September 9, 2005

In this issue

Landscape and nursery

§ Honeylocust trees turning brown from mimosa webworm injury

§ Homeowners can still apply an imidacloprid drench this fall for emerald ash borer

§ Thrips injury to lilac

§ Irrigation do’s and don’ts

§ Conserving soil moisture with landscape mulch

§ Consider herbicide resistance screening

Turfgrass

§ Watch for grub damage in drought-stressed turf

§ Turf remains idle

Christmas trees and forestry

§ Spider mites might increase again

§ Douglas fir needle midge in Michigan

Honeylocust trees turning brown from mimosa webworm injury

Dave Smitley, Entomology

Throughout southern Michigan, especially the counties south of a line from Grand Rapids to Flint, mimosa webworm has built-up to damaging levels. Mimosa webworm does not usually survive our winters very well, but mild winters the last three years have allowed it to expand further north than usual. The small caterpillars web together leaves, stripping the green tissue and leaving brown lace. Heavily infested trees may turn entirely brown, appearing as if they suddenly died from drought. But even trees that are entirely brown should put out a full flush of new growth next spring.

Trees rarely die from a single defoliation, especially late in the summer. However, 100 percent defoliation two years in a row can be crippling or even deadly. The webworms go through two generations per year, with the caterpillars feeding and damaging honeylocust trees in June and July, then again in late August and early September. Insecticides applied now will not help much, since the trees will soon be dropping leaves anyways. Be prepared to apply a Bacillus thuringiensis product, dimilin or Conserve next year, when the injury first begins in late June.

Homeowners can still apply an imidacloprid drench this fall for emerald ash borer

Dave Smitley, Entomology

Research tests have shown that the imidacloprid drench (Tree and Shrub Insect Control) works well on small (less than 12 inch dbh) ash trees, but may not be enough to protect larger trees, especially in the first year or two of treatments. This means that people should start treating their ash trees well before emerald ash borer starts killing trees in their neighborhood.

Anybody living in the 20 infested counties in southeast Michigan can start treating now, if their ash trees still appear to be healthy. The product label allows drench treatments once per year. Drenching in the fall (September or October) will help protect ash trees next year. The trees can be drenched this fall and again next spring, and then once per year from then on. Another product that has worked very well for protecting ash trees against the borer is ACECAP Systemic Implants. The implants can be inserted into tree trunks next spring in early May. If the emerald ash borer is active in your area, you may want to use both the drench and the implants for the first year or two.

Homeowners may also choose to hire an arborist to treat their trees for them, or decide to let the trees die and replace them.

Thrips injury to lilac

Dave Smitley, Entomology

Last week Duke Elsner reported some unusual symptoms on lilac trees in the Traverse City area like looked like damage from eriophyid mites. He returned to the scene of the crime with his camera this week and took some good photographs. The culprit is still a small one, but as small as eriophyid mites. The damage was caused by thrips, which are visible with a good hand lens. If you see lilacs with these symptoms, check them for thrips. There is no need to apply an insecticide this late in the season, but it may be worthwhile to watch for them next year.

Irrigation do’s and don’ts

Bert Cregg
Horticulture and Forestry

For most of Michigan water needs in the landscape are reaching critical levels. Although showers are in the extended forecast, it’ll take a few good day-long soakers to significantly affect soil moisture below the surface of the soil. While many homeowners are diligent to keep their lawns watered and green, trees and shrubs often get ignored. Reducing stress to trees and shrubs this time of year is critical. As we go into the fall trees are shifting internal resources and undergoing physiological changes that will enable them to withstand the rigors of the winter to come. If plants are subjected to severe stresses now, they will be more predisposed to various winter injuries. With this in mind, irrigating trees and shrubs should be on your list of landscape chores. Here are a few “Do’s and Don’ts” to bear in mind as you irrigate your landscape.

Do

Give good, long soakings rather than frequent light waterings. A typical rule of thumb is to provide at least 1 inch of irrigation per week. How many gallons of water this translates into depends on the size of the tree. If we measure the width of crown spread of a tree we can calculate the area under the drip line. We can then figure the volume of water needed to cover this area with 1 inch of water. I’ve done this in the table below and converted the volume to gallons. I’ve also calculated the length of time it would take to apply 1 inch of water assuming a typical garden hose flow rate of 5 to 6.5 gallons per minute.

Increase the irrigation of amount as temperature soar. The 1 inch per week is a good rough guide but peak evaporative demand can approach 2 inches per week in Michigan during extremely hot summer weather.

Apply mulch properly. Mulching is the best way to conserve precious soil moisture in the landscape (See related article in this Landscape Alert.).

Use irrigation bags on newly established trees. Gator bags are designed to provide about 15 gallons of water over several hours, providing an easy way to ensure a slow steady watering.

