Root weevil resistant Rhododendron varieties

My rhodies are being devastated by root weevils. They have stripped many of the branches clean of their vegetation, and have destroyed ~50% of the remaining leaves. My rhodies look like they will require years to recover, if they ever do.

If I replace them with resistant varieties or plants that are not susceptible to these pests, will this eliminate the weevils?

Root weevils are the most common pest attacking Rhododendrons in the Pacific Northwest so they can only be temporarily eliminated from any garden. If the environment is right and their food source returns, so will the root weevils.

If you want to keep your current Rhododendrons, the weevils can be controlled if you’re diligent (forever!?). An article by Caroline Cox in the summer 2005 issue of the Journal of Pesticide Reform discusses their control. However, it sounds as if you’re willing to remove them and start fresh. Some of the most susceptible (host plants) are Rhododendron and Azalea, Heather, Salal, Manzanita and Kinnikinnick, Pieris, Maples, Viburnum, most Conifers, Astilbe, Cyclamen, Helleborus, Hosta and Primrose.

(Source: Root Weevils in the Nursery and Landscape; Identification and Control, by J. DeAngelis and G. Garth, EC 1485, Oregon State University Extension Service).

The extension bulletin from the Washington State University Extension website has an excellent list of resistant Rhododendron varieties.

Rhododendrons–Diseases and pests, Rhododendrons–Varieties, Root weevils

primroses and common pests

I have grown primroses both in Seattle and in South Everett. The first time the plants looked like the leaves were being eaten, and then when I pulled one of the almost-eaten plants out of the pot, most of the roots were gone. When I cleaned out the pot, I found many little white grubs in the dirt. It happened again in my new location. I am mystified, as I grow them in pots on a second floor balcony. What could be causing this and is there a way to grow primroses without this happening?

I wonder if the problem is in the potting soil. Were you using the same batch each time? It might be worth experimenting with a new brand of potting soil to see if you have the same or different results.

Also, you could try purchasing your plants from different sources. The ones you have now may have come to you from the nursery already infested.

There are a number of pests that afflict Primula. Of the culprits listed on University of California, Davis’s Integrated Pest Management site, weevils might be a possibility, as their larvae (grubs) live in the soil. The recommended treatments include parasitic nematodes and trapping of adult weevils. Here is more on this pest, from the Connecticut Agricultural Experiment Station.

controlling root weevils and whiteflies

I have a line of Ward’s ruby azaleas. The three weakest ones have a lot of tiny notches in the leaves. I seem to remember the notches from the root weevil as being larger than these. Are the tiny notches from something else?

I also noticed that some of my dahlias have splotched leaves and that when I disturb the leaves, white-looking insects fly off the leaves. These flies apparently have spread to tomatoes as well. Are these whitefly? Will they disappear after the winter or is there some control I should use to prevent them from taking over?

First you need to get an accurate diagnosis of your problems. If you are in King County, you can bring samples to a Master Gardener Clinic. For information about Clinic hours see their website (Plant Clinic Schedule).

Oregon State University offers this information about root weevils and Rhododendron (which includes Azaleas). It describes using beneficial nematodes as a control.

According to Washington State University Cooperative Extension’s publication, How to Identify Rhododendron and Azalea Problems (1984), root weevil damage to foliage is not usually a serious problem. You can check for weevils with a flashlight at night to confirm that they are the source of the notches you are seeing. There are some Neem oil-based products that may be helpful, but they must be used at the correct times of year. See WSU’s HortSense page.

As for the dahlias and tomatoes, it is important to determine exactly what the insects are before proceeding with treatment. If they are whiteflies, you can put yellow sticky traps around the plants to trap them. University of California, Davis’s Integrated Pest Management site has other recommended control methods, including reflective mulch. You may not want to use insecticidal soap:
“Insecticides have only a limited effect on whiteflies. Most kill only those whiteflies that come in direct contact with them. For particularly troublesome situations, try insecticidal soap or an insecticidal oil such as neem oil or narrow-range oil. Because these products only kill whitefly nymphs that are directly sprayed, plants must be thoroughly covered with the spray solution. Be sure to cover undersides of all infested leaves; usually these are the lowest leaves and the most difficult to reach. Use soaps when plants are not drought-stressed and when temperatures are under 80 degrees F to prevent possible damage to plants. Avoid using other pesticides to control whiteflies; not only do most of them kill natural enemies, whiteflies quickly build up resistance to them, and most are not very effective in garden situations.”

on managing weevils in the garden

What is the pest that eats little notches around my Bergenia
and Heuchera? What can I do to prevent this?

