In this Issue
Spring is the time of year where many pests of ornamental trees are active, so this is a long post! The recommendations for treatment in Utah landscapes only applies if the pest is present or was a problem in the past. Do not treat if the pest is not present! Also please note that the timings of pest activity are based on temperature conditions of the Wasatch Front. Please adjust to earlier or later, depending on your location in Utah.
- Aphids: Watch for newly-forming colonies on undersides of leaves
- Cankerworms: Small green “inchworms” on many hosts
- European pine sawfly: Larvae starting to actively feed now
- Lilac-ash borer: Treat the trunks of ash and lilac now
- Oystershell Scale: Crawlers will be hatching May 10 through mid-June
- Pine needle scale: Crawlers active now through early June
- Plant bugs: Nymphs are feeding now on ash, honeylocust, and sycamore
- Anthracnose: Infections on sycamore, maple, and oak may occur under rainy conditions
- Oak leaf blister: Simply a curiosity
- Powdery mildew: Monitor plants for white infections after leaf-out
INSECT PESTS
Aphids
Watch for newly-forming colonies on the undersides of leaves
Aphid colonies are building on ash, cherry, hawthorn, linden, maple, viburnum, and other shrubs, causing curled and distorted leaves. Honeysuckle aphids snowball viburnum aphids are just now starting to hatch. Aphids overwinter as eggs. They reproduce and feed on their woody host for 6 to 8 weeks, and then leave that host for herbaceous plants until September, when they return to lay eggs.
Treatment
Most healthy trees can tolerate a good deal of aphid feeding; however, the honeydew produced by the aphids can be a nuisance. If you can tolerate the honeydew, you can feel good knowing that leaving the aphids in place is providing food for all our many predatory and parasitoid beneficials.
Otherwise, on small plants, start with a strong stream of water to knock off the nymphs. They will not be able to crawl back up to the tree. Or spray with insecticidal soap (many brands), horticultural oil (1%; many brands), neem oil, or a pyrethroid (which is very strong) such as Spectracide Triazicide, GardenTech Sevin, Aloft, Brigade, Pounce, etc.
Cankerworms
Small green inchworms can be seen now on many hosts
Cankerworms are leaf-feeding caterpillars found on many tree hosts including apple, ash, beech, elm, linden, maples and oaks. There are two species, the spring cankerworm and the fall cankerworm. The fall cankerworm is most common in Utah. It overwinters as eggs on branches and twigs, and caterpillars hatching in spring. They are are greenish in color, and move like an inchworm, traveling from branch to branch on silken threads.
They feed for about 6 weeks in spring, causing irregular holes in leaves, and do not cause serious damage. You may notice them in late June as they swing down from the trees to burrow into the soil. There they pupate and the fall cankerworm emerges as adults in late fall to mate and lay eggs.
Treatment
If control is necessary, Bt (Bacillus thuringiensis; Dipel) is an excellent option. It must be applied before the caterpillars are longer than 1 inch in size. Another excellent option that is safe on beneficials is spinosad (many brands such as Captain Jack’s Deadbeat Brew).
European Pine Sawfly
Larvae are actively feeding now
Larvae of the European pine sawfly have hatched and are actively feeding on last year’s needles of mugo, Scotch, Austrian, and ponderosa pines. In large infestations, they will leave behind bare branches but thankfully, they do not feed on the current season’s buds. As a result, the new needles will often hide the damage.
Sawflies are not caterpillars. They are related to ants, bees, and wasps. They overwinter as eggs laid in slits along the length of needles. The larvae feed for about 4 to 6 weeks. They then pupate, and adults emerge in the fall to mate and lay eggs.
Initial damage looks like brown wilted foliage because the larvae are only feeding on the margins of the needles. As larvae mature, they group together and gorge on entire needles, sometimes causing complete defoliation. When attacked or harassed, the larvae rear back their heads in defense, and are able to eject a repellent substance from a sac off the foregut.
Treatment
Treatment is rarely needed, as feeding is localized and usually not severe enough to harm the plant. Because they feed together, removing the branch removes most of the larvae; hand-picking is another option.
If necessary, spray options include acetamiprid (Assail, Tristar), azadirachtin (Safer BioNeem, Safer Grub Killer), spinosad (Conserve, Fertilome / Monterey), horticultural oil, or insecticidal soap. Bacillus thuringiensis (Bt) does not work on sawfly larvae.
