Many insects do something extremely interesting, and it's something you can probably see right around your house. They make galls...
The short story is that certain behaviors (such as feeding or laying eggs) of certain insects cause plant cells to begin multiplying like crazy until something is developed a lot like a plant tumor. This tumor-like growth is called a gall, and there is simply an amazing variety of galls. Moreover, very often it is easier to determin the presence of an insect by the gall it has formed, than it is to find the insect itself. Typically, when larvae develop inside these galls they eat the gall's tissue, which may be much softer and more nutritious than regular plant tissue. Often galls also provide protective homes for the larvae or even adult insects.
Here are some case histories from some of the most common and conspicuous galls right around my own home:
Notice the fuzzy items along the midvein of the Water Oak (Quercus nigra) leaf above. These items are Vein Pocket Galls, which were caused by the larval (maggot) stages of very small flies called midges. The gall's existence begins when the unfolding leaves start flattening out. At that time, the midge lays its eggs, the eggs hatch and the tiny maggots move to the veins and begin feeding. This seems to cause the vein tissue to start growing, and in a few days the maggots find themselves encased in the fuzzy gall tissue you see. Later in the spring the mature larvae will drop to the ground and remain there through summer and fall, and over the winter. Then next spring the adult midge will emerge, lay its eggs on another Water Oak leaf, and the cycle will repeat.
At the right is a grape-size gall on a Black Oak leaf (Quercus velutina). This is a Roly Poly Gall. The gall is about the size of a grape but it's hollow. Roly-poly galls are caused by gall wasps in the genus Andricus and are called roly-poly because the wasp grub develops in a seedlike shell that lies loose inside the hollow gall, so that if the gall is shaken, it rolls around. Though the gall is common around my trailer, the life history of the Andricus wasp causing it isn't well known. The roly-poly gall is probably an alternate generation for a twig gall not now recognized. One wonders how the rolling grub structure inside the gall gets its nutrients. Evidently it absorbs nutrients directly from gall's wall as it rolls around inside it!
In abandoned, weedy fields, one of the most commonly encountered gall is the one at the left. This is the golf-ball-size Goldenrod Stem Gall caused by the small fly Eurosta solidaginis. In the picture you can see the escape hole excavated by the grub that grew up inside the gall. Obviously the grub is much smaller than the gall, so it has plenty to eat. One reason the grub needs so much food is that when it metamorphoses into an adult, the adult never eats! It just searches for a mate, has sex, and the female sets the stage for the next generation by laying eggs on young goldenrods.
The gall at the right is a very common one on cottonwood trees. It's the Poplar Petiole Gall, caused by the Poplar Petiole Gall Aphid, Pemphigus populitransversus. The inset framed in yellow shows a cottonwood leaf, and the small arrow points to the gall at the top of the leaf's petiole. The main picture shows a close-up of the gall with a hole cut into it so you can see the aphids inside. Before I cut a hole in the gall, all I could see was green petiole tissue, for the aphids were hidden inside. The large arrow points to a kind of natural slit across the gall, but this slit is so narrow you can't see through it.
The aphids overwinter as eggs on the cottonwood's leafless twigs. The eggs hatch in the spring as the leaves develop. When the newly hatched nymphs feed on leaf petioles, they cause galls to form and the small, dark-colored aphids move inside. As shown in the picture, the aphids secrete a white, waxy material which coats their body. After two weeks, the females bear live young that mature into winged females. These females leave the gall and find plants in the mustard family, where they bear more female, mustard-eating aphids. In the fall, winged forms appear on the mustards, and these fly back to the cottonwoods, where a male and female generation is produced, and then one egg is laid by each female somewhere on a cottonwood twigs, that egg overwinters, and the life cycle begins again next spring.
At the right you see a Dogwood Club Gall at the end of twig of Flowering Dogwood, as it appeared near my home in November before the leaves fell. This gall (the swelling at the end of an otherwise green, slender stem) is caused by a member of the Diptera order (flies, mosquitoes... ) and the "Gall Gnat Family," the Cedidomyiidae. Gall gnats are tiny flies with long legs and antennae. The fly making this gall is called a Dogwood Clubgall Midge, Resseliella clavula. It is only about one-sixteenth of an inch long (1.5 mm).
In the fall, Dogwood Clubgall Midge maggots inside galls such as shown in the picture chew their way out of the galls and drop to the soil under the dogwood trees, where they overwinter. In the following spring the maggots metamorphose and in late spring adults emerge and lay their eggs in the tree's terminal buds where the tree's terminal leaves for the current season are still unexpanded and minute. Upon hatching the maggots work their way to the home of their future gall. Feeding causes the formation of an elongate gall, where the maggots live in a central cavity until fall, when they chew their way out, and begin the cycle again. From 1 to 39 maggots may be found per gall.
