Plant Health Care Report April 6, 2012
Tagged as: strange winter, degree days, pine bark adlegids, boxwod psyllid, Cedar rusts, Eastern tent caterpillar, European pine sawfly, Gypsy Moth, Juniper webworm, larch case bearer, Lichen, Pestalotiopsis blight, Rhizosphaeria needle cast on spruce, Wooly alder aphid, Zimmerman pine moth
April 6, 2012 Issue 2012.1
Welcome to the first issue of the Plant Health Care Report (PHCR) for 2012. My name is Sharon Yiesla. I am on staff at the Morton Arboretum Plant Clinic and I will be responsible for compiling the newsletter this year. If you have any comments or concerns regarding the Plant Health Care Report please send them to me at
Our report includes up-to-date disease and insect pest reports, as well as color images, for northeastern Illinois. You'll also find a table of accumulated growing degree days throughout Illinois, precipitation, and plant phenology indicators to help predict pest emergence. The report is published bi-weekly on Fridays in April and August, and weekly May-July.
Arboretum staff and volunteers will be scouting for insects and diseases throughout the season. We will also be including information about other pest and disease problems based on samples brought into the Arboretum's Plant Clinic from homeowners and professionals.
What indicator plants are in bloom at the Arboretum?
Wayfaring tree viburnum (Viburnum lantana) - early bloom
Redbud (Cercis canadensis) - late bloom
Accumulated Growing Degree Days (Base 50): 225 (as of April 5)
Accumulated Growing Degree Days (Base 30): 969.5 (as of April 5)
- Strange winter and spring
- Feature article: What are degree days and why do we care about them anyway? By Donna Danielson, M.S.
- Eastern tent caterpillar
- Gypsy moth
- Woolly alder aphid
- Boxwood psyllids
- European pine sawfly
- Larch casebearer
- Juniper webworm
- Pine bark adelgids
- Zimmerman pine moth
Figure 1 Wayfaring Tree Viburnum (Viburnum lantana)
- Cedar rust diseases
- Rhizosphaera needle cast
Oak and Elm Pruning Advisory
With the early season, pruning oaks and elms should have stopped already. Elm bark beetles, which spread the fungus that causes Dutch elm disease, are already active. The beetles are attracted to pruning wounds. Oaks and elms should not be pruned now through November 15.
Degree Days and Weather Information
As of April 5, we are at 225 base-50 growing degree days (GDD), which is 210 GDD 50 ahead
of 2011 at this time and ahead of the historical average (1937-2011) by 145 GDD50. Since
January 1, we have had 4.64 inches of precipitation since January, including 1.36 inches in
|Location||B50 Growing Degree Days
Through April 5, 2012
|Chicago O'Hare**||246.5 (as of 4/4)||.027 (3/28-4/3)|
|Crystal Lake, IL*||227||--|
|The Morton Arboretum||225||4.64|
**Thank you to Mike Brouillard, Northbrook Park District for supplying us with this information.
*We obtain most of our degree day information from the GDD Tracker from Michigan State University web site. For additional locations and daily degree days, go to http://www.gddtracker.net/
The following charts show 2011-12 winter weather and compares it to previous years. This winter was marked by much less precipitation (especially snowfall) and relatively warm temperatures. March was significantly warmer leading to fast accumulation of growing degree days. This has resulted in early flowering of many plants and early emergence of a number of insect pests.
Strange winter and spring
Overall, the last few months have been very interesting in terms of the weather. A mild winter with very little snowfall was great in terms of less shoveling and better winter driving, but less snow cover can lead to winter damage on ground covers, since they are more exposed to dry, cold winter air. Also with less snow to melt, the ground is a little drier than usual, leading to a deficit that may stress plants. Hopefully spring rains will compensate for this deficit. Mild temperatures may have made it easier for insects to survive although the lack of snow cover also means less insulation for insects overwintering in the soil.
The record-breaking heat we experienced in March has greatly accelerated bloom times for many of our plants. Plants like lilac and viburnum that normally bloom in late April and into May started blooming in late March. This accelerated growth may make it difficult to get pesticide sprays done at the right time. For example, spraying crabapples for apple scab starts when the new leaves begin to emerge. Bud break, flowering and leaf emergence not only came early, but over a shorter period of time than usual. This made the timing of sprays more difficult.
