Psyllids

These insect pests, similar to aphids, can damage the tender growth of trees by sucking out the plant's juices. They also secrete honeydew that coats leaves and may encourage the growth of a sooty black mold. Symptoms of damage include yellowing and dropping leaves.

To control these pests, spray with insecticidal soap every 3-4 days until the insects are gone. In fall, spray trees with a light horticultural oil and, in early spring, spray trees with a dormant oil before the leaves emerge. After the leaves emerge, spray insecticidal soap once a week for 3 weeks.

Specific psyllids

Other images

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Psyllid (Hemiptera) secretions on clematis (Clematis)
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Psyllid (Hemiptera) secretions on clematis (Clematis)
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Feeding by psyllids (Hemiptera) produces these mammiform galls on the underside of hackberry leaves (Celtis) called hackberry nipple galls
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Hackberry nipple gall on hackberry (Celtis) is caused by a psyllid (Hemiptera)
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The upper leaf surface of hackberry leaves (Celtis) showing bleached, circular depressions where feeding by psyllids (Hemiptera) has produced hackberry nipple galls underneath