Huge larvae in oak trees

Asked May 22, 2017, 8:30 AM EDT

Hi there - I have a oak tree next to my deck. A red squirrel lives in it, and kicks rubbish out of the hole onto the deck every week or so. The past couple of weeks, there have been really big grubs kicked out (see attached photo). They seem bigger in person! Any idea what these are? Thanks very much! Karen

Washington County Minnesota oak tree health

1 Response

The grub looking insects are larvae of various beetles we call borers.
Do you know if its a red oak or white oak? What does the leaf look like?
They could have overwintered in a tree cavity, or they could have bored into the tree. Is the oak tree in good health? Is the tree stressed? Did it have normal leaf drop last year? Have there been dead/dying branches in the past year? Does it attract woodpeckers?

Here is some initial research on grubs in trees. After I get your answers, we can look into it a bit more based on what type of oak it is and best tree care practices.

The grub looking insects are larvae of various beetles we call borers.

Many insects feed and make their homes in the bark, trunks and branches of shade trees and shrubs.

Most insect borers are attracted to weakened, damaged, dying or dead plants. These are referred to as “secondary invaders” because they attack only after a plant has been weakened by another stress. Secondary invaders are a symptom of other problems with the health of the tree or shrub, but may contribute to its decline.

Many other insects live in dying or dead trees, including natural enemies (predators and parasites) of the insect borers, sap or fungi feeders, or species which merely use the spaces provided by the tunnels and galleries as living quarters.

Wood-boring insects that attack healthy trees and shrubs are called “primary invaders.” Primary invaders may eventually kill trees.

Damage

Borer infestations often go unnoticed until plants or parts of plants begin to die or show external signs of damage. Wood-boring insects often produce sawdust-like frass (excrement). Their holes are normally round, oval or semicircular and are found in a random pattern on the plant. Woodpecker damage is sometimes confused with that of wood-boring beetles, however woodpecker damage will not produce frass.

Many borers damage plants by tunneling through the inner bark layer (cambium) into the sapwood (xylem) that transports nutrients and water to the leaves. These insects are called phloem feeders. When the cambium layer is completely girdled the plant eventually dies above or beyond the damage site. Partial girdling reduces plant growth and vigor above the site of attack. On occasion, tunneling makes the tree weak, causing limbs and branches to fall.

Managing Wood-boring Insects

Prevention

Since most wood-boring insects are considered secondary invaders, the first line of defense against infestation is to keep plants healthy. Proper care of trees and shrubs discourages many borer pests and helps infested plants survive. Good sap flow from healthy, vigorously growing trees, for example, defends the plant from damage by many borer pests. Good horticultural practices include:

  • Selecting well adapted species of trees and shrubs that are not commonly attacked by wood borers in your area.
  • Choosing and preparing a good planting site to avoid plant stress, freeze damage, sun scald and wind burn.
  • Minimizing plant stress and stimulating growth by using proper watering practices.
  • Avoiding injury to tree trunks from lawn mowers, weed trimmers or construction.
  • Properly thinning and pruning at the appropriate time for oak trees.
  • Removing and destroying infested, dying or dead plants or plant parts, including fallen limbs.