The words Japanese maple and Japanese beetle in the same sentence are usually a signal for panic but the damage Japanese beetles can potentially do to a Japanese maple has to be looked at in perspective. There are several hundred plant species that the beetles feed on and the chances of all the beetles in your garden descending at once on your maple is extremely slim.
It can, however, be somewhat disconcerting to see the leaves gradually disappearing and there are a number of ways to reduce and, depending on your location, eliminate the problem. At the first sign of damage the beetles can be picked off and killed by dropping them in a jar of soapy water. Early morning and late evening is the best time as they are less active. Heavy infestations, particularly if you are surrounded by other gardens, are best dealt with using an appropriate insecticide such as those based on acephate, carbaryl and permethrin. The recently introduced imidacloprid needs to be applied prior to a known infestation about to take place, so some forward planning is required. A non chemical option is Neem oil and whilst the beetles cannot develop resistance to it, unlike conventional insecticides that work on the nervous system, it does require regular applications as the active ingredient in the oil is broken down by sunlight.
One other method of control is the use of traps that are baited with pheromones to attract adults. This can only be recommended if you live in the middle of nowhere; in an urban environment you’ll end up with more in your garden than you had originally!
Controlling the grubs is more important as they have the potential to inflict far more serious damage by eating roots and this is compounded if your Japanese maple is planted as a specimen in your lawn. The range of options for grub control during the period April to June/July when they are close to the surface includes milky spore, nematodes and imidacloprid based insecticides.
Milky spore is a long term control as the bacteria need to build up in the soil over a period of two or three years before they are fully effective. Nematodes, of which strains of Steinernema are preferred, are effective only if the soil can be kept constantly moist as they can only move through the soil in the moisture surrounding individual soil particles. Imidacloprid has the benefit of killing both the mature grubs in the spring as well as the new grubs in summer. There are also natural predators and you’ll know if you’ve got them as they are ripping up your lawn to get at the grubs!
Plastic sheet cut to size and overlaid with pebbles
The best control, though, is prevention rather than cure and the best protection is a mulch, but not the usual such as bark chips or leaf-mould, as these provide an ideal place for eggs to be laid. Rather, an impervious barrier should be used, such as a sheet of plastic held down by a thin layer of pebbles. Don’t worry that your maple won’t get any moisture; the barrier traps the moisture already in the ground and is replenished by sideways movement of rainwater entering the soil beyond the cover. For pots, water will seep through the slot made to accommodate the trunk and won’t interfere with your normal watering regime.
This type of mulch is effective against all soil-borne grubs and larvae and has the added benefit of moderating extremes of moisture levels that can be so damaging to a Japanese maple.
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