Wednesday, August 11, 2010

Hackberry leaf spots may be insect caused.

Hackberry (Celtis spp.) leaves often have spots on the upper surface that looks somewhat like measles or blisters but are light green or yellow in color.

These spots are caused by the Hackberry Blistergall maker (Pachypsylla celtidisvesicula). The blisters are 3 to 4 mm in diameter and only slightly raised from the leaf surface.

There are 25 millimeters (mm) in an inch so these spots are quite small.

When you flip the leaf over you will see and can feel many tiny bumps. The feeding activity of this psyllid stimulates the tissue into forming a gall, an abnormal growth.

[Pysyllids are  known as jumping plant lice due to their tendency to jump when disturbed.]

You may find blistergall makers and nipplegall makers on the same leaf as is seen in the next photograph.  The nipple-like galls are due to the feeding activity of Pachypsylla celidismamma.

 Note the three nipple galls mixed in with the galls of  the hackberry blistergall maker.

Psyllids feed by inserting their mouth parts into the phloem of the tissue and sucking out the plant's juices.  They tend to feed on either a single type of plant such as the hackberry or members of the same family of plants.  Consequently you do not have to worry about these insects feeding on any plants other than hackberries.

There is one generation of these gall makers per year with the eggs being laid as the leaves unfurl in the spring.  A dormant or horticultural oil applied to the tree to coat the overwintering adults helps control this insect.  The use of a product containing the insecticide imidacloprid this fall will solve next spring's hackberry psyllid problems.

While this insect is more cosmetic than damaging, one can easily see why homeowners are concerned when they have an infestation of these leaf-disfiguring insects on their hackberry trees.

To learn more about psyllids check out the Psyllid web site.

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