Description and life history continued…

Eriophyid mites (Eriophyidae)

Eriophyid mites are very small arthropods that usually measure under 1 mm in length and are often found on leaves, flowers, fruits, or buds. These mites tend to form small, finger-like galls, pocket galls, or masses of plant hairs that have a felt-like texture. Eriophyid mites are typically white or yellow, and unlike most arachnids, eriophyid mites have carrot-shaped bodies with two pairs of legs instead of four. These mites develop through four stages that include an egg, two nymphal stages, and the adult stage. Adult females overwinter under the scales of buds or other protected sites, and there are often several overlapping generations produced each growing season. Most species of eriophyid mite feed on a single plant species, which includes various conifers and hardwoods such as birch (Betula spp.), mountain ash (Sorbus aucuparia), and willow (Salix spp.).

Psyllids (Aphalaridae)

One common group of psyllids create galls on hackberry (Pachypsylla spp.). Species in this genus have similar morphology and produce nipple galls or blister galls on leaves. Adult psyllids are small, measuring 2-5 mm long, and hold their wings rooflike over the body. These pests resemble miniature cicadas that jump when disturbed. In Colorado, two notable species include the hackberry nipplegall maker (Pachypsylla celtidismamma), which produces nipple galls, and hackberry blistergall psyllid (Pachypsylla celtidisvesiculum), which produces blister galls. Both species likely produce one generation per year, with nymphs developing inside of galls through the summer and emerging as adults in late summer. There can be considerable variation in the number of galls that form on nearby leaves due to the synchronization of egg-laying and hatching with early leaf development. Different psyllid species tend to feed on one plant species or a group of closely related plant species. Collectively, these two Pachypsylla spp. can attack various Celtis spp., including hackberry, sugarberry, American or common hackberry, and net-leaf hackberry.

Gall-making aphids, adelgids, or woolly aphids (Aphididae, Adelgidae)

These gall-making insects primarily attack coniferous trees, with the most common being the Cooley spruce gall adelgid (Adelges cooleyi) in Colorado, which forms cone-like galls on spruce trees. In addition to the Cooley spruce gall adelgid, some Pemphigus spp. can form stem and petiole galls on certain Poplus spp. such as aspen and cottonwood. The Pemphigus spp. are roughly 2 mm long and have pale green bodies with a dark thorax. Wingless forms are more frequently observed than wing forms, and the nymphs are like adults but smaller. These aphid species have a similar 1-year life cycle that involves two sets of hosts. For example, the poplar petiolegall aphid (Pemphigus populitransversus) lays eggs in the fall to coincide with changes in foliage. Eggs hatch the following spring, and the newly emerged nymphs begin to feed on developing leaf petioles, which results in the formation of a gall that envelops the aphid. The aphid reproduces asexually within the gall until developing wings in late June or July, at which point the gall splits open and the winged aphid migrates to cruciferous plants where it colonizes roots and produces multiple generations of asexually reproducing females through the rest of the summer. The nymphs tend to produce large amounts of white wax when colonizing the roots of cruciferous plants. At the end of summer, winged adults are again produced and migrate back to their winter hosts. Small, mouthless males and females are produced, and the males die shortly after mating. Each mated female lays a single egg on the winter host and then dies. Winter hosts include spruce, Douglas-fir, and Populus spp., while cruciferous plants often serve as hosts for summer generations. High densities of certain species, like the lettuce root aphid (Pemphigus bursarius), can affect the productivity of cruciferous garden crops.

Gall wasps (Cinipidae)

Gall wasps comprise the largest group of gall-making insects that produce a range of galls, including woody, round galls on stems or leaves to woolly or mossy galls. One species, the oak rough bulletgall wasp (Disholcaspis quercusmamma), is native to the midwestern United States and is likely found in most locations that contain bur oak (Quercus macrocarpa). Adults have stout, dark brown bodies, and a rounded abdomen. In Colorado, the wasps emerge from galls in late October or early November, leaving a small circular exit hole in the gall. Females lay eggs in dormant buds, which results in the formation of a minute, light tan gall the following spring after budbreak. Each gall contains a single wasp that emerges in mid-late spring. After emerging, females lay eggs on green stems, which results in the characteristic large, rounded galls that are initially pale brown and soft but gradually darken and harden. Pupation occurs in the center of the gall, and adults emerge in fall. Galls formed on oaks (Quercus spp.) and roses (Rosa spp.) are likely created by gall wasps.

