Native trees for insects: the beetles, wasps, cicadas and fireflies that everything else eats

Plant an oak, leave the leaves under it and turn the lights off, and most of the insect food web assembles itself.

Counted by the caterpillar species they host in the eastern forests, five genera lead the full ranking below:

• Oaks: 436
• Cherries and plums: 340
• Willows: 289
• Birches: 284
• Poplars and cottonwoods: 249

Almost everything people plant a tree to attract eats insects first. Among terrestrial birds, 96 percent rely on insects, and a pair of chickadees needs more than 6,000 caterpillars to raise one brood. The insects are picky in turn: most leaf eaters can only eat the plant lineages they evolved with. So a yard of Asian and European ornamentals has a hole at the bottom of its food web. Caterpillars have an article of their own, and so do bees. The subject here is the rest of the crowd, from sawflies and gall wasps to fireflies and cicadas, and what it takes to keep them.

Insects feed everything else

Insects are the link between leaves and nearly everything with a backbone. In a four-year study of Carolina chickadees in yards around Washington, DC, caterpillars and spiders grew scarcer as yards filled with non-native plants. The birds switched to poorer prey and raised fewer young or skipped breeding. The population held its own only where non-native plants were under 30 percent of the plant biomass. What birds do with all this is in the birds article.

The insects themselves are thinning. Malaise traps run in 63 German nature reserves from 1989 to 2016 recorded a 76 percent fall in flying insect biomass, and 82 percent in midsummer. A 2021 review of the studies that exist found reported declines commonly running 1 to 2 percent a year. One cause is land turned to farms, lawns and pavement. The others are insecticides, introduced species, a warming climate and light at night. A landowner controls more of those causes on an acre than any agency does.

Why a native tree and not just any tree

Plant-eating insects are specialists. A survey covered more than 7,500 herbivorous insect species at 17 sites, from Canada to Brazil and from Britain to New Guinea. It found that on average 69 percent of caterpillar species feed on a single plant family (about 60 percent outside the tropics). The true generalists are a thin tail. The limit is leaf chemistry, and a tree from another continent carries chemistry nothing here has met. Across the country, native oaks host more than 500 species of caterpillars; ginkgo hosts 5.

Across the counties Douglas Tallamy’s team sampled, about 14 percent of native plant genera support more than 90 percent of the butterfly and moth species, with oaks first, then willows, cherries, pines and poplars. The other guilds follow the pattern and most are narrower still. A gall wasp makes its gall on one group of oaks and nothing else. The cone gall aphid of witch hazel lives on witch hazel; the spiny witch hazel gall aphid needs witch hazel and a birch in alternate generations to complete a life cycle at all.

In a University of Delaware trial of native woody plants against their cultivars, most bred traits made no consistent difference to how much leaf-eating insects ate. But leaves bred to come out red, blue or purple were avoided. A purple-leaved ninebark is a ninebark the ninebark leaf beetle avoids. Seed-grown, green-leaved species are the safe choice, and every plant Blue Whale Trees sells is one.

Leaf chewers: sawflies, beetles, katydids and walkingsticks

The list of what eats white oak reads like a census:

• Leaves: orange-striped oakworms, variable oakleaf caterpillars, oak leaf tiers and walkingsticks.
• Wood: the two-lined chestnut borer and the oak timberworm.
• Sap: golden oak scale.
• Acorns: two genera of weevils, the filbertworm and cynipid wasps that gall the acorn itself.

And that is only the insects foresters worry about.

Caterpillars are the largest group of leaf chewers, not the only one. Sawflies are wasps whose larvae look like caterpillars and feed in groups; the dogwood sawfly feeds on silky dogwood from July, then leaves to winter in soft, decaying wood. The common true katydid lives in the canopy, eats tree leaves, sings from July into November and leaves its eggs on bark for the winter. The northern walkingstick eats oak leaves and in a bad year strips scarlet oaks in the northern part of their range. None needs control on a healthy tree. They are what the tree is for.

Sap feeders, from aphids to periodical cicadas

Aphids, scale insects, leafhoppers and treehoppers drink sap and pass the surplus out as honeydew, the sugary waste that everything from harvester butterflies to stag beetles feeds on. The colonies feed the predators below and many songbirds. The oak treehopper guards her nymphs on an oak twig, bumping predators away. The harvester, the only North American butterfly with carnivorous caterpillars, lays its eggs beside colonies of woolly aphids for its young to eat, and the adults drink the honeydew.

Periodical cicadas are sap feeders too. The nymphs spend 13 or 17 years 2 to 24 inches underground drinking sap from tree roots. Then they come up by the hundreds of thousands to the acre (as many as a million), and birds gorge on them. Each female lays 400 to 600 eggs in pockets cut into twigs, preferring oaks, maples and apples; the twig beyond the cut dies and hangs brown.

