Question
we have a pine with blackish green worms hundredds of them. What could this be?

Answer
These are called sawflies. It is the only caterpillar type insect that will eat pine needles. Sawflies are a group of insects related to wasps and bees. Their name is derived from the saw-like ovipositor the adult female uses to lay eggs. Adult sawflies are inconspicuous wasp-like insects that do not sting. The larval or immature stage of sawflies are plant feeders and look like hairless caterpillars (the immature stage of butterflies and moths).

Larvae feed in groups on the previous year's needles and eat all previous-season needles on a single branch before moving to another branch to continue feeding. They will vacate a tree for a new host once all previous-season needles have been eaten. Larvae never eat new needles, but may feed on the bark of new shoots. European pine sawfly seldom kills trees since new foliage is never eaten; however, repeated defoliations can slow growth. Feeding on bark of new shoots may cause twig mortality but it is rarely serious.

The sawfly usually eats the older needles and the tree does not die from this defoliation. These can be controlled by removing the sawfly caterpillar from the tree or by spraying insecticides.

Cultural/Mechanical Control - Larval Destruction - Colonies of larvae can be easily removed by clipping off the infested branch. Place these branches in a plastic bag and destroy. Colonies can also be knocked off by sharply striking the infested branch. Crush the larvae or knock into a pail of soapy water. If few colonies are present, they can be controlled using these methods but large infestations are better controlled by general spraying.

Insecticides with the active ingredients acephate, malathion, carbaryl, esfenvalerate, rotenone and pyrethrin have longer residual properties, providing some extended control against pine sawfly larvae. The foliage will have to be sprayed to kill the caterpillars. Check with your local nursery for one of these products.