In this Issue:
- Sequoia pitch moth: Apply trunk spray now
- Powdery mildew: Monitor plants for white mycelium
- Honeylocust and Sycamore plant bugs: Active now, but treatment usually not warranted
- Root weevils: Feeding damage on foliage will be visible soon
- Euonymus scale: Crawlers are starting to hatch and will continue through mid-June
CONIFERS
Sequoia Pitch Moth
Treat trunks of pines now
The Sequoia pitch moth (Synanthedon sequoiae) is a clearwing moth whose larvae feed on the outer and inner bark of pine trees. Their feeding stimulates the tree to produce copious sap that forms large, drippy masses on the trunk. The larvae will also feed within the mass of pitch and if you remove the glob at the right time, you will often find a larva inside or close to the trunk.
From May through August, adult moths lay eggs on bark, often preferring pruning or other wounds, branch collars, or existing pitch masses. When the eggs hatch, the larvae bore through the bark and create meandering tunnels in the cambium, or may bore into the sapwood. The feeding is usually localized around the pitch mass, and sometimes, the larvae will exit the wood and feed on the pitch.
Smaller trees are at greatest risk of being affected by pitch mass borers. Larger, healthy trees can withstand attack.
Keep in mind that after the moths exit the trees, the pitch mass will remain on the trunk, possibly for several years, unless they are removed by hand.
Treatment
Management should focus on keeping trees healthy. Avoid pruning in July when moths are laying eggs. Manually remove existing pitch masses to prevent further egg-laying or to kill larvae inside. If a spray is warranted on larger trees, it should be applied by a professional.
- Commercial: permethrin or bifenthrin (trunk sprays); start application in mid to late May, so that trunks are protected through early August
European Pine Sawfly
Larvae are feeding now
This pest was mentioned in the May 2 post, and larvae are still active.
Treatment:
- Treatment is rarely needed, as feeding is localized and usually not severe enough to harm the plant. Because they feed together, removing a branch removes most of the larvae; hand-picking is another option.
- If necessary, spray options include acetamiprid (Tristar, Assail), spinosad (Conserve, Fertilome / Monterey), or insecticidal soap. Bt (Dipel) does not work on sawflies.
DECIDUOUS TREES
Powdery Mildew
Monitor plants for infections
Powdery mildew is showing up on roses, maples, ornamental pears, honeysuckle, lilac, crabapple, and other plants. It is important to monitor your own plants to catch it early. Check the undersides of leaves that are lowest on the plant, looking for white, powdery spots.
Powdery mildew is a fungal disease that prevents leaves from photosynthesizing to their fullest ability, resulting in stunted leaf growth, chlorosis or scorching, and leaf drop. In late summer, the fungus produces black fruiting bodies called chasmothecia which allow the fungus to survive the winter on bark or fallen leaves.
Free water (rain) does not spread powdery mildew. Instead, it thrives in the high humidity of the plant canopy, which may increase after rainfalls or overhead irrigation. In Utah landscapes, powdery mildew is found more on crowded or shaded plantings.
Treatment
To lessen infections, prune out or pick off localized infections, avoid overhead irrigation, improve air circulation between and within plants, and rake fallen leave in the fall.
- Residential options: Organic options should be applied every 7 days and include horticultural oil (0.5%), Serenade, potassium bicarbonate (Bicarb, Kaligreen), or neem oil. Conventional options include Spectracide Immunox, Monterey Fungimax, or Fertilome F-stop, and can be applied twice, spaced 14 days apart.
- Commercial options: There are many fungicides for ornamental application against powdery mildew, including AmTide, Banner Maxx, Bayleton, Eagle, Heritage, Pageant, Rally, Trinity.
Honeylocust and Sycamore Plant Bug
Active now, treatment only warranted if nymph populations are high
Plant bugs on honeylocust and sycamore are building in population size now. They can be detected by the leaf symptoms, or by shaking several branches over paper or a cloth tray.
Plant bugs overwinter as eggs that hatch into nymphs during leaf expansion. The honeylocust plant bug has just one generation per year, so the nymphs and adults are only around for about 6 to 8 weeks. The sycamore plant bug has many generations per season, and can thus cause damage throughout the season.
Adults and nymph plant bugs feed with piercing-sucking mouthparts, where they they inject a toxin into the leaves that kills the tissue, causing chlorotic (yellow) and necrotic (brown) stippling, and as leaves expand, they tear and become ragged.
Treatment
If nymphs (insects without wings) and feeding damage is found, and the tree is smaller, try a strong stream of water onto the undersides of leaves to knock them off. Because they are currently flightless, the nymphs will not be able to crawl back to the leaves.
If the tree is large or the population size of the nymphs is high, consider an insecticide application. Any pyrethroid (conventional option) or insecticidal soap or horticultural oil (both organic) is effective.
Root Weevils
Lilac root weevil adults begin emerging May 24
The characteristic leaf-notched feeding by adult weevils is showing up on a variety of hosts including dogwood and lilac. Most weevils that feed on woody ornamentals overwinter as larvae in the soil, pupate in spring, and emerge as adults in June. At night, the adults feed on leaves, where the damage is more aesthetic, while the larvae cause greater damage by feeding on fine roots.
The two most common root weevils in Utah are the strawberry root weevil (Otiorhynchus ovatus) and the lilac root weevil (O. meridionalis).
Root weevil adult and larval feeding is usually not a concern on mature plants. But where populations are high or where young plants are affected, control may be warranted, targeting the larvae.
Treatment
A pyrethroid (Mustang, Asana, Spectracide Triazicide) will target the adults. Spray the foliage and the base of the plant where adults will congregate during the day.
Two options are predatory nematodes (soil must be kept moist), or imidacloprid soil drench.
Euonymus Scale
Crawlers are starting to hatch and will continue through mid-June in Wasatch Front area
Euonymus scale is an armored scale that remains mobile most of its life, resembling whiteflies. It sucks plant nutrients from leaves, stems, and bark, weakening the plant over time. It does not secrete honeydew. Leaves with scale will be yellow and will drop, leaving bare areas of the plant.
Small amounts of this scale should not be harmful, but if your stems are covered, now is the time to start treatment as the crawler stage is active. Crawlers are newly-hatched scales that move for a few days, and are susceptible to treatment.
Treatment
- Residential: Organic options are horticultural oil (1%) or insecticidal soap, and conventional options are GardenTech Sevin (zeta-cypermethrin), or BioAdvanced Insect Killer (cyfluthrin).
- Commercial: Pyrethroids, malathion, imidacloprid (fall application)