What are these red cocoons in my trees leaves?

Question

The tree In my garden as lots of red cocoons on its leaves, I assume these are insects, do I need to be concerned or are they harmless?

Answer 1

Here's some Insect advice from Penn State.

In summary, it sounds like you have a mite problem for your tree (take note of the other trees, that looks like a maple leaf to which this insect is characteristic); they are "commonly referred to as eriophyid mites".

• "these galls are not harmful to the health of a tree. ... Feeding by eriophyid mites appears to stimulate the formation of galls on the upper and lower surface of the leaves... If many leaves on a tree are heavily infested, there may be some reduction in growth."
• "In general, leaf galls do not seriously affect the normal growth of a tree. Thus, chemical control is seldom suggested. If indicated, the best time to treat an infested tree ... is mid-April, before buds swell"
• Note that these mites are predated by ladybugs green lacewings, so if you have an infestation of either of those these might be contributing in some way to that.