Insects are emerging about 4 weeks earlier than normal. Keep this is mind when looking for pests and when considering the use of pesticides. Insecticides for the most part need to be applied when the pest is present and at a vulnerable stage.This will be a very interesting year and one that will certainly keep us on our toes.
What are degree days and why do we care about them anyway?
By Donna Danielson, M.S.
We list the growing degree days we've accumulated at The Arboretum, The Chicago Botanic Garden, and other sites around the state near the beginning of each report. Just what are degree days and why do we care about them?
Accumulated degree days are very important tools used for scouting insect pests. Many living organisms, including plants, insects, and fungi, are dependent on heat energy from their environment to develop. They develop faster as temperatures increase and slower as temperatures decrease. You know from your own experience that plants bloom earlier when we have warm springs compared to cool springs. Insects and many diseases develop earlier when the weather is warmer, too. In fact, many plants and pests have evolved together. So, a tool for measuring this environmental heat can be helpful for determining when to scout for pests. That's why we use accumulated degree days to determine the appearance and growth of insect pests.
Insects don't have calendars, although they probably would like the Far Side calendars created by Gary Larson. Pest outbreaks can be predicted with much more accuracy using growing degree days than the calendar. For example, here at The Morton Arboretum in 1997, we found European pine sawfly larvae hatching around May 12. In 1998 we discovered them hatching on April 16, nearly four weeks earlier. Why? Spring 1997, was much cooler than spring 1998. On May 12, 1997 we were at 159.5 degree days base 50, whereas on April 16, 1998, we were at 165 degree days base 50. Essentially, insects don't care what day or month it is.
There is much less data on predicting the appearance of diseases based on degree days than on the appearance of insect pests, but we intend to collect this information and, over time, identify links between diseases and degree days.
How are degree days calculated? The easiest way to determine daily degree days base 50 is to add the maximum temperature to the minimum temperature for a day, divide by two, and subtract 50. If the resulting number is greater than 0, then that is the number of degree days for that day. Otherwise the number of degree days for that day is zero. For example if the high of the day is 62 and the low is 42, we add 62 to 42 and divide by 2. The result is 52, the average temperature for the day. If we subtract 50 from 52, we end up with 2 degree days. If the result was below 50, we would assign 0 degree days to that day. We add up the total of the daily degree days since January 1, although usually we have very few base 50 degree days until April. That is the number we use to look for many insect pests. 50°F is used as a base because many plants, most insects that feed above the ground, and pathogens begin to grow and develop when the temperature is above 50°F (or 10°C).
Consider microclimates when scouting for insects based on degree days. Some areas may be warmer or cooler than the place where the temperatures are recorded for the degree days. For example, temperatures may be higher on the south side of a light-colored granite building. Insects may hatch earlier in that location than they would in another site. A shady location on the east side of a building may be cooler, so insects may hatch later in that area.
Don Orton's book Coincide is a great reference for use in Northern Illinois to determine what pests to look for at various degree days. In his book, Don also lists indicator plants, which relate pest life cycles to life cycles of common ornamental trees and shrubs. For example, when wild black cherry (Prunus serotina) is blooming, hawthorn leafminers, juniper tip midges, lilac borers, and oystershell scale (brown race) are susceptible to control.
The Plant Clinic has received a number of calls regarding strange growths on tree branches and trunks. These have all turned out to be lichens. Lichens are often flat and scaly and come in many colors (gray, white, blue-green, blue-gray). Lichens are the result of a relationship between a fungus and an alga or cyanobacterium. These organisms are harmless
and no control is needed.
Figure 2 Lichen
Eastern tent caterpillar
Eastern tent caterpillars (Malacosoma americanum) were found on March 28, nearly a month earlier than last year. The caterpillars ultimately grow to two inches long and are hairy with white stripes down their backs and blue spots between longitudinal yellow lines (they are beautiful). The larvae gather at a fork in a tree branch and build a web or "tent". They leave the web to feed during the day, but return at night. Severe defoliation only occurs when populations are high.
Figure 3 Eastern tent caterpillar larva
Eastern tent caterpillars prefer trees in the rose family, such as wild black cherry, apple and crabapple, plum, and peach, but occasionally will feed on ash, birch, willow, maple, oak, and poplar.