It is possible your Bergenia and Heuchera are being nibbled by black vine
weevils or strawberry root weevils. Usually you would begin to notice the
damage in mid-spring. The notches won’t kill your plants, but if you have
a lot of black vine weevils and plants appear to be wilting, you may want
to attempt to control the larvae. Spraying beneficial nematodes
(Steinernema) on the surrounding soil may also help.

Below are links to information about weevils:

Black Vine Weevil from UMass Extension

Black Vine Weevils from University of California’s Integrated Pest Management site

Strawberry Root Weevil from the Pacific Northwest Insect Management Handbook

A document about Black Vine Weevil (and other root weevils) from Ohio State University Extension

Excerpt:

“Adults that feed along leaf margins produce typical crescent shaped
notches. Careful searches should be made to try and locate specimens
since several other weevils and some caterpillars can produce this same
type of notching. Moderate to light notching seems to have little effect
on plant health.

“Black vine weevils are oblong oval in shape, about 1/2-inch long and have
a short, broad snout with elbowed antennae. The body is slate grey to
blackish brown and the wing covers have numerous small pits and short
hairs. This pest is difficult to distinguish from other Otiorhynchus
weevils. The strawberry root weevil is usually half the size of the black
vine weevil, and more brown in color. The rough strawberry root weevil is
only slightly smaller than the black vine weevil but the collar just
behind the head, the pronotum, is heavily pitted.

“Female weevils emerge from soil pupation chambers late May to early July.
These weevils must feed on plant material for 21 to 45 days before they
are ready to lay eggs. After the preoviposition period has passed, the
females place several eggs each day into the soil or leaf litter nearby
suitable host plants. The weevils hide during the daytime at the base of
plants or in mulch and leaf litter near food plants. Adults may live 90
to 100 days and usually lay 200 eggs during this time. The eggs hatch in
two to three weeks and the small C-shaped, legless larvae feed on plant
rootlets. The larvae grow slowly over the summer, molting five to six
times. By late fall the larvae have matured and are about 5/8-inch long.
The mature larvae enter a quiescent prepupal stage in an earthen cell and
pupate the following spring. A single generation occurs each year.

“Strategy 1: Habitat Modification – Egg and larval survival is helped when
soil moisture is moderate to high in July and August. Heavy mulches also
help maintain critical moisture levels. Remove excessive mulch layers and
do not water plants unless necessary. Excessively damp soils in the fall
also force larvae to move up the base of the plant where girdling can
occur. Properly maintain rain down spouts and provide for adequate
drainage of soil around plants.

“Strategy 2: Biological Control Using Parasitic Nematodes – The
entomopathogenic nematodes, Steinernema and Heterorhabditis spp., have
been effective for controlling black vine weevil larvae, especially in
potted plants. Sufficient water must be used during application to wash
the infective nematodes into the soil and root zone. If the nematodes are
to be used in landscape plantings, remove a much of the mulch as possible
and thoroughly wet the remaining thatch and soil before and after the
nematode application. Applications of the nematodes in landscapes has
produced variable results.”

controlling weevils on salal and Rhododendron

Last year I had a big problem with weevils in my salal, rhododendrons and a few other shrubs. I am not sure if they returned after putting down beneficial nematodes last fall.

Weevils are tough! You are on the right track with beneficial nematodes. It might take a few seasons to make a difference. Here is a link to information by entomologist Art Antonelli of Washington State University about controlling weevils, especially on Rhododendron.

Garden Tip #42

A trip through the garden at night with a flashlight will reveal a surprising amount of animal and insect activity. Earthworms crawl across the ground looking for decomposing plants to consume while weevils, slugs and cutworms feed on our prized shrubs and perennials. Remember that the new non-toxic iron phosphate slug baits, such as Sluggo, must be reapplied about every two weeks. More slug-coping advice can be found online at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7427.html