Gall midges (Cecidomyiidae)

Gall midges are common in Colorado and can form a wide range of galls that vary in appearance, from woody, rounded galls on stems or leaves to woolly or mossy galls. These flies are small and have long, slender legs and antennae. The exact appearance and life cycle of the insect depends on the species. For example, adults of Dasineura gleditchiae, commonly referred to as the honeylocust pod gall midge, are about 2.5-3.2 mm (1/10-1/8 inch) long with two black stripes on a gray thorax. The larvae of D. gleditchiae are white or yellow maggots that measure about 2.5 mm (1/10 inch) long, while the pupae are about the same size but have oblong, orange, or white bodies.

Dasineura gleditchiae overwinters as adults near honeylocust trees. The following spring, adults emerge prior to budbreak and lay eggs on individual leaflets. Larvae develop within curled leaves where they feed and cause galls to form. Different species of gall midge feed on one or a few related hosts, which collectively include honeylocust (Gleditsia triacanthos), coyote brush (Baccharis pilularis), dogwood (Cornus spp.), Douglas-fir, ficus (Ficus spp.), oak (Quercus spp.), pines (Pinus spp.), willow (Salix spp.), sagebrush (Artemisia spp.), and creosote bush (Larrea tridentata).

Gall flies (Agromyzidae)

In Colorado, the poplar twiggall fly (Hexomyza schineri) is a native insect in Colorado that produces smooth, spherical galls. Adults are shiny, dark flies with stout bodies that measure about 4.2 mm (1/6 inch) long. The larvae are green or yellow maggots that reside in the gall through the summer, with each gall containing two or three larvae.

Mature larvae overwinter within galls and most drop to the ground in late winter or early spring to begin pupating. The galls are often obscured by leaves and most easily noticed during fall as infested leaves drop. When new growth begins forming, adult flies emerge from the pupae and tend to remain on the sunny sides of leaves during the day. After mating, females insert eggs in developing stems, and the larvae begin feeding after hatching which produces the distinctive swelling. Galls are often formed in the areas below buds, and the egg-laying sites become increasingly swollen as the stem grows. The poplar twiggall fly can attack aspen, cottonwoods, and poplars.

Injury

In addition to the formation of galls, these insects and mites can cause unsightly plant injuries that may reduce plant health. Feeding of eriophyid mites causes plants to have hairy or fuzzy pads on the surface of leaves and other plant structures. Infestations can also cause excessive production of buds, twig elongation, and other forms of broom-like growth in the host plant. Leaves can also roll and blister in response to eriophyid mites.

Cruciferous vegetables with high densities of the lettuce root aphid display wilting of foliage and fail to develop heads, resulting in yield reductions. Sustained high densities of these aphids can kill the plant over time as individual rootlets turn brown and die.

Infestations of D. gleditchiae can cause foliage to turn brown and drop prematurely, leaving entire portions of branches without leaves. Heavy infestations of the rough bulletgall wasp can cause galls to cover much of the infested twig, which reduces the host plant’s growth rate. After leaves drop, the galls can be easily noticed and appear as warts.

Management

Eriophyid mites

Pruning infested leaves and twigs in the early spring can help remove overwintering mites before they begin feeding and reproducing. Clippings should be sealed in a bag and disposed of in the trash. Removing all infested plant tissues is recommended to reduce the likelihood of reinfestation.

Chemical control of eriophyid mites can be challenging, owing to their minute size and lifestyle, in which they remain protected inside of galls. Miticides with high residual activity that contain dormant oil, carbaryl, horticultural oils, and insecticidal soaps can be applied to heavily infested plants to kill exposed eriophyid mites. However, this approach will not kill mites residing within galls, nor will it eliminate galls once they have been formed. Miticides should be applied after spring during early bud break. It is worth noting that some of these, such as carbaryl, are highly toxic to pollinators and should not be applied when pollinators are actively foraging. Generally, chemical control of gall-forming eriophyid mites is not warranted.

