Here are some really revolting insects I recently found feasting on an elm tree. They are called Woolly Aphids. Their "wool" is actually a waxy secretion it is believed they use for protection.
Most of these aphids were bunched up together, like the ones on the bottom of this branch, rather than loners like the ones on the top of the branch or in the first photo. Their wool was billowing back and forth together in the breezes. If I were looking for an easy meal, I would not want to pick through all that wool. Who knows, there could be a sheep under there!
Most aphids, including Woolly Aphids, have a complex life cycle that includes two different host plants, winged and wingless generations, and generations that reproduce sexually and asexually. For Woolly Aphids, their two hosts are trees in the Elm Family and the Rose Family (often apple trees, but sometimes pear, quince, sugarplum, hawthorn or mountain ash).
The first generation of aphids that appear in the spring are all wingless females, which reproduce asexually. For Woolly Aphids, this cycle starts in elm trees. A few more generations of wingless females are born asexually (aphids multiply faster than rabbits!) until sometime in the summer, when a generation of winged females are born.
These females fly to nearby trees of the second host species, and asexually produce more generations of wingless females. Late in the summer, another generation of winged females is born and flies back to the first species of trees. More wingless females are born asexually until sometime in the fall, when (FINALLY!) some males are born and also some females that reproduce sexually.
It's party time in aphid town, until the eggs are laid, winter comes, and all the adults die. The eggs hang out over the winter, and start the cycle again in the spring.
The other strange thing that aphids are known for is producing "honeydew". Since it comes out of their anus, I can think of a better term for it, but apparently it contains a lot of sugar and other insects love to eat it. Some species of ants even have symbiotic relationships with certain species of aphids, where the ants protect and "milk" the aphids like a herd of cows. Some bees eat honeydew, and then produce a type of honey that is darker and stronger tasting than honey made from flower nectar.
Unfortunately, when you get thousands of aphids together secreting honeydew from their anuses, it can seem like it is raining. My hands and my camera were covered with drops of honeydew. I had to stop taking pictures because my lens was getting blurry. I hope that little detail will make you appreciate the above photos even more. Ah, the lengths I go to for my art!