Fruit Trees Mites: The Good, The Bad and the Ugly

Often, you will know when a pest has started to attack your fruit tree. You can see Japanese beetles munching away at the leaves. You can find maggots wiggling around inside the fruit. But some insects are too small to see with the naked eye. They can damage the leaves, slow tree growth, and in some cases even kill a tree. And that is the problem with mites and fruit trees. Just because you can't see the mites, doesn't mean they are not there.

So in this article, I interview Fred Beaulieu, Ph.D., a mite expert and Research Scientist from Agriculture and Agri-Food Canada. We explore what mites look like and how they damage trees and plants. We also talk about beneficial mites and fruit trees and the services they offer in keeping fruit trees healthy. To hear the entire interview, check out the hour-long podcast below.

How do you know if you have a mite infestation on your fruit tree?

But before we dig in, here are some things to look for if you suspect that mites are infesting your fruit trees:

• Are the leaves speckled with yellow or brown spots?
• Are entire leaves turning increasingly yellow or bronze?
• Is your tree growing very slowly or is it not growing at all?
• Is the fruit small, poor quality, spotty or deformed?

If you said yes to any of these questions, it's worth learning more about spider mites. Read on!

What do Mites Look Like?

In the video below you can see images of bad mites – and good mites. You’ll also see what mite damage looks like on leaves. The photos are credited to: Fred Beaulieu, Ph.D., and Wayne Knee, Ph.D, mite experts from Agriculture and Agri-Food Canada. After you watch the video, read the Q&A with Fred Beaulieu to learn more about plant and tree mites.

Can you see mites without a microscope? 

FRED: “People used to say that mites are microscopic or that you can’t see them. But I, disagree. You can see them if they move… Most mites are about half a mm in size. Half a millimetre is about the size of a period at the end of a sentence. So basically, if you see the dot moving, well, it could be a mite. You’ll have some mites that are a bit smaller. And many are beige. So if you see a beige dot on a green background, let’s say on a leaf…and that beige dot is moving then, you realize that something’s living. And yeah, you will believe me that it is a mite.”

What would a mite look like up close ?

FRED: “Okay, say you want to increase (magnify) the size of a mite to the size of a dog. So that would be blowing it up (in size about) two thousand times…(They) would look red or dark brown. They would have a lot of long hairs…(They have) claws at the tip of the eight legs. And then in the front you have the mouth parts… the chelicerae (pincers). So the big predatory mites often have the typical big pincers in the front with teeth on the edges of each of the pincers. And that can be pretty large relative to the rest of body size. So that’s probably the most scary thing for any prey actually.”

How fast do mites move? 

FRED: ”Some (mites) will ambush predators. If prey passes nearby, it jumps in pretty quick. But some are cruise predators and basically it walks along until he finds his prey. But some mites can do it at a pretty amazing speed. One study says there is a mite that is at least ten times faster than the cheetah relative to body size. So actually in North America, the relative to that mite is actually pretty fast. And some will go say fifty kilometres per hour if they were human size. And (they’d go that fast) not only for tens of seconds, like the fastest human sprinters. (Mites could keep this speed) for hours.”

One study says there is a mite that is at least ten times faster than the cheetah relative to body size.

Fred Beaulieu, Ph.D., a mite expert and Research Scientist from Agriculture and Agri-Food Canada 

how do mites eat their prey?

FRED: "(A mite will use its mouth parts to catch prey) using its big pincers like a scorpion would. But they also have the front legs and the palps which they use to push the prey sometimes. Mites in the soil have front legs that are so long that they use them to ensnare or trap the prey. And to sense them or smell or taste them by touching from the prey using the tip of the front legs. But then (they use the palps) to ensnare the prey and push it towards the mouth of the chelicerae… Just one chelicerae would be enough (to catch the prey) and with the other one it can make a hole in the prey. It rips the skin and opens up the hole using the pincers. Then it accesses the meat in there. Then the mite will (inject) saliva which will help digest the meat inside. And then they will suck it up."

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How can mites affect fruit tree health? 