A mature tree loses twig tips and nothing else. A newly planted tree can be badly cut, so in a brood year net anything under 10 feet with half-inch mesh; insecticides do nothing measurable to the numbers. In Maryland, Brood X emerged in 2021 and returns in 2038; Brood XIV came up in 2025.

“Pesticide applications are not recommended for large, otherwise healthy trees as they can withstand the pressure of cicada oviposition with little to no long-term effects and any aesthetic damage is ephemeral.” (Penn State Extension, Periodical Cicada)

Gall makers

A gall is plant tissue grown to an insect’s instructions, and oaks are the world center of it: more than 700 of the roughly 1,000 known oak gall wasp species live in North America. When researchers reared insects from more than half a million galls collected off 32 North American oaks, 155 gall types yielded 151,075 arthropods. Among them were inquilines that move in beside the gall maker, parasitoids that hunt it and hyperparasitoids that hunt the hunters. There were as many as 15 to 25 species to a single gall type. The galls themselves need nothing from you.

“A total of 151,075 arthropods were found in association with these 155 gall types, and of these 61,044 (40.4%) were gall wasps while 90,031 (59.6%) were other arthropods.” (Ward and others, Zoological Studies, The Arthropod Associates of 155 North American Cynipid Oak Galls, 2022)

Oaks are not alone. Hackberry carries at least ten species of Pachypsylla psyllids that make its nipple and blister galls and winter as adults in its bark. Witch hazel has its cone gall aphid on the leaves and its spiny gall aphid at the flower buds. None harms the plant in any way that matters, and each is a meal for something with a beak.

Miners and borers, and the borer that is different

Miners live inside a leaf. The native holly leafminer is a small fly that lays its eggs only in American holly. The larvae tunnel the leaf all winter, and a braconid wasp kills 3 to 58 percent of them on any one tree.

Most native borers work on dead or weakened wood; the two-lined chestnut borer takes oaks after drought, fire, frost or defoliation. The locust borer, a longhorn striped like a yellowjacket, lays its eggs in the bark of black locust and no other tree. The adults emerge in late summer to feed on goldenrod pollen. Healthy trees more than 8 inches through are generally left alone. The twig girdler and twig pruner cut the twigs of shagbark hickory and pecan so the ends drop. The elderberry borer lives out its larval life inside the stems of black elderberry.

The emerald ash borer is what a borer looks like without that shared history. Since it arrived from Asia near Detroit in the 1990s it has killed ash trees, which had never met it, by the hundreds of millions. In a common garden at Michigan State University planted in 2007, every black ash was dead by 2013. In the same garden, blue ash was barely colonized and still healthy in 2022. Its phloem carries hard cells and phenolics the larvae do poorly in. It is the only ash we grow.

Flies, beetles and wasps at the flowers

Bees get the credit and their own article, but flies, beetles and wasps do a good share of the pollination in a native planting. Pawpaw is pollinated by flies and beetles; its stigma ripens before its own pollen and the trees are self-incompatible, so growers plant two unrelated trees and sometimes hand-pollinate. Flower flies, the hoverers that pass for small bees, live on pollen and nectar while their larvae eat aphids, and paper wasps drink nectar between hunting trips and pollinate as they go.

Predators and parasitoids

Everything above is prey. A paper wasp colony catches thousands of insects in a season, caterpillars among them, and feeds them to its larvae. Lacewing larvae (the aphid lions) and the alligator-shaped larvae of lady beetles clear aphid and scale colonies; assassin bugs impale caterpillars and beetles on a beak; ground beetles hunt the litter at night.

Then come the parasitoids, most of the wasps and many of the flies. A braconid lays her eggs inside a living caterpillar, and her larvae eat it from within before spinning cocoons on its skin. The swollen tan aphid “mummies” in any colony are the work of others. Tachinid flies lay their eggs on or in beetles, bugs and caterpillars. Many adult parasitoids feed on nectar, so the shrubs and flowers under the trees keep them close. With aphids on a tree, look for mummies first. They mean the work is being done.

Dead wood, leaf litter and the soil

Much of what a tree feeds, it feeds after death. A standing dead tree is bored by longhorn, jewel and bark beetles, and their tunnels become the nests of mason bees, leafcutter bees and aphid-hunting wasps. A fallen log shelters ground beetles and spiders under its bark.

Once it has rotted about two years, the bess beetles move in: inch-and-a-half black beetles that live in family groups in oak, hickory and maple logs. They tunnel the wood and chew it for their larvae. Stag beetle larvae spend several years in fallen dead trees, and no healthy hardwood forest in Kentucky is without them. Termites and carpenter ants live in the same wood. Stag beetles alone feed birds, toads, raccoons and skunks. The old-growth article counts snags and logs as structure; the habitat article says leave the wood where it falls.

The litter is the densest animal community on the property: about 250,000 mites can live in a square meter of moist forest soil, with springtails, millipedes and snails. Fireflies are litter animals.

