Plant Galls Caused by Insects and Mites

Galls formed on a red maple leaf by the maple eyespot gall midge

“In all things of nature there is something of the marvelous.” Aristotle

Galls that occur on plants may be caused by insects or mites, nematodes, bacteria, fungi, or viruses. I think the more interesting ones are those formed by insects and mites. Plants that have rapid growth in spring are especially vulnerable to gall-forming insects, but often galls go unnoticed until later in the season. The chewing of the insects produces salivary secretions that cause an increase in growth hormone production in the plant tissue. This causes an increase in either cell size or cell number and results in abnormal plant tissue growth called a gall.

Galls likely from Eriophyid mites on poison ivy

Inside developing galls, the immature mite or insect feeds where it has protection from most predators. In the case of stem galls produced on goldenrods, the fly larvae inside may be eaten by woodpeckers who are savvy to the protein morsel that lies within. Leaf galls are normally not a health issue for the plant as enough unaffected tissue can still effectively photosynthesize.

Willow pinecone galls are produced a midge, Rabdophaga strobiloides 

 Black cherry spindle galls are the most common of the galls caused by Eriophyid spp. of mite. As the name suggests, they are found only on native black cherry, Prunus serotina, and especially on young saplings. Small finger-like galls are formed on new leaves in early spring on the upper sides of leaves. They are green at first, and then turn a bright red, and often occur in large numbers that are tightly packed. Leaves produced later in the spring and throughout the rest of the growing season will not be affected, as there is only one generation of this mite per year.

Spindle or finger galls on black cherry

Another unusual gall caused by an Eriophyid spp. of mite is the birch Erineum or velvet gall. This gall looks like pink fairy dust was sprinkled on the tops of the leaves and which then got very hard.  These galls can also be found on maple, linden and certain viburnums.

Velvet birch gall

Grape tube gallmaker galls occur on wild and cultivated grapes. The Cedidomynia viticola midge lays eggs on the grape leaf, and the leaf then responds by producing pointed tube galls. This gall maker is not of economic importance.

Grape tubemaker galls on wild grape

Cockscomb galls on elm are caused by an aphid, Colopha ulmicola. Aphid nymphsemerge in spring from eggs laid on elm twigs and begin feeding on newly emerged leaves, producing a gall that grows around the nymph. Green at first, the gall turns red later in the summer. Winged adults emerge from the leaf galls and fly to nearby grasses where they produce offspring that feed on grass roots. As these nymphs become winged adults in the fall, they fly to elms and lay eggs, and the cycle begins anew.

This elm cockscomb gall will turn red by summer’s end

Aspen petiole galls are formed by the Contarinia petioli midge and are found on cottonwoods and aspens. Each gall contains a single larva, which exits the gall in the autumn to pupate on the ground. This gall is formed at the leaf petiole and does not affect the leaf functions.

This red aspen petiole gall is hard to miss

Oaks seem to have the greatest variety of galls, especially those caused by gall-forming wasps. There are wooly galls, nipple galls, jumping galls and oak apple galls to name a few. The banded bullet gall wasp Dryocosmus imbricariae produces rather attractive twig galls.

Banded bullet wasp galls

The wool sower gall is caused by the tiny Callirhytis seminator wasp, and appears only on oaks, especially white oaks. The wooly galls resemble a toasted marshmallow as it turns from white to brown over time. Inside the gall are small, hairy seed-like structures, each containing a single, developing wasp larva. These structures get darker during the year and by late summer the entire gall is brown.

The wool sower galls later in the summer

Acorn pip galls, caused by the diminutive wasp Callirhytis quercusoperator, are located between the cap and the acorn. When the white gall pushes out, after the acorn has fallen prematurely, there is a tiny triangular hole left. These galls are generally found on red oaks. Ants sometimes chew the pip galls out of the acorn.

Two acorn pip galls pushing out

Colorful maple eyespot galls are caused by a midge, hickories are hosts for a large variety of gall forming insects, and willows have an interesting cone gall that occurs on new branch tips. There are also galls that form on herbaceous plants- the most identifiable ones being those on goldenrod stems caused by a stem gall fly. Downy woodpeckers go after the larva inside the gall.

Left- goldenrod rosette gall right- goldenrod stem gall

The next time you come across some interesting galls, see if you can break one open and find out if an insect is inside. The mystery will be solved, if so. It could be a nymph, a pupa or, rarely, an adult getting ready to chew its way out. Oak apple galls are the most interesting, especially if you find a female wasp that has just emerged from its pupal case.

Oak apple gall

Female oak apple gall wasp emerged minutes before from the apple gall

Galls on hickory leaf caused by a Caryomyia species midge

Pamm Cooper

 

The Remarkable World of Some New England Goldenrods

One of many goldenrod species

Goldenrods, Solidago ssp., form one of the most interesting interrelationships between flora and fauna of the late-season flowering plants in New England. The name solidago is from two Latin words meaning ‘to make’ and ‘whole’, referring to its use as herbal remedies in the form of teas or compresses, among other uses. Goldenrods are perennial herbs that are members of the Asteraceae, or aster, family. Flowering from August through September, they are often found blooming together with Joe-Pye weeds and asters. The time of year that they bloom has made them a scapegoat for many allergy sufferers who believe they are to blame them for symptoms that are actually due to ragweed that flower at the same time.

 

Honey bee on downy goldenrod.