Management: The safest way to control the caterpillar is by tearing out or pruning out the webs. This should be done on cloudy or rainy days or at night when the caterpillars are in the nest and not out feeding. Another option is to remove the overwintering egg masses before spring if you can find them (good luck with that – we've tried it and it isn't easy). The egg masses are dark gray to black and are wrapped around twigs that are about the diameter of a pencil. Bacillus thuringiensis var. kurstaki (Btk) can also be sprayed on young larvae but will not kill mature larvae. For chemical control, refer to the Commercial Landscape & Turfgrass Pest Management handbook if you are a commercial applicator or Home, Yard and Garden Pest Guide from the University of Illinois if you are a homeowner.
Gypsy moth (Lymantria dispar) caterpillars are serious defoliators that feed on over 450 species of trees and shrubs. A mature caterpillar can eat one square foot of foliage per day. Their favorite trees are oak, crabapple, birch, linden, willow, and hawthorn. Although deciduous trees that are defoliated can put out a new set of leaves, the trees use a lot of resources to do so. Trees that suffer a lot of defoliation (approximately greater than 50%) several years in a row may die. Severe defoliation also makes trees more susceptible to other problems. Needle- bearing conifers, including spruces and pines, cannot re-foliate and therefore may die after one season of attack.
The first instar (stage of the insect between molts) of gypsy moth caterpillars is black, hairy, and only about 1/4 inch long. Their head is black, shiny, and large compared to the rest of the body. The second instar has a brown stripe down its back. By the third instar, the caterpillar develops orange spots. Mature gypsy moth larvae (fourth, fifth, and, if females, sixth instars) have five pairs of blue spots on one end and six pairs of red spots on the other end. All instars are hairy. By the time they reach their last instar, the caterpillars are two to two and a half inches long.
Each gypsy moth caterpillar eats leaves for about six weeks. They then pupate at the end of June for one to two weeks, emerging as adults in mid-July through mid-August (these stages may show up early due to the warm spring we have been having). The adults mate, lay eggs on the trunk of the tree, and die.
Management: The gypsy moth is attacked by a number of natural predators and pathogens. The insecticidal bacterium, Bacillus thuringiensis var. kurstaki (Btk), can control young larvae but is not as effective against mature larvae. Other natural enemies include an introduced fungus, Entomophaga maimaiga, which builds up in gypsy moth infested areas and has led to major gypsy moth reductions in the East during wet weather.
Knowing some gypsy moth biology is helpful in control. The first three instars remain in the tops of trees, but mature larvae (fourth instar and later) feed at night and crawl down from the tops of trees to hide during the day in protected spots. A homeowner can trap gypsy moth caterpillars by wrapping a layer of burlap around an infested tree trunk with the top folded over. The folded flap captures the caterpillars as they ascend the tree, and they can then be discarded into a container of soapy water. The burlap also traps female moths as they climb trees to lay eggs (females moths don't fly). Barrier bands act similar to burlap. They consist of double-sided sticky tape or a sticky material such as Tanglefoot™. Tanglefoot™ discolors bark when applied directly to it and so should be applied to the surface of material such as duct tape or tar paper that is wrapped around the trunk. Both the burlap and the barrier bands should be removed after August. The bands should not be so tight as to girdle the tree.
Woolly alder aphid
Small masses of woolly alder aphids
(Paraprociphilus tessellates) were found on
Figure 5 Woolly Alder Aphid
Aphids are small, approximately 2 mm (0.1 in) long, and are identified by their sucking mouthparts, long, thin legs, long antennae, pear-shaped body, and pair of tube-like structures (called cornicles) emerging from their abdomen that look somewhat like tailpipes. Two hosts are needed to complete their life cycle: alders and silver maples. The eggs are usually laid in fall in the bark of the maples. When the young hatch in spring, they collect on leaves and reproduce. Their offspring fly to alders and collect on the twigs where new generations develop. They are small and covered with white waxy filaments. In fall, they will fly back to the silver
maples to lay eggs. They do little damage.
Management: Aphids can be dislodged from plants using a strong jet of water from the hose (syringing). Periodic syringing will keep the aphid populations low and allow the parasites and predators to build up to effective control levels.
