Gadenin日本語Get the appViburnum beetle: two attacks from one insect
This is the problem that actually threatens the plant. A single species does the damage twice a year in two different forms, which is why owners often think they have two separate pests. In spring the larvae hatch on the undersides of the young leaves and feed in groups, eating the soft tissue and leaving the vein network behind, so the foliage turns to brown lace. In late summer the adults, small dull brown beetles, chew ragged round holes through the leaves.
A heavy larval attack can strip a shrub bare within a fortnight. A mature, well established plant survives that, refoliates by midsummer and flowers normally the following spring, because the flowers open before the worst of the feeding. A young plant, or one repeatedly stripped for several years in a row, does not: it loses vigor, dies back branch by branch and eventually fails.
| Stage | When you see it | What it looks like |
|---|---|---|
| Eggs | Present from late summer through winter | Straight rows of small raised brown pits on young shoot tips |
| Young larvae | As the leaves expand in spring | Pale grubs in groups on leaf undersides, tiny holes appearing |
| Mature larvae | Mid to late spring | Yellow grubs with dark markings; leaves reduced to brown lace |
| Pupation | Early summer | Larvae drop to the soil; damage stops abruptly |
| Adults | Late summer | Dull brown beetles, ragged round holes chewed in leaves |
| Egg laying | Late summer into autumn | New rows of pits cut into the current season's shoots |
Beat it in winter, on the shoot tips
The whole of next spring's larval population spends the winter as eggs in capped cavities cut into the tips of the current season's shoots. On a deciduous plant with the leaves gone, those egg rows are in plain sight and completely stationary, which makes the dormant season the one moment when the pest can be removed rather than merely sprayed at.
Work along each stem from the base outwards, turning the young shoots to see every side, and look for straight lines of five to ten raised brown dots on the last few inches of growth. Cut the shoot off an inch or two (3 to 5 cm) below the lowest pit in the row. Cutting through a row leaves half the eggs on the plant. Burn or bin the prunings, because eggs on composted tips hatch perfectly well and the larvae walk back.
This is the exception to the rule that a Japanese snowball is not cut in winter. Removing the last few inches of a shoot costs some flower buds, and that is a real cost, but a stripped plant produces no display worth having either. Take only the infested tips, leave everything else alone, and count them so you can compare against next winter.
If you find the shrub already stripped in spring, do not spray and do not cut it back. Water it in dry weather, keep off the fertilizer, and let it refoliate. Then do the shoot tip inspection properly in the following dormant season, which is where the problem is actually solved.
When the larvae are already feeding
Spring intervention is damage limitation rather than control, and it only works if you are early. Check the undersides of the new leaves as they expand, before any damage is visible from a distance. Larvae feed in dense groups on a few leaves at first, so at that stage picking off and destroying those leaves removes a large share of the population in a couple of minutes.
Once the feeding has spread across the shrub, hand removal is no longer realistic. On a small plant a firm jet of water knocks larvae off the leaves, and many of the ones that fall do not return. Encourage the predators that do the rest of the work: ground beetles, birds and predatory bugs all take the larvae, and blanket spraying kills those along with the target, which is why sprayed plants often have a worse problem the following season.
Accept some damage on mature plants. Skeletonised leaves look far worse than they are, the plant reflushes, and the display was already over by then. Reserve real effort for young plants and for shrubs that have been hit hard two years running.
The other problems, and what causes them
Nothing else on this list is life threatening on an established plant, and most of it traces back to soil, water or site rather than to an organism. Work through the causes in that order before reaching for a treatment.
| Symptom | Likely cause | What to do |
|---|---|---|
| Brown crisp leaf margins in midsummer | Drought, or roots in thin soil over rubble | Deep soak, mulch, check what is under the plant |
| Yellow leaves, soft roots, general decline | Waterlogging or root rot | Improve drainage; a wet site cannot be watered out of |
| Dark spots and blotches, early leaf fall | Leaf spot fungi in a wet season | Clear fallen leaves, thin for airflow after flowering |
| White powdery film on the leaves | Powdery mildew in dry air and poor circulation | Water at the roots, thin crowded branches |
| One branch dies while the rest is fine | Dieback, often after damage or a bad graft union | Cut back into clean live wood, disinfect between cuts |
| Sticky leaves, sooty black film | Aphids or scale feeding on the shoots | Wash off, tolerate light numbers, let predators work |
| Flower buds browning before opening | Late frost on swollen buds | Nothing to do; the plant flowers normally next year |
| Branch split at the fork | Snow load or wind on a long horizontal tier | Cut back to a clean junction; do not leave a stub |
Problems that are not problems
Two things are reported as faults every year and are simply how the plant behaves. The first is the absence of berries on a shrub with fully double, ball shaped white flowers. Those globes are made entirely of sterile florets and contain no fertile flowers at all, so the plant cannot fruit under any conditions. It is not a pollination failure, and adding a second plant changes nothing. Only the flat, lacecap type with a ring of white bracts around small central flowers sets fruit.
The second is the branch that has rooted where it touched the ground. Low horizontal tiers layer themselves readily, and the new plant is perfectly healthy. If you do not want it, sever the connection and lift it; if you do, it is free stock. Either way it is not a sign that anything has gone wrong.
Keep a note of the date you first see beetle larvae each spring and roughly how many infested shoot tips you cut out each winter. Those two numbers, tracked over three or four years, tell you whether the population is being driven back, and they are far more useful than any impression formed in a bad year.