Aphid infestation in spring
Although aphid damage to a Japanese maple occurs in spring, as seen on the example of Taiyo nishiki here, the origins of any attack occur in late summer/early autumn. Generally, if they are noticed at all they are usually ignored as they don’t appear to be causing any problems and any sticky deposits that are seen on the leaves won’t matter as they will be falling off shortly.
Their real purpose at this time of the year, as can be seen below on this example of Sango kaku, is laying eggs and these are going to be deposited in any small crack in the bark as well as around the leaf buds, as this is where their food source is going to be next spring. Once the eggs are laid, they are more or less impervious to sub-zero temperatures and any chemical or biological control apart from visiting birds, so action needs to be taken as soon as the aphids are spotted.
Aphid infestation in autumn
Any contact insecticide such as neem oil or a soft soap solution is effective as well as the common systemic insecticides. If control is left until the spring it’s far more difficult to eradicate them all immediately as there will be successive populations hatching over a period of weeks and they will be concentrated on and within the unfolding leaves. The end result is a mass of sticky and distorted leaves and shoots, which, on a young plant can often result in a severe growth check and possible eventual death, but at the very least, cause stunted growth for the rest of the season.
Chemical control at this point will often add to the damage as repeated systemic insecticide applications over a short period of time can be detrimental to a Japanese maple. Neem oil will provide a more benign control of an aphid infestation but the problem can be effectively neutralized by careful observation of your Japanese maple in the autumn.
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