Mites in pome and stone fruit

An eight-legged animal

Mites are eight-legged invertebrates related to spiders. They’ve been around for about 400 million years and have diversified to occupy many ecological niches. Numerous mite species are beneficial. Some support soil health by decomposing organic material while others assist growers by predating orchard pests, including their plant-parasitic relatives.

Plant-parasitic mites can cause significant losses in a wide range of crops. They include pests such as two-spotted spider mites that attack hundreds of plant species. The most common mites impacting pome and stone fruit in South Africa are listed in Table 1.

“The big problem with mites in fruit trees is that they’re a sporadic pest,” says Saccaggi. “In outbreak years, they can be devastating, and then the next year they’re gone.” Mites can’t fly, and plant-parasitic mites don’t walk far. “They get blown by the wind, and spider mites use their webbing as little parachutes,” says Saccaggi. This practice is known as ballooning and can transport mites up to three kilometres. Mites can also catch a ride on people and equipment.

Female mites lay eggs on the surface of plants. The eggs hatch into larvae that progress through several immature stages before reaching adulthood. Some mite populations – such as most bryobia mites – are entirely female and reproduce asexually.

Two-spotted spider mites overwinter as adults and can continue reproducing on alternative hosts if the weather is mild. European red mites and bryobia mites overwinter as eggs laid in sheltered spots such as bark crevices.

Saccaggi highlights the mites’ potentially explosive reproduction. “The life cycle of spider mites under good conditions can be as short as seven days,” she warns. A female can lay hundreds of eggs over her lifetime of two to four weeks, and spider mites can complete several generations in a season.

Know your mites

Plant-parasitic mites damage plant tissues by piercing cells and sucking out the contents. Researchers have estimated that spider mites destroy about 20 cells per minute. Each feeding site becomes a pale spot, and extensive damage presents as stippling.

“Spider mites have the biggest economic impact,” says Saccaggi. “We do have eriophyoids on pome and stone fruit in South Africa, but not species that have a massive economic impact.” She emphasises that growers must distinguish between mites, as different species require different control measures. “You should be able to tell the common spider mites apart with a hand lens,” she says. The identification pointers below apply only to adult stages.

Two-spotted spider mites, European red mites and bryobia mites are roughly similar in size. They have oval bodies and long legs. European red mites have the heaviest build, and bryobia mites the lightest build of the three.

Both the two-spotted spider mite and the European red mite can be red. Whereas European red mites are always dark red, two-spotted spider mites also come in shades of green and brown. Bryobia mites are dark reddish brown.

European red mites have prominent white hairs that emerge from white spots on the body. Two-spotted spider mites have finer hairs, and bryobia mites appear comparatively hairless. In addition, bryobia mites can be distinguished by their extremely long front legs – at least twice as long as their body – whereas the legs of the other two species are similar in length. Of the three problematic spider mites, only two-spotted spider mites produce prominent webbing.

Eriophyoid mite adults are about 0.2 mm long and difficult to spot. “You would only see symptoms such as rusting or silvering of leaves,” says Saccaggi. “A high infestation will go over to the fruit, which will discolour or have corky lesions, similar to those caused by thrips.”

Growers suspecting eriophyoids can submit affected plant parts to the Stellenbosch University Plant Disease Clinic. Their team includes pest and disease specialists. “It’s very important that scouts can recognise spider mites as different to predators, so they don’t confuse the two,” says Saccaggi. Besides the spider mites’ reddish colour, they are also slow-moving. In contrast, predators are brown or cream-coloured, teardrop-shaped, and zoom about.

First, do no harm

In general, mite problems are more likely to be caused than cured by spray applications. “There are very effective natural predators in the system,” says Saccaggi. “As long as you keep them happy, you typically don’t have a mite problem. But if you spray and kill those predators early in the season, you’ll have a continuous mite problem.” Some of the predators in question are mites themselves. Apart from not being sprayed, predatory mites need food and shelter. Cover crops can provide both. It’s essential to include flowering cover crops, as pollen forms part of the predatory mites’ diet.

Cover crops serve as a vital refuge for predators, as predatory mites are more sensitive to high temperatures than spider mites. Saccaggi has witnessed severe spider-mite outbreaks in netted citrus and speculates that high temperatures may be the reason.

“Spider mites can cope with 35–40 °C, but it’s too much for the predators if they have no refuge,” she says. “The few farms I’ve seen with good cover crops tend to have fewer problems with mites.” Several predatory mites are commercially available as biocontrol agents. They are covered in a previous SAFJ article by Saccaggi and co-authors.

“You have to be careful which ones you release,” says Saccaggi. “Some are specific to spider mites, so if you release them too early in the season, they will die out before the spider-mite population increases. Whereas if you release a generalist predator early in the season, it establishes on pollen and other prey before the spider mites increase.”

Beyond killing predators, some crop-protection chemicals interact with mites in alarming ways. For example, exposure to sub-lethal concentrations of some pesticides stimulates egg-laying in spider mites, thereby boosting rather than reducing populations.

Even with good management, plant-parasitic mites can cause losses. Spider mites tend to be problematic under hot and dry conditions, while eriophyoids are problematic under hot and wet conditions. Heavy infestations can not only lead to fruit damage but also to egg-laying on the fruit, disqualifying it from certain markets.

Given the mites’ rapid reproduction, effective monitoring is crucial to squashing outbreaks before they gain momentum. Monitoring is also foundational to the integrated management of other pests.

Read how to monitor effectively in our article on p. 54 of this issue.