Plant Bugs: Ash, Honeylocust, and Sycamore
Nymphs are hatching and actively feeding now
“Plant bugs” are true bugs in the order Hemiptera. Honeylocust plant bug, sycamore plant bug, and ash plant bug are each unique species that feed only on their namesake host. Plant bugs overwinter as eggs that have recently hatched into nymphs.
- The honeylocust plant bug has just one generation per year, so the nymphs and adults are only around for about 6 to 8 weeks (typically through June). However, in some seasons, there can be hundreds of bugs per tree.
- Ash plant bug has two generations, and is usually not a concern.
- The sycamore plant bug has many generations per season, and can thus cause damage throughout the season.
Adults and nymph plant bugs feed mainly from the undersides of leaves, with piercing-sucking mouthparts. As they feed, they they inject a toxin into the leaves that kills the tissue, causing chlorotic (yellow) and necrotic (brown) stippling, and ragged leaves. Leaves also become distorted, discolored, and dwarfed. The nymphs cause the primary damage; once you see adults, it is too late for treatment.
Treatment
Look for them now and in the next few weeks by shaking them off branches held over paper or a cloth tray.
If nymphs and feeding damage are discovered, a strong stream of water on the plant leaves (of smaller trees) will knock them off, and the flightless nymphs will not be able to crawl back to the leaves.
Otherwise, options include insecticidal soap (Safer, Concern, others), horticultural oil (1%; All Seasons oil; others), or a pyrethroid such as bifenthrin (Bonide Eight, Tundra, Talstar, etc.), permethrin (Hi Yield Lawn and Garden, Aloft, Brigade, Pounce, etc.), or carbaryl, or malathion.
Lilac-Ash Borer
Treat susceptible trunks now
The emergence of lilac-ash borer adults coincides with full bloom of lilacs, indicating the time to protect tree trunks against egg-laying.
Green and white ash (Fraxinus) are the most susceptible. Sometimes, mountain-ash (Sorbus) and privet are attacked.
Lilac-ash borer does not directly kill trees, but repeated infestations can cause branch dieback and can leave trees susceptible to breakage in storms. Infested trees will have round exit holes on the bark, sawdust-like frass near the holes or at the base of the tree, and rough, swollen, cracked bark, mostly near branch crotches.
Treatment
Healthy plants are able to withstand minor infestations, while stressed plants are more susceptible to attack and failure, so give trees optimal water and fertilizer, and prune properly.
Insecticides target the adults. Small trees can be treated by the home gardener, but in order to get thorough coverage on large trees, treatments should be made by a licensed pesticide applicator.
Applications should protect the bark for 6 weeks (usually until mid-July), so repeat as needed.
- Residential options: Hi-Yield Lawn and Garden (containing permethrin), Spectracide Triazicide, or GardenTech Sevin. Spray the trunk twice, spaced 3 weeks apart. An organic option is any product containing spinosad or azadirachtin, but applications must be made weekly for a total of 5 applications.
- Commercial options: Acelepryn (chlorantraniliprole), permethrin (Astro, Covert, Prelude), or bifenthrin (Onyx)
Oystershell Scale
Crawlers start hatching May 10
Oystershell scale is an armored scale (an immobile insect) with two generations of crawlers. This scale occurs periodically in landscapes, and is been killing forested aspen trees in southern Utah.
The first generation of crawlers will be emerging in the next week in Wasatch Front areas, and slightly afterward in cooler areas. Dozens of deciduous plants can be attacked by oystershell scale. In Utah, common hosts are ash, aspen, cottonwood, lilac, hybrid poplars, maple, and willow.
Treatment
The best time to treat for this scale is when the crawlers are active, which will require a 10x magnifying lens to see.
- Residential: For organic options, which include insecticidal soap (many brands), horticultural oil (1%; many brands), or neem oil, four applications spaced one week apart will be required. For convention options, which include Hi Yield Lawn and Garden, Spectracide Triazicide or GardenTech Sevin, make two applications spaced two weeks apart.
- Commercial: Dinotefuran (Safari) as a soil injection (imidacloprid is not effective) is very effective. Distance (pyrifoxyfen) or Talus (buprofezin) are insect growth regulators and best when used early. They also have sublethal effects in that surviving females lay fewer eggs the following year. Any pyrethroid (Tempo, Astro, Pounce) is also effective.
