WSU Extension

Hortsense

Rhododendron
 
Disease
Armillaria root rot 
Botrytis shoot blight 
Gray blight 
Leaf spot 
Lime-induced chlorosis 
Marginal leaf necrosis 
Physiological leaf spot 
Phytophthora blight 
Phytophthora root rot 
Powdery mildew 
Ramorum leaf and shoot blight (Sudden oak death) 
Rust 
Salt injury 
Sunburn 
Tissue proliferation 
Insect
Aphids 
Azalea bark scale 
Caterpillars 
Lecanium scale 
Rhododendron lace bug 
Rhododendron whitefly 
Root weevils 



print version| pdf version| email url    
Caption: Root weevil damage on rhododendron
Photo by: A.L. Antonelli
  
Rhododendron : Root weevils
(revision date: 3/10/2017)


Biology
Several species of weevils feed on rhododendrons. Characteristic damage includes notching of leaf edges by adults and minor to severe root and bark damage by larvae. Occasionally the larvae girdle the trunk at or just below the soil surface, killing the plant. Adults are wingless and begin feeding around June to August, depending on the species.
Management Options


Non-Chemical Management
  • Plant resistant types of rhododendrons. Your county extension agent or WSU Master Gardeners can furnish a list.
  • Application of beneficial nematodes which kill the larvae. They are used as a soil drench in moist soils over 52 degrees F (late summer to early fall in western Washington).
Select non-chemical management options as your first choice!

Chemical Management

Adult control should begin when adults emerge late May-June or when first signs of notching appear on new leaves. A follow-up application may be necessary about a month later. Beneficial nematodes can be ordered from certain nurseries. They can be applied to control the larval stage if soil temperatures are warmer than 52 F. Do not apply nematodes in direct sunlight and be certain soil is saturated with water before application. Homeowners should not make foliar applications to trees over 10 ft tall. Consult a commercial pesticide applicator for treatment of trees and shrubs over 10 ft. tall.

Listed below are examples of pesticides that are legal in Washington. Always read and follow all label directions.
  • Bonide Systemic Insect Control
    Active ingredient: acephate  |  EPA reg no: 239-2461-4
  • Naturalis L
    Active ingredient: Beauveria bassiana  |  EPA reg no: 53871-9
  • Safer Brand BioNEEM Multi-Purpose Insecticide & Repellent Conc [Organic]
    Active ingredient: azadirachtin  |  EPA reg no: 70051-6-42697
  • Safer Brand Garden Defense Multi-Purpose Spray Conc [Organic]
    Active ingredient: clarified hydrophobic extract of neem oil  |  EPA reg no: 70051-2-42697
  • This list may not include all products registered for this use.
Images

+ Show larger images

 
Caption: Root weevil damage on rhododendron
Photo by: A.L. Antonelli
Caption: Root weevil larval damage
Photo by: A.L. Antonelli
Caption: Adult root weevil damage
Photo by: E.P. Breakey
Caption: Root weevil larvae
Photo by: A.L. Antonelli