Rhododendrons are generally immune to most insect pests, the concentration of toxic substances in their leaves is so great that most insects avoid eating them. Aphids will feed on the young leaves when they emerge, but by the time they are more mature even aphids avoid them. Slugs may eat a little, but there is one pest that really attacks them.
This last few weeks has seen the annual emergence of Root Weevils, the worst pests of the Rhododendrons. The adult forms of the insect are inch to inch and a half beetles that hide during the day and emerge at night to feed on the leaves of their hosts, including Rhododendrons, Azaleas, and Clematis, their signature is the feeding marks of the adults. The edges of the leaves show notching as if a ticket has been punched, (see the pictures following the article). The leaf damage is worrying, but it is really only a symptom of the greater worry. Rhododendron leaves only stay on the plant for a few years, they are replaced as the new growth adds more to the tops.
The real damage is being done by the juvenile Weevils, the larvae are feeding on the roots. If you see indications of the adult feeding, check around the base of the plant by brushing away the soil around the trunk and looking at the zone where the roots emerge. Look for ½ inch little white grubs, usually curled into a “C” shape, no legs, with soft bodies. They may be found within a couple of inches of the surface, and in particularly bad infestations they will have girdled the main trunk leaving a ring of soft sawdust like material.
Rhododendrons are not their only targets, but the characteristic feeding pattern on the big evergreen leaves is the best indication of their presence, and if they are eating your Rhodies, you can be assured they are sampling other pleasures in your garden. I looked at a yard this week where we had planted beautiful new dwarf Yakusamina hybrid Rhododendrons this spring, there were no other Rhodie plants in the yard. In just the one summer the weevils had eaten their indicative notches in all the new leaves, so much so that they looked like lace doilies. This indicates to me that some other plant in the garden was hosting their party, even though I could see no indication of their presence.
We have seen them for the last 10 years or so. I imagine they arrived with some ornamental plants as unwanted hitchhikers, but no matter how they arrived I have seen them in increasing abundance over the last few seasons.
Weevils are incredibly tough, and since they feed on what are very toxic plants — Rhodies and Yews — they are resistant to all but the most toxic of insecticides, and very few people want to deal with them. They prefer to live with the damage since they rarely kill the plants. Alternative methods of control include trapping the adults by placing snares that mimic hiding places, or using sticky substances on the trunks that immobilize them as they try to climb up during the night.
The search for biological controls of insect pests worldwide has led to the resurgence of raising bugs that eat bugs. European greenhouse growers quit using pesticides in their crops a decade ago, and as the technology develops, a method of controlling weevils has emerged. This is not another insect, but something much smaller. Microscopic nematodes have been discovered that focus on the larval stages of these damaging insects, and since the same weevils attack strawberries, much attention was focused on developing the nematode method.
Facilities that raise insects to protect crops are all over the world, and these nematodes, though not insects, are raised by the billions. They are sent via UPS, in coolers like fish, overnight to growers, nursery people, and gardeners for control without poison. The microscopic creatures are stirred into a watering can of water, and poured onto the surface under the plants, watered in and they go on the hunt.
Nematodes seek out the larvae and attack them. While the weevil population persists, the nematodes do too. When they’re gone the nematodes die out. There is no evidence of them being in any way dangerous to any other life forms, microscopic or larger, and as they are dependent on the weevils to survive, when the host is gone, so are they.
This is the future, and it’s here now.
• David Lendrum and Margaret Tharp operate Landscape Alaska, a nursery and landscape business located on the Back Loop Road in Juneau. Visit their website at www.landscapealaska.com, or reach them at landscapealaska@gmail.com. This column “Landscape Alaska” appears every other week.