Nearly 170 species are described from the United States and Canada. Most of a life that runs two or three years in the North is spent as a larva in leaf litter, soil or rotting wood. There the larvae hunt snails, slugs and earthworms. Pupation is underground or in rotting logs, and one genus pupates in the bark furrows of big oaks, hickories and tulip poplars. Of 132 North American species assessed against the Red List criteria in 2021, at least 18 are threatened and more than half could not be judged for lack of data.

Leaf litter, not wood chips, is the rule under the canopy. Its ants carry the spring flowers: bloodroot, trilliums, violets and wild ginger fit each seed with a fatty attachment. The ants haul the seed home, eat the attachment and drop the seed in a waste pile. The seed grows there. Salamanders and toads hunt the same layer.

Keystone trees and shrubs: the genera that feed the most insects

Only the caterpillars have been counted plant by plant, so a ranking leans on them.

The keystone genera

The counts for the Eastern Temperate Forests, with the plants we grow in each genus:

The last column names what we grow in each genus. Black cherry is the big native member of the cherries. If you already have common willows, poplars, maples, pines or elms, keep them. The three chestnuts named are native, which Chinese chestnut is not.

Six of ours with an insect found on nothing else

Beyond the keystone genera, six of ours carry an insect found on nothing else:

• Black locust, the only tree the locust borer lays its eggs in.
• American holly, the only leaf the native holly leafminer tunnels.
• Witch hazel, with its cone gall aphid and its spiny gall aphid.
• Pawpaw, pollinated by flies and beetles rather than bees.
• Black elderberry, whose stems the elderberry borer lives in.
• Ninebark, the green-leaved plant the ninebark leaf beetle eats.

Pesticides, lights and the other ways to undo it

Mosquito spraying is the commonest way a good planting is wasted. When the Xerces Society tested yards after treatments, residues were high enough to kill bees and butterflies, and 75 percent of the samples from nearby unsprayed yards were contaminated by a neighbor’s spray. Emptying buckets, gutters and birdbaths weekly does more than a truck. Nursery stock is the second route: systemic insecticides, neonicotinoids among them, move into a plant’s pollen and nectar. They reach higher concentrations and last longer in woody plants. Residues have been found months to years after a soil drench or trunk injection. “Bee-friendly” on a tag has no legal definition. Ask what was applied, and prefer seed-grown plants from a grower who can answer.

Light is the third. About 30 to 40 percent of the insects that fly to a street lamp die soon after. At least 80 percent of the firefly species in the United States and Canada find their mates by light signals at dusk or after dark. Turn outdoor lights off in summer; where one is needed, make it shielded, on a motion sensor and red or amber rather than blue-white.

“Laboratory and field studies reviewed here demonstrate that ambient light can inhibit the courtship flashing of several firefly species.” (Owens and Lewis, Ecology and Evolution, The impact of artificial light at night on nocturnal insects: A review and synthesis, 2018)

Then the smaller things. Leave an unmown strip, since tall grass shelters butterflies, moths, fireflies and lacewings. Clear the invasive shrubs and vines, which feed little and shade out what does.

Where to start

• Plant an oak, and under it a plum, a hickory, a dogwood or serviceberry and a shrub layer of hazelnut, elderberry, viburnum and witch hazel: a species for each guild. Put one of each on a request list and we reply with what is ready and current pricing.
• Choose seed-grown, green-leaved species over cultivars, and ask any nursery, us included, what insecticides its plants have seen.
• Leave the leaves under the drip line and the logs where they fall; leave a snag standing where it threatens nothing; build a brush pile.
• Do not spray for mosquitoes; empty standing water weekly instead.
• Turn the lights off at night from May to September.
• Leave the galls, the mummies and the chewed leaves alone. A tree with holes in its leaves is a tree that is working.

Common questions

Which tree supports the most insects?

Oaks, by every measure: 436 caterpillar species in the Eastern Temperate Forests, more than 700 kinds of gall wasp across North America, and the borers, scales and weevils of the white oak census above. Cherries and plums come second, with 340 caterpillar species.

What are keystone plants?

Keystone plants are the small number of plant genera that carry most of a place’s caterpillars: about 14 percent of the local genera support more than 90 percent of the butterfly and moth species. In the eastern forests the oaks lead, then the cherries and plums, then the willows.

Should I spray my yard for mosquitoes?

No, because yards tested after mosquito treatments carried residues high enough to kill bees and butterflies, and three quarters of the neighboring unsprayed yards were contaminated too. Empty buckets, gutters and birdbaths every week instead.

Are galls and chewed leaves a sign that my tree is sick?

No, a gall is a wasp’s chamber grown from the tree’s own tissue, and a mature tree loses nothing that matters to a gall or a chewed leaf. A tree with holes in its leaves is a tree that is working.