Figure 6 Psyllid damage on boxwood
Boxwood psyllids (Cacopsylla buxi) began hatching this week on the Arboretum grounds. The psyllids overwinter as tiny orange eggs in the bud scales of the boxwood. As the buds open, the psyllids hatch and begin to feed. The nymphs are about 1/16th of an inch long, yellowish, and partially covered with a white, flocculent secretion that protects them from parasitoids and chemical sprays. Their feeding causes cupping of the leaves. Winged adults normally appear in late May to early June, but this stage may show up earlier this year. We sometimes see ladybird beetles (also known as ladybugs), feeding on the psyllids.
Management: Damage is mostly aesthetic. Shearing boxwoods reduces the population as the insect or the eggs are removed in the process. Chemical insecticides can be applied, but if using a spray, it is important to spray inside the cupped leaves.
European pine sawfly
Fig 7 European pine sawfly
The Illinois Department of Agriculture reports that
European pine sawfly was already out on
March 23. These insects are interesting to watch. Groups of sawfly larvae rear up their heads simultaneously when disturbed, making the group to appear to be one much larger organism. This is a great defense mechanism. When fully grown, the sawflies will be about ¾ - 1 inch long and will have several light and dark green stripes on each side of their bodies. Their heads and the first three pairs of legs are black. Their mouths are so small after hatching that they can only eat one side of each needle, making the chewed-on needles look like straw. Eventually as the insects mature, they are able to eat entire needles. The larvae feed for weeks on old conifer needles but are finished feeding before current year's needles emerge. Then they drop down into the ground to pupate, emerging in September as adults to mate and lay eggs. The eggs look like small gold dots along the needles. In an extremely heavy infestation, trees could be entirely defoliated or stunted. But because new growth is rarely attacked, the trees survive.
Management: Birds feed on the larvae and rodents eat the pupae in the soil, but these predators are usually inadequate to control the larvae. If you can find the needles before the larvae hatch, remove the needles. Larvae can be removed by hand or washed off with a strong stream of water from the garden hose. They have no hooks on their feet like caterpillars do, so they can't hang on very well. Since the larvae are not caterpillars, Bacillus thuringiensis (Bt) does not control them.
Figure 8 Larch casebearer
Larch casebearer (Coleophora laricella) was found by our scouts on March 21 this year. The larvae hollow out needles causing them to first wilt and then bleach to a light off-yellow color. The needles will soon turn reddish-brown and drop prematurely within a few weeks.
The caterpillars of this species are very small and overwinter as larvae within tiny tan-colored cases made of hollowed out needles lined with silk. Larvae emerge and begin feeding in early spring as needle growth begins. They feed for several weeks, pupate on the twigs, and emerge as adult moths in late May and early June. The adults lay eggs on needles, and in a few weeks, eggs hatch (late June and July) and larvae begin to mine inside the needles (remember that this schedule may be accelerated a bit this year because of warm weather). Larvae mine the needles for about two months before making their cases from hollowed-out needles. These cases will be carried around on their backs (like a backpack) for the remainder of their larval period.
Management: Unlike most other conifers, larches can develop a second set of leaves. However, repeated defoliation can weaken trees and make them more susceptible to attack by other insects and pathogens. There are various natural controls, such as weather, predators and parasites, and needle diseases that usually keep populations in check. For severe or repeated infestations, insecticides may be needed.
The overwintering juniper webworm
(Dichomeris marginella) larvae have been Figure 9 Juniper Webworm damage
found feeding this week. Infestation is
evident by brown needles bound together with silk, primarily in the thick, inner foliage of the tree. The larvae are small, light brown caterpillars, with dark reddish-brown stripes and dark brown heads. They reach 3/4 inch in length at maturity. The adult moths will emerge in June (or perhaps earlier this year) and lay eggs on current year's growth. Young larvae feed as needleminers and often build silken tubes around their feeding sites. Juniper webworm prefers Juniperus horizontalis, J. depressa, J. aurea, and many of the Chinese junipers.
Management: Prune out and discard webbed needle masses now and whenever you see them. Pfitzer juniper (Juniperus chinensis var. pfitzeriana) and Savin juniper (J. sabina) are reportedly immune. Insecticides are most effective against young larvae in summer.
