Elm leaf beetle

Trees affected

Siberian elm mainly in northern Nevada; other elms can be attacked

Larvae

Pale yellow with two dark stripes; skeletonize leaf surfaces

Adults

Chew round holes (shot-holes) in leaves; enter buildings in fall

Generations

At least one a year in northern Nevada

Usual outcome

Weakened tree, not usually killed by the beetle alone

Siberian elms line the older streets of Carson City, Reno, Sparks and many valley towns, and in mid summer many of them look scorched. The cause is often the elm leaf beetle. This page helps you tell beetle damage from a dying tree and explains when an elm should be pruned or removed. Battle Born Tree Service does not spray, inject or apply products to trees. We identify the problem, prune out dead or hazardous wood and remove trees that cannot be saved.

What does elm leaf beetle damage look like?

University of Nevada, Reno Extension describes the year in four steps. Adults wake in spring and feed on new elm leaves, then lay eggs: yellowish, in double rows on the underside of a leaf. The larvae hatch pale yellow with two dark stripes and eat the leaf surface in summer without breaking through the opposite side or the veins. That is what leaves the lace-like brown netting. Adults chew all the way through leaves and leave a shot-hole pattern. Leaves curl, turn brown and drop early, so shade thins out by August.

In late summer and fall the adults look for shelter and sometimes come into homes to spend the winter. Seeing them on a window sill is another sign the elm outside is the source.

Which trees, and where?

UNR Extension says the beetle feeds mainly on Siberian elm (Ulmus pumila) in northern Nevada, although many elm species can be attacked. Siberian elm is the tree on many old lots in the valley towns around Carson City and Reno. See Siberian elm and Siberian elms on old Nevada lots. The beetle has one generation a year here and two to three farther south in Nevada.

Is the tree dying?

Not necessarily. The UNR publication says an elm leaf beetle attack is not usually life-threatening, but the feeding weakens the tree, and heavy defoliation year after year can cause decline over several years. Healthy, well-watered trees stand it better than drought-stressed ones, and the extension advises supplemental summer water for elms. See watering mature trees in the high desert.

To tell beetle damage from a dead tree, look at the twigs. A tree with brown lace leaves but live buds and flexible twigs is stressed. A tree whose branches are bare in summer, brittle, with bark that is peeling or cracked, has another problem. Wilting that starts in one branch of the upper crown in summer is a different disease; see Dutch elm disease.

What does an arborist check?

An arborist checks whether the damage is only in the leaves or has reached the twigs and branches, and looks for dead limbs and trunk defects that matter more to an elm's safety than leaf loss. They also check root and sidewalk conflicts, lean and how many years of decline the crown shows.

• Whether the browning is leaf damage only or has reached the twigs and branches
• Dead limbs and dead wood in the crown that could fall
• Included bark, splits and cavities in the trunk, which matter more to an elm's safety than leaf loss
• Root and sidewalk conflicts, and whether the tree leans over a house or a line
• How many years of decline the crown shows

When is pruning or removal the answer?

Pruning is the answer when an elm has dead limbs, weak forks or crossing branches. Hazards and dead wood can be pruned at any time, and most deciduous trees are best pruned in late winter. See tree trimming and pruning and dead wooding. An elm that is dying back every year may do better with its dead wood removed and the crown kept in balance, never topped. See why topping is not pruning.

Removal is the answer when the elm is dead, when decay or a split has made the trunk unsafe, or when a large elm leans toward a house. Elm leaf beetle by itself is not the reason. Ask an arborist to look before you decide, and see tree removal when the answer is to take it down.