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How to identify it
- Adult females are about 1/8 inch long, with globular red bodies covered in white wax. Males are smaller and winged and are seen in spring.[1][2]
- The young (crawlers) are red or brown, smaller and more elongated than adult females, and feed on the leaves in early summer.[1][2]
- In winter look on the trunk: the overwintering nymphs sit in bark crevices and produce a little mealy secretion that gives infested bark a speckled look.[2][1]
- Infested trees often show conspicuous sticky honeydew and black sooty mold.[1][2]
Where it lives
It is widely distributed through the U.S. Midwest, the North Atlantic states and some western states. Colorado State describes it as introduced in Colorado, where it has become an important pest as hawthorns have been planted more widely in landscapes.[2][1]
Biology and life cycle
It feeds only on plants in the rose family. Hawthorn is its most common host, and as of the 2000 study it had been reported only from hawthorn in Colorado; other recorded hosts include mountain ash, juneberry (Amelanchier), cotoneaster, quince and certain Prunus.[2][1]
Late-stage nymphs overwinter in bark crevices, mostly on the trunk. In Fort Collins in 1998 they became active around bud swell in late March, then migrated to feeding sites at the bases of buds and in twig crotches from mid-April, with the peak in early May. Winged males appeared on the trunk in early April, and mating went on over the following month.[2][1]
Females mature in late May or early June and swell with eggs; in the Colorado study they averaged 180 maturing eggs at the peak, around June 10. Crawlers appear from late May, and some keep appearing into October.[1][2]
Crawlers feed on the leaves, then move to sheltered spots on the twigs for much of the summer. In fall the late-stage nymphs return to the trunk and larger branches to overwinter. Wet, cool weather delays the spread of the crawlers.[1][2]
The 2000 study found one generation a year in Colorado. Colorado State notes that egg-laying females and hatching have been seen in late September, which suggests either a second generation or a long egg-laying period.[2][1]
Damage and signs
- The mealybugs suck sap from twigs and small branches and produce large amounts of honeydew, which grows black sooty mold and makes trees unsightly.[1][2]
- Heavy infestations can cause severe leaf loss, twig dieback and weakened trees, and sometimes kill trees, although Colorado State says the insect does not usually cause significant injury.[1][2]
- Hawthorns vary in how badly they are hit. English, Arnold and thornless cockspur hawthorns appear most susceptible, and landscape trees are more often attacked than native stands.[1]
Health risk
Colorado State and the 2000 study describe it only as a pest of hawthorns and related trees and mention no risk to people or pets.[1][2]
Control
Prevention
Choose resistant hawthorns. In the Colorado study, Crataegus phaenopyrum 'Cordata' stayed highly resistant, and Crataegus x mordenensis 'Snowbird', Crataegus succulenta macracantha and Crataegus ambigua also showed substantial resistance.[2]
Biological control
Lady beetles and green lacewing larvae prey on hawthorn mealybugs and can often hold them to acceptable levels without insecticides. In the Colorado study, lady beetles fed heavily on the nymphs as they left the trunk in spring; no parasites were found.[1][2]
Pesticides
Timing is critical. The wax coating and bark crevices protect the mealybugs, so sprays work best in spring when the overwintered nymphs migrate from the trunk to the twigs. Dormant oil on the trunk and the undersides of branches, where they overwinter, can be effective, and in the 2000 study adding horticultural oil consistently improved results. Insect growth regulators, relatively safe for beneficial insects, can be used at that spring migration, and late summer or early fall treatments give some control.[1][2]
Systemic insecticides can control the mealybug but can accumulate in nectar, and hawthorn flowers attract honey bees, so apply systemics only after bloom.[1]
Common questions
- Why is my hawthorn sticky and black?
- Hawthorn mealybugs are one cause, especially in Colorado. They produce large amounts of sticky honeydew, which grows black sooty mold. Look for small red, white-waxy mealybugs on the twigs and at the bases of buds, and for speckled patches in the bark crevices of the trunk in winter.[1][2]
- When should I spray for hawthorn mealybug?
- In spring, when the overwintered nymphs move from the trunk to the twigs; in Fort Collins in 1998 this ran from mid-April to late May, peaking in early May. Dormant oil on the trunk and branch undersides can be effective. Don't apply systemic insecticides until the tree has finished blooming.[2][1]
- Which hawthorns resist hawthorn mealybug?
- In Colorado trials, Crataegus phaenopyrum 'Cordata' was highly resistant, and Crataegus x mordenensis 'Snowbird', Crataegus succulenta macracantha and Crataegus ambigua were substantially resistant. English, Arnold and thornless cockspur hawthorns appear most susceptible.[2][1]
Sources
- Colorado State University Agricultural Biology, “Hawthorn mealybug, Phenacoccus dearnessi.” agsci.colostate.edu/agbio/ipm-pests/hawthorn-mealybug/ (accessed 2026-11-08)
- International Society of Arboriculture, “Observations on the Biology and Control of the Hawthorn (Two-Circuli) Mealybug, Phenacoccus dearnessi (King) (Cranshaw, Jevremovic, Sclar and Mannix; Arboriculture & Urban Forestry 26(4):225-229, 2000).” auf.isa-arbor.com/content/26/4/225 (accessed 2026-11-08)
- iNaturalist, “Phenacoccus dearnessi taxon page (order and family).” www.inaturalist.org/taxa/898096 (accessed 2026-11-08)