Pine bark adelgid
Figure 10: Pine bark adelgid eggs
Adult pine bark adelgids (Pineus strobi) have been found by scouts this week on white pine. Adult females secrete a protective white, woolly mass, which covers the light-yellow eggs and can be found at the bases of needles and on the bark of limbs and trunks. Crawlers should begin to emerge within the next two weeks. The adelgid prefers white pine but also attacks Scots and Austrian pines. Healthy trees are not usually harmed by this adelgid.
Management: Eggs should be washed off now with a high-pressure water spray. Do the same to the crawlers if you see them. In severe or repeated infestations, an insecticidal spray can be applied when the crawlers are out. Lady beetles, hover flies, and lacewings feed on adelgids, so if these predators are present, it is best to use an insecticidal soap or high pressure water spray.
Zimmerman pine moth
Figure 11 Zimmerman pine moth pitch
We are finding white to pinkish pitch masses caused last summer by Zimmerman pine moth (Dioryctria zimmermani) larvae on pine. Larvae damage trees by tunneling just beneath the bark of the trunk and branches. The tunnels can girdle and weaken the trunk or branches, so they are easily broken by wind or snow. Heavily infested trees are often deformed and are sometimes killed. Common hosts include Austrian, Scots, and ponderosa pines.
Larvae overwinter in cocoon-like structures under bark scales. They become active in the spring and tunnel into the bark and sometimes the terminals. In late spring, they migrate to the base of branches, tunneling into the whorl area where pitch masses exude from the wound site. The larvae continue to feed, pupate within the pitch mass, and emerge as adults in August. After mating, female moths lay eggs, often near wounds or previous pitch masses. Eggs hatch in about a week and the larvae feed for only a brief time before preparing to overwinter.
Management: Larvae are very difficult to detect by scouting, so you have to focus on symptoms and phenological indicators. Damaged wood should be pruned out as soon as dieback and pitch masses are seen. Larvae can be controlled by spraying bark and foliage with insecticides when saucer magnolia is in pink bud to early bloom (70 –160 GDD50), or when panicled hydrangea is pink (2700 – 2900 GDD50). We have already missed the first treatment time, so be prepared to treat this summer.
Cedar rust diseases
Figure 12 Cedar apple rust on juniper Figure 13 Cedar quince rust on juniper
All three cedar rusts have been found on common juniper (Juniperus virginiana) within the last ten days. There are three main rusts on juniper: cedar-apple, cedar-hawthorn, and cedar-quince. Cedar-apple rust and cedar-hawthorn rust both form golf ball-shaped galls on junipers, with the cedar-apple rust galls being larger than the cedar-hawthorn galls. During spring rains, gelatinous tendrils called telial horns expand from the galls. This is what is happenign now. Spores are released from the telial horns as they dry and are blown to a host in the rose family, e.g., apples, crabapples, quince and hawthorns. Later in the season, orange leaf spots subsequently develop on the rose family plants. Spores from the large galls, (the cedar-apple rust), create orange spots on the leaves of apples and crabapples. Spores from the smaller galls, (the cedar-hawthorn rust), create orange spots on hawthorn leaves.
Cedar-quince rust is not well-named. Of the three cedar rust diseases, cedar-quince rust can cause the most damage by infecting fruits and twigs on trees in the Rose family, especially hawthorns. Although cedar-quince rust spends part of its life cycle on junipers similar to cedar-apple and cedar-hawthorn rust, it does not form galls on the junipers. Cedar-quince rust appears as spindle-shaped swellings on twigs and branches of junipers. In spring, the swellings turn orange and release spores. We will discuss symptoms on the alternate hosts in a later issue.
Management: The disease is usually not serious on the juniper host. Management is usually based on the hosts in the rose family. The best management is to plant resistant varieties of crabapples and hawthorns. Remember, resistance is not the same thing as immunity. "Resistant" does not mean that the tree will never get rust. It only means that, in an average year, it is not likely to have much problem with the disease. In a year that is very favorable to the fungus, even "resistant" trees may show some signs of disease. When considering the purchase of a new crabapple, check with your local nursery about which rust-resistant (and scab-resistant) cultivars they offer. We will discuss scab in a future issue.