At the left the brown, spherical item with a dead brown leaf dangling from it is the Hackberry Petiole Gall. These galls are produced by members of the Homoptera (aphids, cicadas...) in the "Psyllid Family," the Psyllidae, which are small, plant-eating insects. The psyllid causing the gall at the left is the Hackberry Petiole Gall Psyllid," Pachypsylla venusta,which is about ¼-inch long (5-6 mm) from nose to wing tip, and looks like a very small cicada.
This insect has incomplete metamorphosis, so nymphs look like small editions of adults. Nymphs overwinter in galls such as the one in the picture, found on a Sugarberry tree (very closely related to Hackberrys) in November. In the spring the nymphs emerge as adults, the adults mate, and eggs are laid. On leaf petioles (stems) of new leaves, subspherical galls form around the young nymphs as they feed on the plant tissue around them. Galls contain several developing individuals in separate compartments, and each compartment is lined with wax. There may be as many as 13 nymphs per gall or more. Nymphs develop throughout the summer and overwinter inside the galls. Often these nymphs fall victim to parasites of other Hymenoptera larvae. Infested leaves die in the fall but do not fall from the trees, and thus are easy to spot during the winter when other leaves have fallen off.
At the right the bulging, woody, brown thing is a Horned Oak Gall, a rather hard growth about the size of a golf ball growing on the twig of a Black Oak tree. This gall is made by a tiny member of the Hymenoptera order (ants, wasps, bees...) and the "Gall Wasp Family," the Cynipidae. It is the Horned Oak Gall Wasp, Callirhytis cornigera.
This gall and its wasps, though common, are especially interesting because of their complex life history. Adult wasps emerge from stem galls such as the one at the right in May and June to lay eggs on the larger veins of leaves of various oak-tree species. The resulting larvae cause oblong, blisterlike galls to develop in the leaves' veins. Adult wasps emerge from these leaf-galls about three months later, they mate, and the females lay eggs in young oak twigs. At first a series of small, brown galls about the size of marbles appear along the twig but these eventually coalesce into the roundish gall pictured here, up to 2 inches in diameter. A study in Kentucky found that the wasps inhabited the twig galls for about 33 months. The galls are covered with short "horns" through which the adult wasps eventually emerge. In the picture you can see three or more of these exit-horns. When the wasps emerge from the twig-galls they attack leaves and leaf-galls are formed, starting the cycle over.
That's the Wool-sower Gall at the left, a fuzzy ball about an inch in diameter on a White Oak twig. If you split the gall open you find seedlike items. Wool-sower Galls are fairly common in some parts of the country. This picture was taken in northern Mississippi on a tree with maybe a dozen such galls on it. The gall was made by a Cynipid Wasp, Callirhytis seminator. Cynipid Wasps, members of the Family Cynipidae, are 4-6 mm long (0.16- 0.24 inch), so they are much smaller than the wasps you see buzzing around gardens and house eaves. They are black and shiny with roundish abdomens and thoraxes, with a sort of hump on the back. Lots of gall-making wasps are found in this family. Some species live in galls made by other organisms!
The gall at the right is very commonly found in eastern North America. It's the Hackberry Nipple Gall, and that's a Hackberry leaf the galls are on. These galls are made by small, winged insects about the size of aphids (2-5 mm long), and closely related to them. Homopterous members of the family Psyllidae, they are Pachypsylla celtidis-mamma. The adult Pachypsylla overwinters in tree-bark cracks, then mates in the spring. Females deposit their eggs on the undersides of expanding leaves, then nymphs hatch in about ten days and begin feeding. This feeding process causes leaf tissue to grow rapidly into a pouch or gall surrounding the insect. Inside the gall the developing insect passes through several stages (instars) before emerging as an adult in the fall.
At the right you see another gall occurring on the upper surface of an elm leaf. The ruler at the left is divided into millimeters, so the little gall is only about 5 mm high (0.2 inch). The inset at the bottom left shows the bump on the leaf's surface.
Though this gall is very much like the ones described above -- there's an egg inside which will produce a larva that will eat the gall's soft tissue -- this gall is not produced by an insect! The gall is called the Elm Finger Gall and it's caused by a mite called Eriophyes ulmi. And we all know that mites aren't insects, but rather are related to eight-legged ticks and spiders!
So that picture, as well as the one at the left showing Maple Spindle Galls caused by yet another species of gall mite, are here just to remind us that all galls aren't produced by insects.