General Information
Phylloxera are small (up to 3/32 inch long), aphid-like insects in the genus Phylloxera of the family Phylloxeridae. They feed on the developing leaves of pecan and hickory and cause galls to form. Phylloxera are yellowish-tan with dark heads and a dark band across the thorax. The transparent, gray wings are held flat on the back. Phylloxera eggs are very tiny and yellow or greenish. Young phylloxera are yellowish-tan, wingless and smaller (up to 1/16 inch) than adults. Phylloxera galls on hickory and pecan are usually large (up to 5/8 inch) and spherical, somewhat flattened or irregular in shape. The galls are hollow and are green outside and white inside. As the galls dry out, they darken and split open. When the phylloxera escape, they lay eggs that develop into males and females. These insects mate and the females of some species crawl to protected places on the bark of the host tree and die. With these insects, a single egg inside the body of the female survives the rest of the summer, fall, and winter. Other species lay eggs on the leaves and twigs. These eggs hatch the following spring and the tiny, new phylloxera feed on the developing buds and form a new generation of galls. Phylloxera galls apparently occur wherever hickory and pecan grow, but they may not found on every tree. Some years these galls are rarely seen. On the other hand, occasionally certain trees may have numerous galls on just about every leaf.
Phylloxera are aphid-like insects that form galls on specific host plants.
Phylloxera galls on hickory leaves are relatively large.
Leaf stem galls usually cause more damage as the leaf may die from the gall outwards.
Biology
Various species of hickory and pecan (pecan is actually a species of hickory) are susceptible to phylloxera galls. Damage caused by phylloxera is primarily aesthetic. Galls on the leaf stem distort the leaf, sometimes grotesquely. As these galls mature and dry out, infested leaves die and drop prematurely. The canopy of heavily infested trees becomes filled with distorted, dying leaves, and the area around the tree becomes littered with fallen leaves. The winter is spent in the egg stage inside the dead female on the bark in cracks and crevices and in the crevices of old galls or as eggs on fallen leaves and elsewhere. Nymphs hatch from these eggs in spring as the buds swell. The tiny new phylloxera nymphs crawl into the expanding buds and feed on the tender tissue. Feeding causes the plant tissue to form galls around the insects. When mature, the phylloxera then lay eggs inside the galls from which hatch more phylloxera. The gall ultimately becomes filled with these insects as they grow. Finally, the galls split open and the phylloxera emerge to lay eggs on the leaves. From these eggs hatch male and female phylloxera that mature and then mate. These females lay eggs and die or they hide in bark crevices die each with a single egg inside the abdomen that manages to hatch the following spring.
Phylloxera reproduce inside their galls until each gall is packed!
Phylloxera galls on hickory eventually crack open.
Phylloxera that lay their eggs on leaves tend to lay hundreds along the midrib.
Control
By the time the galls are noticed, it is too late to treat during the current season. As far as the basic health of an infested tree is concerned, no pesticide treatment is actually needed. The vigor of infested trees can be increased by proper fertilization (following the recommendations indicated by a soil test) and watering during prolonged dry spells. For an extreme infestation on a valuable landscape specimen, if the tree is examined closely, it is often possible to detect scale insects or aphids or mites for which horticultural oils are labeled. If horticultural oils are applied in the dormant season, scales (and phylloxera eggs) can be controlled. Such applications should be made during the fall, winter or early spring before the buds open. Spray the trunk, main branches and smaller branches as thoroughly as possible. Rake up and destroy leaves from beneath the tree in case they are infested with overwintering eggs. Be sure to follow the directions for safe use found on the label of whatever pesticides are used.
Other Resources
- Hickory Galls Induced by Phylloxera. Anonymous. 2012. Div. Plant Industries, Missouri Environment & Garden. MU IPM Prog.
- Pecan Pest Management: Insects and Diseases. Smith, G. S., M. H. O'Day and W. Reid. University of Missouri Extension. MP711.
- Pecan Phylloxera, Phylloxera spp. Anonymous. No Date. Entomology & Plant Pathology. Digital Diagnostics. Information on Insects/Arthropods and Plant Diseases. Div. Agr. Sci. and Nat. Resources, Oklahoma State Univ.
- Phylloxera Gall on Pecan. Goodwinn, C. 2008. AgrLife Extension, Texas A&M System.
- NC State Extension Plant Pathology Publications
- NC State Horticultural Science Publications
- North Carolina Agricultural Chemicals Manual
For assistance with a specific problem, contact your local N.C. Cooperative Extension center.
This Insect Note has not been peer reviewed.
Publication date: Dec. 18, 2000
Revised: Oct. 10, 2019
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