Chemical control for rosaceous hosts, if used, needs to start as leaves are emerging and when the telial horns are expanding on junipers (now).
Rhizosphaera needle cast
Figure 14 Spores of Rhizosphaera
In 2011, there were numerous reports of Rhizosphaera needle cast on blue spruce (Picea pungens) and we should be prepared for more reports of it this year, due to the long time it takes for symptoms to develop. Rhizosphaera needle cast is a disease caused by the fungus Rhizosphaera kalkhoffii.
Rhizosphaera kalkhoffii infects needles on the lower branches first and gradually progresses up the tree. The needles become infected in
May and June, when the new needles are emerging. However, symptoms do not usually appear until late summer to late fall or the following spring. Infected needles initially turn yellow, and small dot-like fruiting bodies (pycnidia) can be seen (with a hand lens) erupting through the stomata. Later, the needles turn purple to brown and begin to drop. Although trees are not usually killed by this pathogen, branches which lose needles for 3 to 4 consecutive years may die. If left unchecked, the disease can turn the tree into an undesirable landscape specimen in two to three years. Colorado blue and Engelmann spruces are highly susceptible to Rhizosphaera needle cast. White spruce is moderately susceptible and Norway spruce is relatively resistant. Hosts in other genera include true firs, Douglas fir, and pines.
Management: Rake and dispose of infected needles to reduce the source of inoculum. Prune off lower branches, provide adequate spacing between trees, and control weeds and unwanted shrubs to improve air movement. Chemical controls are most effective if the disease is detected early. Fungicides should be applied when needles are half-grown (as soon as bud caps fall off) and again when fully elongated. Two years of applications are usually required.
Pestalotiopsis blight was found on juniper and Chamaecyparis in late March. The blight is caused by the fungus Pestalotiopsis sp., which attacks injured or weakened foliage and causes foliage to turn yellow, then dark brown to almost black. The disease starts at the tips of the foliage and progresses toward the leaf base. Damaged foliage is usually near the base of the plant where snow or mulch has accumulated keeping moisture conditions high.
Management: This is a minor disease and can be controlled by pruning out the damage as soon as you see it in the spring. Do not allow snow to accumulate at the base of the plant for an extended period of time. Keep mulch about two inches from plant base.
The Plant Health Care Report is prepared by Sharon Yiesla, M.S., Plant Clinic Assistant and edited by Stephanie Adams, M.S. Research Specialist (Plant Heath Care); Fredric Miller, Ph.D., research entomologist at The Morton Arboretum and professor at Joliet Junior College; Doris Taylor, Plant Information Specialist, and Carol Belshaw, an Arboretum Volunteer. The information presented is believed to be accurate, but the authors provide no guarantee and will not be held liable for consequences of actions taken based on the information.
Thank you...I would like to thank the volunteers that scout for us and who found most of the insects and diseases that are in this report. The Scouting Volunteers include: Mary Carter Beary, Davida Kalina, Ann Klinglele, LeeAnn Cosper, Kathy Stephens, and Loraine Miranda. Your hard work is appreciated.
Indicator plants are chosen because of work done by Donald A. Orton, which is published in the book Coincide, The Orton System of Pest and Disease Management. This book may be purchased through the publisher at: http://www.laborofloveconservatory.com/
The 2010 Commercial Landscape & Turfgrass Pest Management Handbook (CPM), for commercial applicators, and the Home, Yard & Garden Pest Guide (HYG) for homeowners from the University of Illinois, are available by calling (800-345-6087).
This report is available on-line at The Morton Arboretum website at http://www.mortonarb.org/tree-plant-advice.html
Copyright © 2012 The Morton Arboretum
Not printed on recycled paper, or any paper for that matter.
- April 30, 2010 Plant Health Care Report 24%
This issue contains details on many items including: European elm flea weevil, oak apple gall, maple bladder gall, larch casebarer, ash plant bug, boxwood psyllids, honeylocust plant bug, gypsy moth, crown rust of buckthorn, cedar quince rust,...
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This issue contains information on Euonymus scaleEgg massesPestalotiopsisAlternaria blightFlooding To download the PDF of the Plant Health Care Report click here