Why Does My Larch Tree Look Sickly?

Are the leaves mined, yellowed, and shriveled?

Have the branches been defoliated?

Does the trunk or branches have oozing lesions?

Are the needles curled and distorted?

Are the needles eaten and there are visible masses of brownish-orange eggs?

Are the needles yellowed with undersides that show fungus?

Mined, yellowed, and shriveled leaves are caused by Larch casebearers. Tiny reddish brown larvae of this moth feed and overwinter inside the leaves. Adult moths emerge in May or June. Natural parasites usually keep this pest under control. For severe infestations, apply a dormant spray of lime-sulfur to branches.

Larch sawflies often leave whole branches of the larch trees defoliated. In June, the small, wasplike adults lay eggs in the side of larch shoots, causing shoots to twist. Grayish green, caterpillar­like larvae emerge about a week later. They feed on the leaves, starting with lower branches. Remove debris from around trees, which provides an overwintering site for the pest. Spray shoots with superior oil to control heavy populations.

Larch branch canker produces sunken areas on the bark that are surrounded with drops of resin. Cut out and destroy affected parts as soon as detected. Once established, there is no control. European larch (Larix decidua) is prone to serious infections and Japanese larch (L. kaempferi) is less susceptible to the disease.

Woolly larch aphids are soft-bodied, pear-shaped sucking insects, about the size of the head of a pin. They cluster on tree foliage and suck cell fluids, retarding or distorting tree growth. You can control them by washing them off the plant with water spray. Insecticidal soap should be used with severe infestations.

Gypsy moth caterpillars sometimes mass on larch foliage and devour it, defoliating the tree. The tree may die after repeated defoliation. Mature caterpillars have 5 pairs of blue spots and 6 pairs of red spots along the back. In light infestations destroy eggs or pick caterpillars by hand, in more severe cases spray plants with BTK or pyrethrin as the last resort.

Several kinds of fungi cause rust on larch needles. One rust develops on nearby willows and then affects larches, another rust requires poplars as its alternate hosts. If the affected larch tree is the valued ornamental tree in the landscape, remove the alternate host of the rust to interrupt the life cycle of the fungi. Be sure that the problem is rust before cutting down the suspected host tree.

If the larch tree fails to thrive, the cause is probably lack of sunlight. Larches, like most trees, need many hours of sun per day to thrive. Take care that neighboring plantings have not grown to block the exposure of young larch trees to the sun.