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Sunday, July 15, 2018

Leaf Chewing Damage to Plants Present Special Problems

Q. We've noticed leaf damage this year on our rose bushes, apple and pear trees. More than previous years. But, peach, apricot, and pomegranate seem unaffected. Some of our roses were decimated. Any idea what pest causes this and how to control it?

Leaf damage from chewing insects. Very typical of  adult June beetle damage.

A. The picture of leaves you sent show uneven missing parts, many areas with parts missing down to the veins of the leaf.  This type of damage is from insects that have mouths specialized for chewing leaves. Heavy damage means there is a large population you have not seen in the past.
Typical June beetle
            Unless you see this type of damage throughout the year, there is no reason to spray chemicals now. Damage from these types of insects is probably over. Instead, concentrate on getting the plant producing replacement leaves. Let me explain why.
            Common insects that chew leaves include grasshoppers, many different types of beetles, weevils, and others. Some of these insects chew leaves throughout their entire lifecycle (e.g. grasshoppers) and others only during part of their lifecycle (most of beetles). Sometimes the damage progressively gets worse (grasshoppers and some weevils) and other times it lasts only two or three weeks and it’s gone (many beetles).
Metallic Green June Bettle
            Most of this type of damage that begins in summer lasts perhaps three weeks and stops. This is because the damaging stage of this insect is over. The lifecycle of this insect has moved to another stage of development and most likely will not be a problem. If you spray now, you wasted your money and time.
Grape fleabeetle
            When this type of damage suddenly occurs this time of year, it is usually from the adult stage of large insects like scarab beetles. They feed for two or three weeks before they die. An example are June beetles. Chewing damage from smaller beetles, like flea beetles, occurs earlier in late spring. Smaller insects like flea beetles can produce a second “crop” of adults so sometimes damage is seen in the fall as well. But the damage comes on fast and it’s over fast.
Grape fleabeetle damage
            Exceptions are grasshoppers where all stages of its lifecycle have chewing mouthparts. With insects like grasshoppers, we see leaf damage early in the season that progressively gets worse as the season wears on. Grasshopper control is best done as soon as possible if there have been problems in the past.
Probably root weevil damage
            Chemical pesticides reduce damage next year if you kill the chewing adults as soon as damage starts. If you wait until the damage is extensive, it’s too late. The leaf-damaging adults are gone, the damage has been done and have laid their eggs for next year.
            Instead, concentrate your efforts in rebuilding what was lost. Apply fertilizers to the soil and water them in. During hot summer months, use half of the recommended rate on the bag or container. Apply it twice, four weeks apart. Avoid applying fertilizers after August 1 to plants that are tender during cold winter months.

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