Psyllids

Passerine birds feed on hackberry psyllids by extracting nymphs from within galls. Migrating birds feed ovipositing adults in the spring and after adults emerge from galls in the fall. Vacant galls can provide shelter for other natural enemies such as spiders and green lacewing larvae.

Several insecticides can be used for managing gall-forming psyllids on hackberry. Trees can be treated with insecticides when leaf buds begin turning green and again after leaves are about 50% expanded. Before applying any insecticide, it is important to check the label to determine whether it is registered for psyllid management.

Gall-making aphids, adelgids, or woolly aphids

The severity of poplar petiolegall aphid infestations can be influenced by planting date since heavy infestations are more likely to occur on hosts planted before the aphids move to cruciferous crops in the fall. When possible, planting after the fall migration will greatly reduce the likelihood of an infestation. Practices that promote rapid, vigorous plant growth, such as proper watering and fertilizing, can also help mitigate some of the feeding injuries produced by aphid infestations.

Insecticides can be used to manage infestations of the poplar petiolegall aphid when populations exceed acceptable levels and rapid control is desired. Some of the traditionally used broad-spectrum insecticides include organophosphates, carbamates, and pyrethroids. Opting for selective insecticides can help protect beneficial insects and may also provide high levels of control over a longer residual period. These selective insecticides are often systemic and do not need to be applied to the whole plant, unlike the more traditional contact insecticides.

Systemic insecticides can be especially useful for late-season treatments of cruciferous crops, which often have distorted plant structure that shelters aphids against contact insecticides. It is important to note that rapid asexual reproduction allows resistance to quickly develop in aphid populations. Therefore, it is recommended that insecticides with different modes of action be rotated to maintain their efficacy.

Gall wasps

On small trees, removing galls before wasps emerge can help suppress the pest population. Several species of chalcidoid parasitoid wasp have been reared on colonies of the oak rough bulletgall wasp in Colorado and can help suppress populations of the oak rough bulletgall wasp, including Sycophila dubia, Torymus denticulatus, Mesopolobus spp., and Pteromalus spp. These parasitoid wasps emerge in spring from galls produced by the oak rough bulletgall wasp. It is important to note that removing galls in the winter and spring after the gall-forming wasps have emerged can inadvertently destroy parasitoid wasps.

Gall midges

In nearby states such as California, natural enemies include predatory beetles, predatory bugs, and spiders. Hymenopteran parasitoids have been observed emerging from galls, with the greatest proportion of parasitoid wasps belonging to the family Pteromalididae. Thornless honey locust varieties are more susceptible to attack by D. gleditchiae. Pod galls that fall to the ground can be destroyed to reduce the pest population. Other Dasineura spp. are managed by targeting the soil-dwelling overwintering stage. Some heavy infestations can be managed with residual insecticides applied when buds begin to break.

Gall flies

Limiting irrigation and fertilization can help slow gall production in succulent aspen, where the poplar twiggall fly is most problematic. Eurytoma contractura is a species of parasitoid wasp that attacks eggs or young larvae of the poplar twiggall fly, with parasitism rates typically ranging from 20-30% in Colorado. This parasitoid is most active for about two weeks after budbreak. Other natural enemies include birds that feed on the poplar twiggall fly by tearing galls. Removing galls should be avoided in late winter, as pruning during this period may remove developing E. contractura.

In nurseries, the soil over the root zone can be treated with a drench application of imidacloprid shortly prior to or after budbreak to prevent plant injuries. Immediately prior to the drench application, mulches should be raked away to expose the soil. After the drench application, the treated soil should be sufficiently watered for a minimum of two weeks to facilitate uptake of the insecticide. Foliar insecticide applications have not been effective against the poplar twiggall fly and may reduce numbers of E. contractura.

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