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Japanese Beetles by Martha Smith

Yes – this news is a gardener's dream come true – for at least the 2014 growing season! All the winter data is in and the experts now say Japanese beetle numbers in northern Illinois will be much lower this year.

Many Japanese beetle larvae did not survive the winter – particularly in the northern half of the state. "Earlier predictions stated that number should be at normal levels due to the insulating effect of the deep snow cover", explains Martha Smith, Horticulture Educator, University of Illinois Extension. "But the latest data shows the cold temperatures challenge this early prediction."

This past winter the cold temperatures penetrated deeper in the soil than what was first thought. Japanese beetle grubs do not migrate deeper than 11 inches into the soil for the winter. They die if the soil temperature reaches 15 degrees Fahrenheit or if they are subjected to freezing temperatures for two months. Last winter the soil was frozen to 15 inches deep in central Illinois and 30 inches deep in northern Illinois for several weeks. Based on previous experience, it is likely that about two-thirds of the larvae died during the winter in the northern half of the state. That is good news for crops and landscapes!

A double hit for Japanese beetle numbers has to do with summer 2013 rainfall amounts. In addition to cold temperature mortality, Japanese beetle larvae require approximately 11 inches of water from egg hatch in late July into the fall. Although we received abundant rain in spring of 2013, much less rain fell from July through October, averaging 9.5 inches during this critical time in their lifecycle. On turf areas that were not irrigated and allowed to go dormant, Japanese beetle larvae had a very low survivability rate compared to irrigated turf. Japanese beetle larvae had a greater chance of late summer survivability on irrigated lawns, athletic fields and golf courses. Because late summer 2013 was dry the numbers of larvae were reduced only to be hit with very cold winter temperatures for extended periods of time. This sounds like the perfect storm for 2014 Japanese beetle populations.

Other pests such as the northern and southern masked chafer grubs require less water and tunnel deeper than Japanese beetle grubs. In areas of central Illinois and other areas where these grubs are numerous, neither drought nor cold is likely to have reduced their numbers. Where Japanese beetle adults are few, masked chafers are likely to invade those turf areas. These adult beetles do not inflict the feeding damage on landscape plants like Japanese beetles, but the grub stage can cause considerable turf damage.

Source: Martha A. Smith, Horticulture Educator, University of Illinois Extension, #309-756-9978