Wool Sower Gall

Visiting America is always a rewarding experience from a naturalist’s point of view. Though New Zealand has some spectacular wildlife, it simply doesn’t have the sort of diversity one finds in North America.

This visit’s best find so far has been the wool sower gall.

This spectacular fuzzy ball, found on the twigs of white oak trees, is caused by the tiny wasp, Callirhytis seminator. The structure of the gall is reminiscent of a fluffy seed head of a plant. Small ‘seeds’ inside the fluffy exterior house wasp larvae.

Galls are fascinating structures. They are made by the plant in response to chemicals produced by an insect or mite. Galls are incredibly diverse in structure and location on the plant, but all provide food and protection to the insects or mites as they grow and develop. Essentially, the insect has hijacked the plant’s biology to create a perfect home with a built-in food source.

Gall-making evolved several times in different groups of insects and mites; it’s clearly a successful survival strategy.

But the plants aren’t entirely defenceless. In response to the gall insects, many plants produce chemicals that attract predators to eat the damaging insects.

In turn, some gall-inducing insects can turn off these chemical defences in their host plants. Host and insect are constantly evolving, each trying to get the better of the other. The galls are a spectacular result of their arms race.

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