Question
I was wondering if you could give me any info about some very large white grubs I have been finding in maple trees I have recently removed. They are the size of my thumb and have all been found in rotten areas of the the trees. I am in Nebraska.  The trees are all at least fifty years old. I am wondering if the grubs are there because the trees are rotten or are the trees are rotten because of the grubs.

Answer
One of the flathead borers, a beetle. They enter wounds and expand the decay that the fungus causes. Here is one of the borers that attacks maple. They can be controlled by spraying the trunk with an insecticide called Merit.  
The sugar maple borer, Glycobius speciosus (Say), a long-horned wood boring beetle, is a common pest of sugar maple (the only known host) throughout the range of the tree. Although borer-caused mortality is rare, infestations lead to value loss through lumber defect caused by larval galleries, discoloration, decay, and twisted grain.

The adult borer is an attractive black and yellow beetle about 25 mm long . A prominent "W" design appears on the wing covers. Although it resembles the much more common locust borer, the sugar maple borer can be distinguished by its yellow legs and two black dots near the end of the wing covers (the locust borer has reddish-brown legs and no dots). Eggs are white and about 3 mm long. Larvae are robust and dirty white and have brownish chewing mouthparts. They are nearly 50 mm long when fully grown. Pupae develop deep within the wood and are seldom seen.

The sugar maple borer has a two-year life cycle. Most eggs are laid in midsummer in roughened bark locations-in cracks, under bark scales, or around wounds. Upon hatching, the larva makes a meandering mine beneath the bark. Mining continues until early fall when it excavates a shallow cell in the sapwood. Here it spends the winter. The following spring, the larva resumes mining, etching a deep groove in the sapwood. The mine partially encircles the bole or branch as it spirals upward.

With the coming of winter, the second-year larva bores a J-shaped tunnel deep into the wood . In the tunnel's far end, the larva forms a chamber for overwintering. Before spring pupation, the larva chews a hole to the outside through which it will emerge as an adult in June or July.

The first sign of attack is a wet discoloration on the bark. Later, coarse boring dust and frass can be found exuding from the hole in the bark, or scattered in bark crevices below the hole. Infestations occasionally cause branch death or sparse crown foliage. Old borer scars can be classed as horizontal or vertical. The former results when the larva does not complete its life cycle. Horizontal scars  generally indicate much less internal injury than vertical scars. Vertical scars attest that the larva survived into the second year. Bark ridges form over larval galleries, causing the bark to raise and crack . Sometimes the bark sloughs off, forming an open-face scar which exposes the old larval gallery in the sapwood .

Bole and occasional branch infestation almost always occur in the lower 30 feet of the tree (most in the lower 20 feet). Multiple attacked trees are common. These signs appear in all diameters of sugar maple, but they are most pronounced on larger trees.

The gallery provides an infection court for wood inhabiting micro-organisms which interact with the tree and result in discolored wood. When wounds stay open for long periods of time, wood decay fungi may invade the tree. In reacting to close the spiral wound, the tree may develop a twisted wood grain which further compounds the defect.