Insect and Mite Pests of Honeylocust

Introduction

Several Insects and mites feed on honeylocust trees in Colorado. Their feeding can cause conspicuous injuries that may make the trees unattractive. Heavy infestations can reduce tree growth rate and vigor and cause some twig dieback. However, it is extremely rare that these injuries alone permanently injure the trees. Plant diseases, notably Thryonectria and Tubercularia cankers, are far more important to the health of honeylocust.

Honeylocust Plant Bugs

The honey locust plant bug (Diaphnocoris chlorionis) is a small, pale green insect that feeds primarily on honey locust trees (Gleditsia triacanthos). Honeylocust plant bugs commonly infest honeylocust in late spring. This common damaging species of plant bug is generally green. When immature, it superficially resembles an aphid but is much more active and slightly larger. After they feed and damage plants, injured foliage shows yellow or brown spotting. Leaves become twisted, and twig dieback can occur following heavy infestations. The honeylocust plant bug particularly favors new spring foliage. 

This insect can indirectly limit honeylocust podgall midge by destroying gall-susceptible new growth. A single generation of these Insects occurs, with damage peaking in late May or early June. New growth produced after this time emerges normally. 

Chemical controls are recommended only when spring growth is seriously threatened, because regrowth occurs in June when insect populations diminish. Avoiding the use of broad-spectrum chemicals across the entire tree canopy helps protect beneficial insects, reduces the risk of pesticide resistance, and minimizes environmental contamination while still allowing for targeted pest control. Orthene, carbaryl (Sevin), pyrethroid insecticides (permethrin, bifenthrin, cyhalothrin), and imidacloprid have been shown to control plant bugs. 

Figure 1

Honeylocust Podgall Midge

The immature stages (larvae) of a small fly, the honeylocust podgall midge, commonly cause distortions of new honeylocust growth. Infested leaflets curl and thicken, forming into swollen finger-like “pod galls” instead of expanding normally. Inside each small pod gall, two to eight cream-colored larvae or pupae may be found. After the adult midges emerge, the galls drop, leaving bare leaf stalks. 

The honeylocust podgall midge has multiple annual generations. It is most active from the time of first shoot growth through midsummer. Infestations occasionally can be severe enough to temporarily destroy all new growth. Leaflets that have already expanded are not injured by the insect. All common honeylocust varieties appear to be susceptible. 

Natural controls include severe weather, predatory plant bugs, and a small parasitic wasp that attacks young midge larvae. Heavy midge infestations often collapse when new growth ceases and the trees are no longer attractive to the egg-laying adult midges. In the absence of suitable egg-laying sites, the adult midges apparently go into an inactive diapause stage, renewing activity the following spring. 

Chemical controls have been only moderately effective. Some reduction of gall production can result from applications of pyrethroids (permethrin, bifenthrin), Spinosad, or imidacloprid during egg-laying periods. Control of the earliest midge generations is most important because that is when most leaf growth is produced. The first generation appears in spring with the first flush of new growth. The second and subsequent generations follow at about three to four weeks intervals. Repeat insecticide applications to protect the newly expanding leaflets. 

Figure 2

Figure 3

Honeylocust Spider Mite

The honeylocust spider mite is closely related and similar in habits to the common twospotted spider mite. Both are barely visible to the unaided eye and feed on the undersides of leaves. During midsummer, honeylocust spider mite populations greatly increase. The foliage of infested trees turns bronze in coloration. Injured leaves often drop prematurely. 

Problems with spider mites tend to be much greater on street trees and in other dry, drought-stressed, and unfavorable sites. Regular watering during hot summer months helps to reduce mite populations and lessen tree damage. Natural spider mite predators (predatory bugs, predatory thrips) often contribute to a great reduction of mite populations by late August, but chemical controls may be needed to prevent injury. Fortunately, honeylocust spider mites are controlled more easily than the twospotted mites that cause problems on other ornamental plants. 

Bifenthrin (Talstar) and Orthene are among the most widely available miticides. Dormant season oil treatments can also suppress honeylocust spider mites. Mites are small enough that oil application can cause them to suffocate. However, they need to be applied to both the trunk and the crown, because overwintering mites are present in both sites. Repeat applications of insecticidal soaps or spray oils can also be effective. Good coverage during oil applications is essential to ensure that pests and their eggs are thoroughly smothered, maximizing effectiveness of application efforts.