 

Goldenrods naturally produce rubber, and Thomas Edison actually experimented with the cultivation process to increase the rubber content in the plants. George Washington Carver and Henry Ford devised a process to make a much needed rubber substitute from goldenrod during World War II. It was rather tacky and not as elastic as true rubber, but goldenrods and other native plants such as Asclepias and Chrysothamnus have rubber in sufficient quantity that may one day prove worthwhile. Tall goldenrod (Solidago altissima) had the most rubber content at 6.34 %.

Goldenrods have a unique type of inflorescence that consists of many tiny flowers that aggregate together in a flower head and form a ‘false flower’. The individual flowers are most commonly in the form of ray flowers or disk flowers. Identification of species is often done by observing the hairs on the seeds, which may be visible when the plant is still in flower. Goldenrods vary in height, with the tallest (Solidago altissima) at six feet. Some, such as sweet goldenrod (Solidago odora) have pleasant odors.

Joe- pye weed and goldenrods blooming together at Harkness Park in Waterford, Connecticut

One of the most common goldenrods in New England is the Canada goldenrod (Solidago canadensis). It is considered alleopathic to sugar maple seedlings, producing chemicals that inhibit their growth. Habitat is disturbed areas like meadows, fields or roadsides. This is a tall plant with hairy stems and a plume flower arrangement.

Asters and goldenrods growing together in a waste area

It is associated with the goldenrod gall fly (Eurosta solidaginis) whose larva feed inside a round gall on the stem which is formed by the reaction of the plant to the larva’s saliva. You can easily find these galls when green or later in the season when stalks turn brown. The larva chew an exit hole before the plant tissue hardens up for the winter. In the spring, the adult fly will exit through this hole. Downy woodpeckers and chickadees will peck at these galls to access the larva, especially in harsh winters. Studies have shown the larger the larva inside the gall, the less likely it is to be parasitized by other insects or eaten by birds like downy woodpeckers in the winter. The goldenrod gall moth also causes a stem gall, but this is a spindle- shape rather than a ball. The caterpillar hatches from an egg laid the previous autumn and feeds its way into a stem.

goldenrod bunch gall on left and stem gall on right, caused by the goldenrod gall fly (Eurosta solidaginis)

Licorice goldenrod (Solidago odora) has a licorice or anise scent and the leaves were used in a tea by the Cherokee for colds, coughs, and fevers. This plant is found in the southernmost parts of the New England states, but is absent in Maine. Found in woodlands, along roadsides, disturbed sites and old fields, the flowers have been used to make deep yellow dyes and attract beneficial insects such as lady beetles and lacewings.

White goldenrod (Solidago bicolor) is found at the edges of woodlands. It is also sometimes called ‘silverrod’ in reference to its white flowers. It is the only goldenrod with white flowers in the eastern part of the country. The stamens and pollen will give it a slightly yellow look. Sometimes the spectacular brown hooded owlet caterpillar can be found on this plant where it primarily eats the flower buds and flowers. Found more often on any goldenrods with longer flower spikes, this caterpillar is a favorite of many lepidopterists.

Silverrod at the edge of the woods.

Early goldenrod (Solidago juncea) gets its common name from its bloom time, which can be as much as a month prior to many other goldenrod species. This attractive, slender plant has a very delicate appearance and can be distinguished from other goldenrods by the lack of, or near lack of hairs on the stems and leaves. White-tailed deer, woodchucks, cottontail rabbits and livestock may feed on the plant if less desirable food is available.

Goldenrods provide a source of seeds for eastern goldfinch, tree, swamp and song sparrows as well as some migrating warblers such as the yellow- rumped warblers. Mice and other rodents eat the seeds throughout the winter and have a better time of it when seed heads are pressed down against the ground by heavy snows.

The asteroid caterpillar

Any insects still around in late summer that have an interest in flowers may be found on goldenrods, especially pollen and nectar seekers and their predators. Some of the many insects and other arthropods that rely on goldenrods for survival are bees, wasps, butterflies, moths, flies, beetles, grasshoppers and spiders. Many of these visit for the pollen and nectar often in shorter supply as the season winds down. Migratory butterflies, especially along their shoreline routes, depend upon goldenrods for food sources as they travel south for the winter. Bloom periods are extended for at least two months as different species of goldenrods bloom in succession or coincide with each other.

Gray hairstreak butterfly

Black and margined blister beetles are often found on these plants in the late summer and early fall. Many beneficial insects, such as soldier beetles and assassin bugs use the flowers as either food sources or hideouts where they wait to ambush other insects. If you see a butterfly hanging upside down without moving, check and see if an ambush bug or crab spider is feeding on it. Caterpillars such as the asteroid and flower moth caterpillars, aphids, tarnished plant bugs, and many other insects feed on flowers, stems and leaves. Wasps, goldenrod and crab spiders, praying mantids, lacewings, ambush bugs, assassin bugs, spined soldier bugs and birds prey on insects that visit or live on the plants. Cucumber beetles also feed on goldenrod pollen. Some flies cause galls on stems and upper foliage as their larvae feed.

brown hooded owlet caterpillar on goldenrod

Chinese mantids also hang out around goldenrods, and often lay their egg masses on its stems. Look for these in the winter if heavy snows have not mashed the plants into the ground. I sometimes take a stem with the mantid egg case and stick it in my garden. The mantids usually emerge by mid- May, and they disperse quickly

Mantid egg cases are often found on goldenrods where the adult females were hunting the year before. These are mantids just hatching

.

There is a great interconnection between goldenrods and vertebrates and invertebrates, and nature reveals such things to the careful observer. If you happen upon some goldenrod, or seek it out on purpose, just a few moments of careful observation will be rewarded with a peek into the drama that is on display in a simple stand of yellow flowers.

By Pamm Cooper, UConn Home & Garden Education Center