To help develop the best control measures for your landscape, it is important to understand the life cycle of this insect pest.
Japanese beetle has a one year life cycle. Adults begin to emerge from the soil from late June through early August.
As adults, Japanese beetles are found feeding and mating on foliage and flowers of their host plants, especially roses, grape vines and Virginia Creeper. Periodically, mated females will move in late afternoon to areas of moist turfgrass to lay small clusters of eggs, 2-3 inches below soil surface. They subsequently emerge and will resume feeding on host plants, returning to turfgrass later to lay more eggs. A total of 40-60 eggs may be laid by each Japanese beetle female during the course of her 4-8 week life span.
As we discussed last week, this is why controlling the adult stage through hand picking is so important. If the adult is removed and eliminated from the environment, she cannot lay eggs. The most effective means of control in your landscape, is to keep up with the physical removal of the adults! The beetles are easily picked or dislodged into a container of soapy water especially in early morning and again in the evening. The regular removal of beetles prevents the feeding damage by those beetles and also deters other beetles from aggregating on the plant. Beetle presence on plants and prior injury is attractive to other beetles.
Upon hatching from the eggs in the ground, the grubs (larvae) seek out nearby plant roots and feed. During the time Japanese beetles are in the egg and earliest grub stage they are quite sensitive to drying and may die if soils temporarily dry during this period. Later stage larvae are less sensitive to drying. Japanese beetle larvae become nearly full-size by early September and their rapid development during late summer can cause extensive root pruning. Surface symptoms of injury may be present at this time, with damaged grass appearing drought stressed.
Cultural controls of lawn management will help prevent Japanese Beetles from laying eggs or their successful hatching and development in your lawn. Watering can have several effects. Japanese beetle eggs and the tiny early stage larvae are very susceptible to drying. If the top couple of inches of soil in a lawn can be allowed to dry a bit during the time eggs are being laid and hatching – July and early August – then many may be killed.
Some cultural practices can limit damage and applied chemical or biological controls may also be useful. However, control of Japanese beetle larvae in a yard will have very little, if any, effect on the number of Japanese beetle adults feeding on trees, shrubs and garden plants. The insect is highly mobile so that problems with adult beetles typically involve insects that have moved a considerable distance.
Larvae continue to feed until soil temperatures drop to about 60oF at which time the larvae move deeper in the soil where they remain through winter. All activity ceases when soil temperatures drop below 50oF. Activity resumes as soils warm in spring and, after a feeding period of about 4-6 weeks, the larvae form an earthen cell and pupate. A few weeks later the pupal stage is completed and the new adults emerge the following year.
The new adults are emerging from a moist lawn area, moving within a 3-5 mile radius to invade our landscapes. Adults feed on leaves, buds and flowers of many common garden and landscape plants. On leaves feeding is usually restricted to the softer tissues between the larger leaf veins, which results in a characteristic feeding pattern known and described as ‘skeletonizing’. More generalized ragged feeding occurs on softer tissues, notably flower petals; rose flowers are particularly susceptible to Japanese beetle injury. Damage on individual plants may be patchy, concentrated where aggregations of feeding beetles occur.
In addition to hand-picking, adult Japanese beetles can be effectively controlled by use of sprayed insecticides which either kill and/or repel beetles. Bonide Eight or Captain Jacks’ Deadbug are particularly effective and may provide protection of plants for 2-3 weeks following a single application. As with all pesticide applications, read the label and follow the directions carefully. Make your application in the evening, after the sun has gone down so as not to disturb the honeybees.