Imagine a home where the refrigerator stocks itself, the temperature regulates itself, and the trash takes itself out. If only, right? Well some social aphids (Nipponaphidini spp.) have managed to engineer such a home by forming specialised galls on plants. These fleshy growths on the plants house the developing aphids until such time as they are ready to emerge. They feed on the phloem sap of the plant host and, adding insult to injury, they even get the plant to transport their waste (honeydew) away from the gall. This unique system allows hundreds to thousands (depending on the species) of aphids to hole up in a closed system for months on end. This behaviour has been recorded in a study published in Nature Communications.
Most social aphids form galls that are not completely closed off from the outside world; instead forming galls where openings remain for waste disposal purposes. In these species a caste of altruistic soldier-aphids perform tasks such as waste disposal, gall repair and defence against predators. Aphids have wax-producing, dorsal plates, which they use to coat their waste, forming little spheres (The wax also helps protects the aphids from dessication and parasites). Soldier aphids in these galls routinely collect these spheres and dispose of them via openings on the gall. The soldiers’ efforts are further facilitated by a hydrophobic, waxy, surface on the inner walls of the gall. This repels the waste and makes it easier for the soldiers to transport to the gall opening.
For researchers studying aphids of Nipponaphidini species, the big question is : How do they persist in a closed system without drowning in their own waste? The answer appears to lie in the structure of the gall’s inner surface. Unlike galls formed by most species, closed galls have a spongy, hydrophilic inner surface which absorbs the aphid’s waste and then transports it away from the gall via the plants vascular system. To observe the difference yourself, take a sheet of paper and place a drop of water on it. The water will be absorbed and will wet the paper, mimicking a closed gall. Now try the same thing but use a crayon, or a candle, to coat the paper with wax. This time the water will sit in little spheres on top of the paper, much like it would in an open gall.
The researchers postulate that wax production by aphids in both open and closed galls facilitates waste disposal, but in different ways. As we’ve already seen, aphids in open systems will coat their honeydew in wax to aid its removal. In closed galls the aphids don’t do this, but wax production utilises sucrose, reducing the sucrose concentration of the aphid’s honeydew, and thus aiding its removal via osmosis (Sucrose concentrations in the plants vascular tissues are higher and so the honeydew is drawn into the vascular system and away from the gall).
This is great for the soldier aphids whose duties are now restricted to gall repair and providing protection against gall-boring predators. Now they can just concentrate on stuffing their faces at the expense of their unfortunate host and letting that same host clean up the resultant mess (insert clumsy dig at bankers here…).
Aphids are not the only species to form a parasitic relationship with a host that protects them, feeds them, and cleans up after them until they reach maturity. Humans are frequently made unfortunate victims of equally freeloading parasites. Unfortunately for the host, while the plants only have to survive a few months of this burden, we must endure ours for close to 18 years.
Aphid close up by Shipher Wu (photograph) and Gee-way Lin (aphid provision), National Taiwan University.
Aphids in gall by Alaskanent