Pine Needle Scale
Crawlers active now through early to mid June
Pine needle scale (not to be confused with black pineleaf scale) is an armored scale insect pest of Scotch, Austrian, and mugo pines. The scale body forms a white waxy covering, making it easy to identify. We usually do not see heavy populations in Utah except in localized areas.
This insect is immobile most of its life except the crawler stage, which is just beginning now along the Wasatch Front. Crawlers are so tiny you need a 10x magnifying lens to see them, and each individual crawler settles within a few days of hatching, and develops a white waxy covering. As the scale sucks plant juices, the needles turn yellow and may drop prematurely. Most pines can tolerate a small population.
Treatment
The timing for treatment in northern Utah is now through early June.
Two sprays of horticultural oil (1%) or insecticidal soap, one week apart, is very effective and safe on natural enemies. Be sure to get good coverage of the needles.
Other options that are stronger include a pyrethroid such as bifenthrin, permethrin, or cyfluthrin, or dinotefuran (Safari) can be applied as a soil drench in spring.
PLANT DISEASES
Anthracnose of Sycamore, Maple, and Oak
Plan to treat foliage before or between rains
Anthracnose is a fungal-caused disease that thrives in cool, wet weather. It causes blotch and blight symptoms on leaf shoots. Leaves that are infected in early spring will drop, and new infections may occur with additional cool, wet weather.
When temperatures increase and rains stop, anthracnose will become inactive, and infections will stop for the season. In addition, the tree will produce a new flush of healthy foliage.
Maple and oak anthracnose overwinter in fallen leaves, so these diseases are the most severe in natural or wooded areas where the fallen leaves collect from year to year.
Sycamore anthracnose is more harmful because the fungus invades the wood of twigs, causing small cankers. Each spring, these cankers produce spores which infect foliage.
Treatment
- Maple and oak anthracnose are treated by applying a preventive fungicide as leaves expand. For commercial applicators, there is a long list of fungicides (Abound, Heritage, Luna Sensation, Reliant). For backyard applicators, options include Spectracide Immunox, Monterey Fungi-Max, or Natural Guard Copper Soap (organic). Raking leaves in spring and fall will also help.
- For sycamore anthracnose, the same fungicides listed above can be used. For specimen trees, a professional applicator can apply a trunk injection (Arborfos, Arbotect) in spring or fall that will provide up to 2 years of protection.
Oak Leaf Blister
No treatment needed at this time, but watch for puckered leaves
Oak leaf blister is a fungal disease caused by Taphrina caerulescens. Due to the recent cool, moist weather, this disease may be visible in the next few weeks on leaves of oaks in both the red and white oak families in the next few weeks.
The fungus overwinters on buds, and new infections in spring occur under cool temperatures and moisture. Infected leaves will form raised, yellow blisters that may coalesce and cause the leaves to pucker and curl.
Treatment
Since this disease causes only unsightly damage to leaves, and is usually not serious in Utah, fungicides are not necessary. But on specimen trees, or situations of high infestations, a fungicide application may be warranted.
The timing has passed, and would be recommended for next spring, when the leaves are just starting to emerge from their buds. In this case use a single application of chlorothalonil (Daconil, Bravo, Echo, Ferti-Lome, etc.).
Powdery Mildew of Ornamentals
Treat foliage as necessary
Powdery mildew is showing up on a variety of plants including crabapple, lilac, and rose. There are dozens of different mildew species, and many of them are species-specific. For example, powdery mildew on your lilac will not affect your crabapple tree, and vice versa.
Powdery mildew prevents leaves from photosynthesizing to their fullest ability, resulting in stunted or twisted leaf growth, leaf chlorosis or necrosis, and reduced winter hardiness. In late summer, the fungus produces black fruiting bodies called chasmothecia, which allows the fungus to survive the winter on the trunk or on fallen leaves. Free water (rain) does not spread powdery mildew. Instead, it is humidity, which occurs naturally in the plant canopy.
Treatment
Where powdery mildew occurs every year, focus on a preventive fungicide application before the disease is visible. Apply once now, and repeat two weeks later.
- Residential: Avoid overhead irrigation, improve air circulation between plants, and rake fallen leaves in the fall. Fungicides include horticultural oil (0.5%), potassium bicarbonate (Monterey Bicarb), BioAdvanced Natria, neem oil, Spectracide Immunox, Monterey Fungi-max, or Fertilome F-stop.
- Commercial: There are many fungicides, including Banner Maxx, Bayleton, Heritage, Regalia, Tourney