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Insect Pests of Pine Trees

July 29, 2019

Written by Dave G.

Pines – big and small – were the subject of a recent blog, and they are beautiful trees that bring a unique look to any garden, with so many to choose from. Pine trees are usually trouble-free, but, like all plants, they can be attacked by insects from time to time. Often these attacks are unsightly, but not fatal, unless the tree is very small, but they can weaken it, leaving a tree more susceptible to disease attacks, or environmental problems. Some are more serious, and these have become significant problems in some parts of the country. An idea of what a pest is, and how serious (or not) it might be, is useful knowledge for every gardener, so let’s look at some of the insect problems you might see on your pine trees.

Something is eating the needles on my pine tree!

Pines such as Mugo Pine are popular garden plants, and since they are dwarf and low to the ground, we notice their shoots and needles a lot. If your Mugo Pine looks thin, with needles only on the last few inches of the stems, then you could have a problem. You will probably see that many of the needles have been reduced to short stumps. In spring, when the new shoots are just beginning to show, look at the older needles below them. If you see tiny caterpillars, then you probably have Sawflies. If they are disturbed you will see them rear up on their hind legs and wave their bodies in unison to scare you away – something true caterpillars would never do. These are the young of a small, fly-like wasp, not a butterfly, called a Sawfly. These are found on a wide range of pine trees, as well as on many other trees in the garden. On pines the females lay their eggs in a slit along the needles, and evolution has taught the babies to only eat older needles, not the newly emerging ones, so trees will usually not die, just become thin and unattractive.

Most pine trees can be attacked by one species or other of sawfly, but in gardens the most commonly seen one is a European species that attacks not only Mugo Pine, but Japanese pines and Scots pine. Once they are fully grown – it only takes a short time – the larvae drop to the ground, pupate in the soil, and emerge as adults to lay more eggs in fall. Any kind of insect spray, or hand picking, or even a strong water jet, will remove the larvae, and since females don’t travel far, doing this for a couple of seasons will usually end the problem.

There are holes in the bark of my pine tree!

Holes in the bark can indicate serious problems, but first, are the holes at random, or in rows across the trunk? If they are in neat holes in rows, these are sapsuckers – a woodpecker relative that feeds on the sap. Although a little unsightly, they rarely cause problems unless the attacks are very severe. If, on the other hand, the holes are scattered at random, then your tree is probably being attacked by bark beetles. These are mostly small, brown beetles that burrow under the bark and lay eggs. The grubs chew burrows in the sap wood, eating as they go, and damaging the circulation of the tree. Some, like the Pine Sawyer Beetle are larger, about ¾ of an inch long, and the adults have very long antennae. If you see a lot of gum and resin on the trunk where the branches come out, this mixture of sap, sawdust and frass (insect poop) comes from the burrowing caterpillars of the Zimmerman Pine Moth, which can be found on Austrian and Scots pine.

These insects are a serious problem, since they carry nasty diseases – blue-stain fungus and pine wilt nematode – that will kill trees completely. The presence of these pests is a sign of stress – healthy, vigorous trees don’t usually suffer from them to any significant degree. Drought, lack of nutrients, covering the root-zone with soil – all these are factors that weaken trees and lead to them being attacked. Keeping your trees watered during dry periods and fertilizing when young is the best way to protect them. Never plant a pine tree deeply and keep the area at the base of the trunk free of extra soil and accumulated debris. Keep mulch well away from the trunk.

If you do have a tree die from bark beetles and the diseases that follow it, don’t just leave the tree in place, or stack it for wood. The beetles continue to live in the pieces, and then migrate to healthy trees growing nearby. Instead, burn it, chip it or bury it, and break the life-cycle of these dangerous pests, for everyone’s benefit.

What are these spots on the needles and white fuzz on the stems of my pine tree?

People often ask about white spots on the needles and stems of their pine trees, or clusters of white ‘fuzz’ on stems and around shoots. Those white spots are scale insects – some are brown, and much harder to spot – which suck sap, and weaken the trees, as well as causing distorted growth. The fuzz is mealy-bugs, an insect related to scale, an they also feeds on sap. These insects spend most of their lives as stationary, legless blobs protected by a hard, protective covering, or those masses of white powder. Over time their numbers build up on a tree, until it can only produce small, yellowish needles and very little growth. It may die in time, but this is rare.

There is one pine which has natural white spots on the needles – and they are even considered to be an attractive feature. This is the Rocky Mountain Bristlecone Pine, Pinus aristata, the most commonly-grown of three species of these rugged trees, some of which are among the oldest plants on the planet. If you see white spots on the needs, just make sure this is not the pine you have, as of course it doesn’t need controlling!

These pests are naturally controlled by a whole range of parasitic wasps and other insects. If you spray with insecticides you will make the problem worse, since you will kill the predators and parasites too. On larger trees scale and mealy-bugs are usually not a serious issue, but they can be on smaller trees, and on plants in pots, such as bonsai. The best method of control is to use neem oil or horticultural oil (look for grades called ‘superior’, ‘premium’ or ‘narrow-range’). Spray in winter, and more importantly, in spring, during the stage when these insects move around, and when they are therefore more vulnerable. This will leave most of the predators alive, and over a couple of seasons the natural balance will usually be restored. If you only have a problem on one or two branches – which is common – pruning them off and destroying them can be the easiest solution.

Knowing what you have is the first step. As you can see, some things are more serious than others. If you take good care of your trees you will probably avoid these pests, but it pays to keep an eye out for them.

Comments 2 comments

  1. March 15, 2020 by david smith

    What are the white worms inside sap like bulges on my pine trees? What can I use to kill them?

    1. March 16, 2020 by Dave G

      Hm, not sure about this, from your description. Could it be Pine adelgid? https://extension.psu.edu/pine-bark-adelgid ? You will see some suggested controls on that page.