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Larry Sagers Horticultural Specialist Utah State University Extension Service Thanksgiving Point Office © All Rights Reserved
The stems on our globe willows are covered by masses of small, black insects and are covered with wasps and other insects. Should we worry about them? Will they kill our trees?
The insect you described is the black willow aphid. It is a very large aphid and may have winged or wingless forms.
The reason you are seeing so many on the branches of the trees is that the predators are not as active during cold weather. Once the predators become inactive the insects grow to massive numbers and cover the branches of your trees.
The wasps and other insects are of no concern because they are just coming to get the sugar that the aphids are secreting. If you get rid of the aphids, the wasps will no longer be attracted to the trees.
These aphids have another interesting characteristic. Their body fluids are bright red. If they are crushed, they appear to have red blood.
Normally cold weather destroys these insects. Washing them off the stems will normally keep them under control. The water also removes the sticky sugar that attracts the wasps.
If they are extremely numerous, they may warrant control to reduce problems the following year. A dormant oil spray or an application of an insecticidal soap usually controls these soft-bodied insects.