FRED: “(When the population of rust mites or spider mites is low) they just jab a cell at a time on the leaf… But then mites can jab twenty cells per minute. So if there are many spider mites on your leaves, then after some weeks there is yellow stippling or yellow dots. The leaf will become yellow or silver brown. So then this leaf (becomes) dysfunctional and it can drop…So there is less photosynthesis and the tree’s health is a bit hammered, and the fruit harvest can be affected and so on. Now, if winter is difficult or severe afterwards, it could further weaken the tree because it’s already weak after (the mite infestation)."

Do mites have eyes? 

FRED: “Many mites don’t have eyes at all in part because they live in cryptic habitats in the soil beneath the leaf litter or in cow dung or in animal nests. Or they live inside the skin of animals. So they don’t have eyes many of them. But even those mites can actually sense the light. They can react to light, perhaps because there is a sensor or an optical nerve somewhere. They can have reception of light somehow.” 

Which mites are fruit tree pests? 

FRED: “The two spotted spider mite is probably the most common mite all over the world on plants and including fruit trees. In part, it’s because it reproduces so fast. But it’s (able) to start a colony on most (different types of) plants. So it’s a generalist and that’s one of the reason why it’s bad – because it’s everywhere. It can move from one host to another easily as well. The two spotted spider mite is at times a major pest on some fruit trees and smaller fruit crops. The European red mite is another one that at times can be worse than the two spotted spider mite. But the European red mite is less common on other plants. So it’s more of a fruit tree pest than the two spotted spider mite.”

The two spotted spider mite is probably the most common mite all over the world on plants and including fruit trees. In part, it’s because it reproduces so fast.

Fred Beaulieu, Ph.D., a mite expert and Research Scientist from Agriculture and Agri-Food Canada 

How do I know if my fruit tree is infested with mites? 

FRED: “I think you’d have to look at the leaves. So if you see stippling – yellow little dots – on the leaf, then you could suspect mites. Unfortunately, some insects can do similar damage. So I’m not always entirely sure. Some homoptera or some plant hoppers say could do something that appears similar… But to be sure you just look using a hand lens to see beneath the leaf (usually beneath, but it can be on the upper surface as well).  If there is a mite there or if there are a few mites. So if you see spider mites on a leaf stippled with a bit of yellow then boom, it’s almost certain that mites caused the stippling. "

What do mites eat?

SUSAN: “But it sounds to me like they eat different things. Some of them prefer a meat diet (of other creatures) and some of them like leaves because they can penetrate the cells and get some good stuff out of that. And now it sounds like some of them like fungus. So is that the three different types of mites you’ll have? They will eat only one of those three things?”

FRED: “If you want to gather the three main categories of what mites feed on, you’re missing only one. The parasites. So the parasite is somehow a bit like a predator. It attacks animals. But instead of eating many animals and killing them all, it is just slowly feeding on a single host on a single animal. So, yeah, you have hundreds of thousands of species of parasitic mites. Most of them are not described (named). But these parasites can feed on vertebrates, like birds I’m told. And even amphibians and reptiles. And even in sea animals like seals and turtles.  But also invertebrates. Many mites are parasitic on insects.”

What are gall mites? 

FRED: “The gall mites. It’s from the super family Eriophyidae. It’s the scientific name for all these mites that we call gall mites, rust mites, blister mites, or bud mites. Each species sometimes has a common name. They are often defined or named because of the gall that they produce. So the bud mites will produce enlarged buds, for instance on hazelnuts. They get into the bud and the bud gets much larger and then it probably won’t produce a hazelnut. If you say blister mite, that’s a type of gall, the blister gall. It’s when the one of those Eriophyidae mites get inside the stomata (pores in leaves), the breathing hole in the leaf of the plants. So it gets inside the leaf and starts a colony there.”

How are galls formed?