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Eriophyid Mites

Minute eriophyid mites commonly are observed on honeylocust and are thought to contribute to leaf bronzing. Eriophyid mites are extremely small, worm-like mites with only two pairs of legs, visible only under magnification. They feed on plant tissues and can cause a variety of symptoms including galls, leaf curling, bronzing, russeting, and other deformities. Many species are host-specific, and while some cause minimal damage, others can significantly impact plant health and appearance when populations are high. They easily spread on plant material and cuttings that are untreated since the easy go unnoticed being microscopic. Because these eriophyid mites occur on leaves (versus in galls), they are called “leaf vagrant” mites. Carbaryl (Sevin, etc.), insecticidal soaps, and oils can control these mites on honeylocust.

Leafhoppers

Leafhoppers, belonging to the family Cicadellidae, are small, wedge-shaped insects that move quickly and often jump or fly when disturbed. They feed by piercing plant tissues and sucking sap, which can lead to stippling, yellowing, curling, or stunted growth in plants. Some species also transmit plant pathogens, including viruses and phytoplasmas. High populations can cause significant damage, particularly when feeding causes leaf burn or hopperburn symptoms. 

There is a common species of green leafhopper (Empoasca sp.) that infests honeylocust and many other trees. Peak populations of honeylocust leafhoppers occur in late spring, often together with plant bugs. Visible damage is minimal and rarely results in more than a scattered yellow spotting of the foliage. A temporary honeydew problem may also occur during heavy infestations. 

Leafhoppers on honeylocust appear to have two generations per year. However, some adult leafhoppers are found in late July. Control is never necessary but is possible with most shade tree insecticides. 

Blister Beetles

Ash-gray blister beetles suddenly may occur in large numbers on trees, occasionally causing serious defoliation over parts of the tree. Infestations tend to occur most frequently in late June or early July and end as suddenly as they begin. If severe defoliation is threatened, control can easily be accomplished using any of the insecticides commonly used for insect control on trees and shrubs.  

Cottony Maple Scale

Cottony maple scale (Pulvinaria innumerabilis) is a soft scale insect that infests a variety of deciduous trees, especially maples, producing distinctive white, cottony egg sacs on twigs and branches in late spring to early summer. They can infest honeylocust as a host. Heavy infestations occasionally lead to leaf yellowing and sticky honeydew buildup, which may attract sooty mold and nuisance insects. 

Cottony maple scale occasionally builds up in large numbers on branches and leaves of honeylocust, becoming quite conspicuous as egg sacs swell in June and July. The overwintering stage of this scale (mated female) can be controlled with heavy coverage dormant oil applications made before new growth in spring. These treatments also can help suppress spider mites, eriophyid mites and plant bugs that overwinter on trees. 

Newly hatched crawlers generally appear in late June through July and can be controlled at that time with Sevin, Orthene or various pyrethroids. Imidacloprid (Merit) applied as a soil systemic treatment should also be effective, provided sites are irrigated. Natural controls generally maintain cottony maple scale at nondamaging levels, and sustained infestations lasting more than one season are very rare. 

Honeylocust Borers

There are various honeylocust borers, either metallic or longhorned beetles, that spend larval stages tunneling under the bark.  

Honeylocust borers, primarily Agrilus difficilis, are metallic wood-boring beetles in the family Buprestidae. Larvae tunnel beneath the bark of stressed honeylocust trees, disrupting nutrient flow and weakening the tree. Infestations often occur in trees already under stress from drought, poor site conditions, or previous damage, and can lead to branch dieback or tree decline if not managed. 

This injury can contribute to honeylocust decline, but borer problems are rarely the fundamental cause of the decline. The most common species of honeylocust are thought to emerge and lay eggs during June. 

Honeylocust borers can attack and develop successfully only in trees already stressed due to drought, root pruning, disease, or other causes. Most borer activity occurs in areas of existing cankers. External evidence of a honeylocust borer infestation includes “weeping” at wounds and the small circular oval exit holes made by the adult beetles as they emerge from the trunk. 

Proper watering, tree care and, in particular, wound prevention are the most important techniques for reducing problems with honeylocust borers. Usually, limiting any stressors is sufficient. 

Supplemental insecticidal controls should consist of maintaining a protectant insecticide on the tree trunk during the egg-laying and egg-hatch period in early summer. Permethrin (Astro) is used for control of related borers attacking other shade trees. Alternately, soil drench applications of imidacloprid can be effective if applied in spring to trees not already badly damaged by borers.

Figure 6