IPM Files: Cottony Ash Psyllid

Introduction

Cottony ash psyllid adult. © David Fenwick, 2013

The cottony ash psyllid (Psyllopsis discrepans) is an exotic, invasive insect species that was introduced into North America from southern Europe in the early 21st century. This relatively new landscape pest was first detected feeding on black ash trees in Alberta (c. 2000), where its population slowly spread eastward through the prairie provinces and southward into the United States. Psyllids, also known as “plant lice”, are one of several horticultural insect pests belonging to the order Hemiptera, which includes aphids, leafhoppers, shield bugs, scales, and whiteflies. Combined with the emerald ash borer (Agrilus planipennis), these pests are slowly extirpating populations of black ash in North America. As of 2014, the IUCN (International Union for Conservation of Nature) has classified black ash (Fraxinus nigra) as critically endangered. Hosts

Black ash (Fraxinus nigra) leaf. © Keith Kanoti, Maine Forest Service, USA, n.d.

Despite being a monophagous (host-specific) pest, the cottony ash psyllid only targets two species within the genus Fraxinus. Black ash (F. nigra) and its cultivars ‘Fallgold’, ‘Northern Gem’, and ‘Northern Treasure’ are particularly susceptible, as is Manchurian ash (F. mandshurica) and its cultivar, ‘Mancana’. Green ash (F. pennsylvanica), white ash (F. americana), and blue ash (F. quadrangulata) are not affected. Identification

Adult

Cottony ash psyllid adult. © Joe Botting, 2008

Adult psyllids resemble tiny cicadas and are very small, reaching a total length of 2–3 mm at maturity. Due to their small size, they are often only detected by the naked eye when they jump or hop between leaflets. And even though their primary locomotion is jumping, cottony ash psyllids are capable of flight. Of its four wings, the two forewings are clear, membranous, prominently veined, and black-tipped while the two hindwings are thin and transparent. At rest, these wings are folded into a characteristic gabled or peaked position against the body. A magnifying glass can help identify its other anatomical features such as a wide head, large red eyes, a yellow thorax, a segmented yellow-green abdomen, and a maculation of black spots, stripes, and bands.

Nymph

Cottony ash psyllid nymph. © David Fenwick, 2013

Nymphs undergo five developmental stages called instars, where they gradually metamorphose into imago (adults). While nymphs are difficult to see with the naked eye, they can be observed under the aid of a magnifying glass. Nymphs exhibit a soft, flat, wingless, and pale green or yellow body covered in secretions of honeydew and white, waxy filaments. Life Cycle

The cottony ash psyllid’s life cycle is bivoltine, meaning it produces two generations per year. Like many insect species, its phenology largely depends on the number of growing degree days (GDD), which is a measurement of the average heat accumulation over time. In northern climates, this can translate to a later seasonal emergence than warmer, southern climates. This is especially true of cottony ash psyllid in the prairie provinces, where overwintering eggs typically hatch just after bud break in June. After emerging, first generation nymphs will feed on buds and leaves until midsummer before maturing into adults. First generation adult females will lay fertilized eggs along the leaflet’s midrib (central vein), where progeny will hatch in late July to early August. Second generation nymphs repeat the feeding cycle, protecting themselves in a cottony, waxy substance before reaching maturity in late summer, near the end of August. After copulating, second generation adult females will oviposit eggs on bud scales to overwinter until the following spring, thus completing the life cycle.

Damage

Examples of damage from cottony ashy psyllid. Left: Pseudo-galls in the canopy of a black ash; Centre: A black ash tree showing signs of defoliation from cottony ash psyllid; Right: Close up of a pseudo-gall. Note the cottony secretions on the underside of the leaflets. © Trees Winnipeg, 2022

The most conspicuous symptom of cottony ash psyllid damage is sparse crowns of pseudo-galls, which are clumps of severely distorted, curled, shrunken, or crumpled leaves. Often, the undersides of these galls are encapsulated in cottony secretions. Other signs and symptoms to look for include leaf discoloration, nymphal castings (skin moults), and premature leaf drop. A magnifying glass may also reveal tiny eggs laid on the edge of bud scales or along the leaflet’s midrib. The risk of tree mortality is increased after several years of consecutive damage.

Pest identification is very important in distinguishing cottony ash psyllid from other, similar pests. Leafcurl ash aphid (Prociphilus fraxinifolii) is often mistaken for cottony ash psyllid due to their similarities in morphology, life cycle, and damage. A major difference between these two pests is the location of damage, with leafcurl ash aphids only attacking branch tips. Comparably, ash flower gall mite (Eriophyes fraxiniflora) does not attack leaves but instead causes pseudo-galls to form in the staminate (male) flowers of ash trees. Lastly, ash plant bug (Tropidosteptes amoenus) can superficially resemble cottony ash psyllid damage, but lacks the cottony secretions in favour of copious amounts of frass (excrement). Unlike cottony ash psyllid, these three pests are considered to be cosmetic, meaning they only cause aesthetic damage to the tree and do not necessarily require treatment. Control

• Monitor trees for presence of adults, nymphs, and eggs during the active phases of the pest’s life cycle (June – August).
  

• Consider planting species that are resistant or unaffected by cottony ash psyllid, such as green ash (Fraxinus pennsylvanica) and white ash (F. americana).
  

• “Right tree, right place”. In the wild, black and Mancana ash trees can be found growing in low-lying moist areas or along rivers and streams. Therefore, it is important to plant these trees in a place where they will thrive to reduce drought stress and susceptibility to pests.
  

• Maintain tree vitality with good cultural practices such as watering, fertilizing, mulching, pruning, and aeration.
  

• Homeowners can use an assortment of pesticide alternatives like insecticidal soap to combat the early stages of cottony ash psyllid infestation. However, timing of applications is critical as chemicals are rendered ineffective once leaf curl damage occurs. Nymphs should be sprayed when they first emerge in the first, second, or third week of June, and then again in the first or second week of July.
  

• If the infestation is severe, persistent, or recurring, conventional chemical pesticides may be required. Please note that all chemicals registered for use against cottony ash psyllid can only be used by a licensed commercial pesticide applicator.

When in doubt, contact a professional to ensure your landscape can be enjoyed for years into the future. Contact us for a consultation today!