Summer is here, the heat is on. One of our least desirable imports, Japanese beetle, is foraging in gardens and laying eggs that will hatch 10-14 days later as root-feeding grubs. I'd like to share some of the collective wisdom I've heard over the years. These facts may help you refine your plan of attack and minimize damage.
While "first emerging" adult JBs clamber out of the ground early July (in Chicago) it can be mid-August before they have all materialized. This means at least 6 weeks or more of potential adult feeding damage. Not good news.
|Voracious activity on grape|
- Trees such as linden, birch, Japanese maple (logically enough), crabapple, Norway maple
- Climbing Hydrangea (from personal experience)
- Hardy Hibiscus
- Raspberry and blackberry
Adults fly to the top of preferred host plants growing in sun to feed. If undisturbed they tend to feed on the same plant for 3 days at a time. At that point some signal causes them to move on, flying up to 1/2 mile to a new host plant. Consider this fact when thinking about putting up a JB trap (with their 2 scent lures). Consensus is traps attract larger numbers to your garden, with only a percentage meeting their maker in the trap bag.
Adult JBs have a quirky habit late in the afternoon and evening. When disturbed at that time they fold their legs and drop straight down from their feeding perch. Earlier in the day they will tend to fly up, up and away. If you have the time and patience there's an easy way to capitalize on this PM drop-and-roll behavior. Put soapy water in a wide mouthed jar. Place it under the feeding beetles(s), tap and they should drop right into the jar. Boom, done. Consider you may have to do that a lot of times...
Interestingly, they are attracted to plants that are/or have already been fed upon by other JBs. Taste-tested leaves evidently emit a tantalizing odor that serves as an attractant. In addition, satisfied early feeders emit a hormone that also serves as a **** review drawing in additional diners. If you feel control in any form is warranted consider knocking down (your choice, physically or chemically) the first adults so they don't leave their calling card. This should mean less feeding damage for the season.
When feeding and mating are accomplished females scout for sunny areas with good soil surface moisture to deposit their eggs. This is generally done around dusk. So, yes there's a reason that in years with high JB populations and low July-August rainfall heavily irrigated lawns are more apt to have grub infestations than unwatered grass. If your lawn has historically had JB issues you might consider irrigating less frequently, but more deeply, during the mid-July through mid-August egg laying period. Let the surface dry out. I personally like the idea of my lawn having to develop a deeper root system while seeking water.
Eggs hatch in ten to fourteen days (depending upon soil moisture and temperature). Grubs start feeding on the roots of grass and other plants, but especially the former. Grubs can affect turf turf health and vigor by feeding on roots. There is general agreement that control should be considered when grub populations reach a 10-12 per square foot threshold.
Japanese beetles, game on!