are present on the lower leaf surface, which may appear tan or yellow and have a crusty texture. 

Feeding by the cassava green mite leads to stunted and deformed cassava leaves. Severe attacks cause the terminal leaves to die and drop, and the shoot tip looks like a "candle stick". 

Under severe infestations leaves redden, whither, and drop. Some spider mites (e.g. T. evansi) produce large amount of webbing. Heavy infestation will result in a fine cobwebby appearance on the leaves and the whole plant. Plants die when infestation is severe. 
 

Initial infestation of spider mites on tomato. Note mites seen as reddish specks on underside of a tomato leaflet.

© A. M. Varela, icipe

Spider mites on tomato. Note the mites and their webbing visible between the leaves.

© Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org

Affected plant stages

Vegetative growing stage, flowering stage, post-harvest 
 

Affected plant parts

Leaves, inflorescences, fruits. 
 

Symptoms by affected plant part

Leaves: lesions, abnormal colours, abnormal leaf fall, yellowed or dead.
Inflorescences: yellow or abnormal colour, abnormal flower fall, premature fall of young fruits.
 

Introduction

Mites are not insects. They are related to spiders and ticks and they are very tiny (they rarely exceed a size of 0.5 mm). Spider mites are normally active within a temperature range of 16 to 37°C. They are more numerous in hot, dry weather. They are normally less numerous after rains. Wind plays an important role in the dispersal of spider mites. The lifecycle of a spider mite may take 10 to 30 days depending on temperature. It includes five stages: egg, larva (first instar) two nymphal stages and adult. A female may lay over 100 eggs during its lifespan. Spider mites spin silk threads that anchor themselves and their eggs to the plant. This silk protects them from some of their enemies and even from pesticide applications. 
 

Description

Eggs are tiny, spherical, pale-white, and are laid on the undersides of leaves often under the webbings. They can only be seen with a magnifying lens. Eggs hatch in 4 or 5 days. 

The larvae are light green or pinkish, slightly larger than the eggs and have six legs. 

The nymphs look similar to the adults but are smaller. They are green or red in colour and have eight legs.

The adults are oval and have eight legs. They are very tiny (they rarely exceed a size of 0.5 mm) resembling tiny moving dots to the naked eye. The male is usually smaller than the female and have a more pointed abdomen. Spider mites are variable in colour depending on the species. Many of the species are bright red in colour; and that is reason why spider mites are sometimes referred to as red spider mites. Others are yellowish, greenish, pinkish, orange or reddish in colour. The two-spotted spider mite has a large dark blotch on each side of the body. 
 

Female of the cassava green mite (Mononychellus tanajoa), real size 0.8 mm

© F. Haas, icipe

Eggs and larva of the cassava green mite (Mononychellus tanajoa), real size 0.2 mm

©  F. Haas, icipe

Male of the cassava green mite. (Mononychellus tanajoa), real size 0.8 mm

© F. Haas, icipe

Major species of spider mites in Africa:

Pest and Disease Management

Pest and disease management: General illustration of the concept of Infonet-biovision

This illustration shows the methods promoted on infonet-biovision. The methods shown at the top have a long-term effect, while methods shown at the bottom have a short-term effect. In organic farming systems, methods with a long-term effect are the basis of crop production and should be with preference. On the other hand methods with a short-term effect should be used in emergencies only. On infonet we do not promote synthetic pesticides.