“Whether it’s a blister or pouch gall, finger galls, (the galls feeding on the leaf triggers a change in the) plant tissues made by plant. It’s the plant that creates this development. It’s complicated. But genes are involved. But it happens only when the given plant feeder feeds on it. Because you have other insects that can produce galls. But the plant has evolved somehow to react to that specific plant feeder to create this gall. So say if you have a gall mite on prunus (cherry trees) and some of these gall mites may have overwintered in the bark or on a twig or in a bud. But when the spring comes and the leaves open, then the mite will feed on the underside of the leaf. And then (in the case of finger galls) the leaf will react in making this little finger-like house. And so, basically it will be bulging from the top of the leaf but beneath the mite this feeding on the tissue but will get invaginated inside the gall.”

Why are gall mites hard to control on fruit trees?

FRED: “With the gall mites, those making the little houses, you’ll have hundreds of eggs inside the galls. And hundreds of adults in there at times anyway. So, yeah, these guys will never be easy to control whether by pesticide or by a biocontrol agent because only a few mites or a few predators could get in these little galls. It’s a pretty tight entrance. But the thing is these mites that are making galls on fruit trees will not necessarily be a problem. From the orchard people I talk to, they say the damage of gall mites isn’t as bad as the damage from spider mites or gall mites. So rust mites are free living (underneath the leaf). So these could be affected by pesticides and by predatory mites crawling around. But the thing is though, we need to do more research and find solutions.”

How do organic growers deal with mites on fruit trees?

SUSAN: ” I have another email here (sent to The Urban Forestry Radio Show during the live interview) and I want to read it to you. Robert from central Utah, writes, “The mites love it in my orchard. Every year. I can see them moving about in the spring. Since learning about them a few years ago. I’ve been watching and observing them in my orchard.  While the mites caused some damage to the younger trees, on the whole they aren’t much of a problem for me. From what I’ve read orchardists who try to control them are committed to continued spraying, and it seems to lead to a nasty cycle of sorts. I’ve kept my orchard floor in native grasses and weeds. And there’s a vibrant population of native bees, ladybird, beetles, and mantis. I feel that when they first hatch they’re so small, they probably depend on the mites, and their eggs for their initial diet. In addition, there are two or three native species of predatory mites that control the spider mites through the seasons. For those reasons. I don’t spray instead depending on mother nature to take her course.”

FRED: “That’s fantastic….We know in the past that if there is a pest that you have to deal with…the only short term solution is a broad spectrum pesticide or something else. Then you’ll see if it causes a problem for the mites. If all the predatory mites are killed (the good mites) and then you have an infestation of bad mites you have a problem. But indeed, what he described there sounds like an ideal solution. In Quebec there are some good stories like that in orchards where they barely use, or don’t use pesticides at all. They use just a little bit of selective insecticides for some insect pests. And indeed, as he says, it’s a question of how you manage your yard … perhaps to attract other predators – including predatory mites. But this guy knows a lot.”

Most mites are good. Some directly to us but others indirectly because they help with decomposition of nutrients and the recycling of nutrients in the soil.

Fred Beaulieu, Ph.D., a mite expert and Research Scientist from Agriculture and Agri-Food Canada 

What are beneficial mites?

FRED: “Most mites are good. Some directly to us but others indirectly because they help with decomposition of nutrients and the recycling of nutrients in the soil. A very large portion of mite species live in the soil. And a large portion of those that live in the soil are detrivors. Basically, they will be like earthworms eating the detritus and fungi and they help the soil maintain structure and recycle nutrients so that trees can eat. So that’s one large group of beneficial mites or good mites. But also in general mites are part of the food network. They are there to be food for other mites or other insects or animal. As part of the food chain they are very important and help with the stability of the ecosystem. 

To listen to the entire interview, listen to the podcast below. And to learn more about fruit tree pests, listen to episodes 20, 33 and 46 of The Urban Forestry Radio Show and Podcast.

Susan Poizner

Director, OrchardPeople.com Fruit Tree Care Education Online

Susan Poizner is an urban orchardist in Toronto, Canada and the author of Grow Fruit Trees Fast and Growing Urban Orchards. Susan trains new growers worldwide through her award-winning fruit tree care training program at Orchardpeople.com. Susan is also the host of The Urban Forestry Radio Show and Podcast and an ISA Certified Arborist.