What exactly is causing the defoliation? Dormant oil sprays may or may not be the appropriate treatment.

In general, petroleum based oils are considered safe to mammals, birds, reptiles and amphibians but are toxic to fish. They are non-selective so could be toxic to any beneficial insects present at time of spraying but not after application. Vegetable based oils have a similar mode of action and should be equally effective. They also tend to leave no residue.

Directions for use, including timing of spraying and info regarding specific toxicity will be on the product label. But you need to know what you are spraying for and if the dormant or hort oil will be an effective control.

Lisa, the problem is Viburnum beetles, which are steadily heading south after an outbreak in BC. They are already a major scourge in the eastern part of the country. They feed exclusively on viburnums, and lay their eggs in holes they drill in their branches. I think the idea is that the oil is supposed to smother the eggs and prevent hatching. I'm just undecided if the risks to wildlife outweigh the viburnums so I wanted to know how toxic these products are and if they're practical to use on really large shrubs.

Timing of the spray would be critical in this case. 2-4 weeks before the eggs hatch is the targeted date for maximum effectiveness. Horticultural/dormant oils are considered approved organic controls and in the grand scheme of things, are relatively benign on the environment. You might also want to investigate pesticides that are formulated with spinosad, a biological control derived from a bacterium. This has been found to be virtually non-toxic to non-target organisms and you have a much wider window of application. There are also resistant species of Viburnums you could elect to replace your current shrubs with.

And I'm not sure who Lisa is, but it's not me :-)

Hort oils are pretty safe, although they kill all soft-bodies insects they contact, not just the pestiferous ones. And they're not at all safe for aquatic life, which means not just fish, but a lot of larval amphibians and insects too (like dragonflies). So you can use them safely, once in a while, just so long as you're not spraying near open water.

But, from the reading I've done on viburnum beetle, it sounds like this is not the time to spray, because the eggs are sealed inside the wood of the viburnum twigs. Hort oils need to contact the eggs to kill them and that won't happen when the eggs are inside the wood. Cornell recommends spraying when the viburnums first leaf out as that seems to be when the eggs hatch and the larvae emerge.

Here is a link that might be useful: Cornell U Viburnum beetle page

( FYI, Lisa is "sedumgal")

Brody, wow, sorry to hear about your viburnums. But I do hope you can keep those beetles safely in check up north so they won't spread down here to my viburnums. Keep us posted.

PNWJudy

Dormant Oil is a very safe form of pest control, but if you don't want to kill anything, well, you just have to let the bugs take their toll.
Nature is a battlefield that makes the Iraq war look like a picnic...... lots of bugs and diseases eating each other and plants and such.
If you are afraid of 'collateral damage', the fact is this.
You will kill a few non target bugs.
If you can't live with that, don't use any sprays.
Oil is way down the list on toxicity of pesticides---- but do not get it on conifers, they are very sensitive to Oils and will get real damage where the Oil lands. Probably the tree will live, with a 'brown spot'.

Oops, sorry about that gardengal. Judy's right, I did get you mixed up with sedumgal/Lisa from the plant trades. I think I'm going to visit Bakerview nursery and give the hort. oil a go since it seems to be fairly benign. Thanks for the helpful information.

Judy, I hope those critters don't spread south too. Both the larva and the adult bugs eat are harmful, so they actually defoliate the host shrubs twice in one year. Yikes. On the plus side, the beetles attract birds. I had a host of waxwings last year for the first time feasting on them.

What type of Viburnams do these bugs eat, deciduous or evergreen? Both?!!

Mike, they seem to focus on certain deciduous species: V. opulus, dentatum, sargentii, and trilobum are most frequently mentioned. And the resistant species include carlesii, burkwoodii, juddii, doublefiles and any of the evergreen forms.

Have not seen any sign of these critters locally so far but Bellingham is not all that far away.