We live on the waterfront in BC in a zone 8 and have a snowball bush approximately 50+ years old. It has always been very healthy until this year. About half the plant (a very large one) appears to be almost dead, the rest seems attacked by something. All the leaves are being eaten and have taken on a laced look. The plant is being watered adequately. This is beautiful plant that is stunning when in bloom. Help!!!!
So sorry to hear about your devastated bush! Since there is no picture I will assume that we are dealing with a Viburnum plicatum f. tomentosum as this is a more common one that has been around for a long time.
Firstly, it sounds like your Viburnum has been eaten by a beetle. It could be that you have a Viburnum Leaf Beetle thriving on your bush. With this bug damage can be done at two stages of it’s life: in mid May to early June the larvae will hatch from eggs that have been laid on the viburnum branches and feed off of the leaves and then head down the branches and stems to the soil where they head into the pupa stage (very hard to find in the soil). In late June the pupae have grown into beetles and head up the plant to eat more of the leaves until frost. The adults will mate and lay eggs on the branches. The full life cycle is 8-10 weeks. I have attached a couple of links to the Cornell University website that has a full description with pictures of the beetle in it’s different stages and some of the ways that the beetle can be managed.
Since it is already August probably your best bet for managing the beetles right now would be to try spraying the bush to knock the beetles off. You could also try some safe insecticidal soap but it may not be very effective because it works best of younger pests. The most effective things you can do would need to take place in fall and winter/early spring. In fall you could cut out branches that have the eggs on them but since you have such a large shrub it is probably more practical to spray the shrub with a dormant oil spray before the leaves emerge. The oil will smother the eggs killing most of them. If the oil is applied when the leaves are present the leaves will also be coated and smothered as well, killing them.
There is a chance that the damage could also be caused by Japanese beetles as they leave behind skeletonized leaves as well (Sawflies also cause this effect but not as often on Viburnums). Japanese beetles have a metallic blue-green body with bronze wing covers. They are actually quite pretty but you really don’t want them around. The adult beetles also chew on buds and flowers. There are pheromone traps that you can buy or you can hand pick the beetles, best done in the morning.
If the damage is being done by sawflies then you are looking for bugs in the larvae stage. They range in size from 1/3″ to 1 1/2″ and look like caterpillars with more than 5 pairs of legs. An insecticidal soap can be sprayed on the leaves to coat the larvae. You can check at the local nursery for natural soaps/sprays.
The second issue that you have is the health of the plant. Since the shrub is unhealthy it is weak and more susceptible to pests or diseases. Your shrub will need extra care to be able to recover from this attack. Give it extra water this year and put down a nice thick layer (3-4 inches) of mulch around the base of the shrub. The mulch should extend out as far as the circumference of the shrub as this is usually how far the roots reach out as well. Mulch benefits shrubs by helping the soil retain water, keep a constant temperature, cooler in the summer and warmer in the winter (good for the roots) and it degrades feeding the soil with nutrients that are available for the plant. A bit of composted manure mixed in would be a good boost as well. Your shrub will appreciate the extra care.
Since your shrub is old and large it is smart to take a look the bones of the shrub and see if it is overcrowded in the middle. Shrubs need to have space for air to circulate and to prevent branches or stems from rubbing together, otherwise pathogens may have a chance to settle in and enter through wounds in the bark. When pruning try to take out the oldest branches first. This way you will be regenerating the shrub, basically getting the same shrub but younger. Normally the rule of thumb is to make sure that you do not take out more than 1/3 of the shrub as this causes stress on the plant. In your case the shrub is already stressed so I would suggest that you wait until next year to see how much of your shrub survives. It would be a good idea to remove any dead material for better air circulation in the meantime.
I hope this has been helpful and I wish you lots of success with such a special old beast.