Editor’s note: Kelly R. Jackson takes questions called into his phone recently at the Christian County Extension Office and answers them in the Garden Corner column.
Question. Why is my maple tree dropping all its leaves?
Answer. If you have observed a sudden leaf drop, your tree likely has maple petiole borers. Although trees drop leaves for other reasons (i.e., drought, squirrels, heavy aphid infestations), defoliation caused by the maple petiole borer is distinctive. When these insects attack, individual leaves, with part of their petiole still attached, fall to the ground. The ends of the leaf petioles are often black and the leaves are usually still green.
The maple petiole borer is the larvae of a sawfly, a type of non-stinging wasp, about a sixth of an inch long. The adult sawflies emerge in mid-April. After mating, the female will puncture a leaf’s petiole and lay a single egg near the leaf blade of a maple tree, preferably a sugar maple. After the egg hatches, a yellowish grub with a brown head emerges and tunnels inside the petiole. The larvae may tunnel and feed in the leaf stem for 20 to 30 days. This process weakens the connective tissue and causes the leaf blade to fall. Often many will fall suddenly. The leaf petiole may remain attached to the tree where the larva will continue to develop and feed for about 10 days. During May the petiole will break and fall to the ground. The larva, now about a third of an inch long, will chew a hole in the side of the petiole and burrow 2 to 3 inches below the soil, where it will pupate and remain inactive until next spring when the adult sawfly emerges.
Controlling maple petiole borer is difficult and generally unnecessary. Although leaf drop may at times be large enough to cause concern, rarely does more than 25 percent of the tree’s leaves fall. While this can be overwhelming to the homeowner, it has little effect on the health of the tree. If you want to reduce the number of borers for next year, one technique is to collect each infected petiole by raking and hand-picking. This should begin when leaves first start to drop and be continued throughout the leaf drop period. Raking just the leaves will not be effective as the borer does not live in that portion. If you live in a neighborhood with many maple trees, this technique may not be practical since adult sawflies can fly in from adjacent areas to lay eggs on your tree next spring.
Maple petiole borers have very sporadic outbreaks and chances are good that in some years they will not cause problems. The good news is that since the effect is temporary, your tree should recover quickly, fill in with a flush of new growth, and be healthy for the rest of the season.
Q. There are mushrooms in my yard. Is there anything I can spray to get rid of them?
A. Mushrooms (some of which may also be called toadstools or puffballs) live on organic matter in the soil. The mushroom is the above-ground reproductive structure of a fungus. In wet weather mushrooms will often sprout overnight. Most mushrooms don’t damage the lawn but many people find them unsightly. There is no practical or permanent way to eliminate mushrooms. However, the easiest temporary solution is simply to mow them off. Annual de-thatching to reduce the organic matter buildup also helps. When possible, remove any buried roots, stumps and lumber that mushrooms might live on.
Q. There are holes in the leaves of my Knock-Out roses but it’s too early for Japanese beetle. What caused it?
A. Damage early in the growing season is caused by an insect called the “rose slug sawfly.” They are really small and can be easily overlooked as its coloration will be similar to the leave itself.
The insect will feed on the underside of the leaf in early stages of life. When young, its mouth is too small to be able to chew all the way through the leaf. That is why the damage can appear to be browned spots at first (they leave only the epidermis tissue of the leaf) that may later fall out as it dries to become a hole in the middle of the leaf.
As they get older and larger, the insect can start actually removing all layers of the leaf much like a Japanese beetle would damage it.
When the damage is minor, just ignore them. The rose will recover. If their damage becomes severe, they are easy to control with carbamate (Sevin) or bifenthrin (Talstar).
Kelly R. Jackson is the Christian County Extension Agent for horticulture. He can be reached at 270-886-6328 or visit Christian County Horticulture online at www.christiancountyextension.com.