Take a look at the plants and trees in your area over the next few weeks and you might notice some strange shaped swellings on the leaves and stems. In most cases, these abnormal growths are the places where botany has met entomology, or more exactly, where an insect has invaded the plant or tree.
The swelling is called a gall, and the gall is literally the way the plant has reacted to having an insect laying an egg on its leaf or bud. The plant produces a growth spurt of cells around the egg, so isolating it from the rest of the plant. This is exactly the effect the insect wanted.
If you've ever read the book 'James and the Giant Peach' by Roald Dahl, you'll know that living inside a giant peach had its rewards. There is a part where James and the creatures are inside the giant peach and they're bobbing on the sea. They're all extremely hungry so they eat some of the inside of the delicious peach, being careful not to make any holes in the skin - in case they sink. The insects which live inside galls are doing a similar thing, albeit on a much smaller scale. The gall provides a safe place to sleep. The inside walls provide food, and the outside walls protect the insect from predators. They can literally eat the walls of their bedroom, as long as they don't eat through to the outside until they're ready to fly.
Oak trees are a great place to look for galls in springtime. In May you sometimes find what look like small apples, about 4cm in diameter. These are known as oak apples, and they're home to the larvae of the oak apple gall wasp. The female gall wasp lays her eggs in a leaf bud causing the oak tree to produce this peculiar apple-like growth.
The second photo above shows what goes on inside the oak apple. The apple has been cut in half, and you can see the way the grubs have chewed their way from the centre to the outside. You'll notice that most of the grubs have pupated and flown away, but there is still one near the centre at the 12 o' clock position.
If you want to know what the adult wasps look like, try hand rearing the inhabitants. Each gall type requires its own particular conditions. Oak apples, which normally develop on twigs need reasonably dry and airy conditions, so place the gall inside a container with a bit of muslin or netting over the top, and some dry sand in the bottom to help prevent mould forming. Check the container a few times each week. The tiny gall wasps usually come out in June and July but they only survive for a few days, so you need to let them out soon after they emerge.
Some other galls you might encounter on the oak tree (see photos above) include the cherry galls and spangle galls which appear on the undersides of leaves.