Can You Believe the Gall of These Aphids?

What am I looking at?

The main image shows a female aphid nymph that just started inducing the formation of a gall in the tissues of a budding witch hazel leaf. The aphid is the green and red insect at the center of the image (1). The hair-like structures around the aphid are called trichomes (2), a product of the leaf’s epidermis that serves a protective function. The cells of the leaf are blue (3). Notice the difference in the structure and density of the cells around the gall (4), indicating their more rapid division compared to those in the rest of the leaf (3). Click on the right arrow to see some different views of aphids in their galls.

Biology in the background

Aphids are tiny insects that feed on the sap of plants, mainly through their leaves and stems. They can be found all over the globe but are most diverse in temperate regions of the Northern Hemisphere and are less common in the tropics or colder regions near the poles. Aphids have a relatively soft exoskeleton, but they have evolved other forms of protection. In this case, they recruit the plant to help defend them.

As aphids feed, they use their sharp proboscis to pierce the leaf and reach its vascular tissue (the phloem) and suck out the sugar-rich sap. In addition, as they feed, they inject their saliva into the leaf, which contains a cocktail of molecular factors that alter the metabolism and growth of the leaf. These molecules force the leaf to grow around the aphid, encasing it in a protective ball called a gall. These images highlight the difference between normal, healthy leaf tissue and the tissue of a gall.

Gall-making isn’t an exclusive specialty of aphids. Many other insects – including psyllids (relatives of aphids), wasps, beetles, and midges – can also hijack plant development to induce galls that provide them with nutrition and a temporary home. Galls result from a dramatic reprogramming of the cell biology of the plant, though the specific roles of the insect molecules in gall development have not yet been determined.

An aphid at this stage is about half a millimeter long, or roughly seven times larger than the width of a human hair.

Technique

This image was created using confocal microscopy.