Gallons of water needed to provide 1 inch of irrigation under the dripline of trees of various sizes

Tree crown spread (ft)	Gallons	Minutes of watering*
6	20	3 - 4
8	30	5 - 6
10	50	8 - 10
12	70	11 - 14
16	125	20 - 25
20	200	30 - 40

*Assuming 5 to 6.5 gallons per minute from typical garden hose

Don’t

Allow water to run-off. Water that runs off is wasted water. If you’re watering by hand and notice water running off move from tree to tree to allow water to soak in before resuming watering.

Ignore signs of drought stress in landscape plants. Wilting leaves, leaf scorch, dropping leaves and drooping leaders in conifers are your tree’s way of saying, “What’s a guy gotta do to get a drink around here?!”

Water during hot midday periods to reduce water loss to evaporation. Some experts argue against watering late in the evening due to possible disease problems associated with wet foliage. Morning is the best time to water – unless you have to be at work.

Conserving soil moisture with landscape mulch

Bert Cregg
Horticulture and Forestry

As we’ve noted elsewhere in this Landscape Alert, drought conditions continue to be severe in much of Michigan. Perhaps the single best way to conserve soil moisture and reduce moisture stress in landscape trees and shrubs is by proper mulching. Mulch provides several important benefits to trees and shrubs. Mulch conserves soil moisture by reducing evaporation from the soil surface. Mulch aids in reducing weeds, saving additional soil moisture. If you’re using an organic mulch material, the mulch will breakdown and add organic matter to the soil, improving the soil’s ability retain nutrients. Mulch around trees in lawns will also prevent lawn mowers from running into them and keep the weed-eaters away from the trunk.

A common question we receive is “Which mulch should I use?” Over the past two years we’ve conducted a trial looking the effectiveness of various mulch types in conserving soil moisture and the response of various landscape shrubs to the mulches. In the summer of 2004 we established a series of 12’ x 12’ landscape plots at the MSU Horticulture farm. Within each plot we planted 10 common landscape shrubs including spirea, viburnum, hydrangea, weigela, arborvitae and yews. Each plot then received one of six ground surface treatments:

1) No mulch, no weed control

2) No mulch, complete weed control

3) Three inches pine bark mulch

4) Three inches cypress mulch

5) Three inches hardwood mulch

6) Three inches dyed ground recycled pallets

What have we learned?

All of the mulches were effective in conserving soil moisture (Figure 1). Weekly measurements of soil moisture at a 6-inch depth show that soil moisture on the no mulch plots is closely tied to weekly rainfall. Applying any of the mulches greatly increased soil moisture throughout the summer. Periodic surveys of weed cover by Dan Little, MSU Department of Horticulture, indicated that the mulches were effective in controlling most common weeds and there was little difference among mulch types in weed control. All of the mulches, except the cypress mulch, increased growth of the shrubs in the plots compared to the no mulch treatments. (Figure 2). Why the plants grew more slowly in the cypress mulch is not known at this point. Periodic measurements of photosynthesis showed a similar decline in plants in the cypress mulch plots. Cypress wood is noted for its resistance to decay, which is associated with the presence of phenlolic compounds in the wood. It may be that some of these compounds are leached from the mulch. We are continuing to monitor other responses of the plants and soil to the various mulches.

In the final analysis, most decisions on mulch come down to aesthetics and personal taste. Most mulches will benefit landscape trees and shrubs, especially during the hot, dry summer months. Regardless on the mulch material you select, proper application is essential. Three to 4 inches of much is adequate. Make sure that the mulch is note built up in a cone around the base of the tree to avoid the ‘Mount Vesuvius’ effect. Also, avoid using grass clippings or uncomposted leaves that can form a thick mat and ultimately prevent moisture form reaching the soil.

Consider herbicide resistance screening

Rob Richardson
Horticulture

Editor’s note: Because this information is valuable to growers at this time, this article is being reprinted from the July 30, 2004 Landscape Alert. MSU now has a GREEEN grant to evaluate herbicide resistance in Christmas tree fields. If you are a Christmas tree grower and suspect herbicide resistance, please contact Jill O'Donnell at 231-779-9480.

Weeds can develop resistance to herbicides after repeated exposure to the same herbicide mode of action. This has happened in many instances including common lambsquarters resistance to triazine herbicides (atrazine and simazine), marestail resistance to glyphosate, and resistance in pigweed species to ALS inhibiting herbicides. Knowing whether or not resistance is present is very important for planning future weed management programs.

In a recent screen of a Christmas tree field, triazine-resistant marestail and ALS-resistant common ragweed were both present. The presence of two weed species resistant to two modes of action will severely restrict the weed control programs that may be used. This is also cause for concern as simazine has historically been the most commonly used preemergence herbicide and ALS-inhibiting herbicides are not labeled for Christmas tree applications. Other instances of resistant weeds in Michigan Christmas trees and nurseries need to be documented so that sustainable weed management programs can be developed.

Possible signs of resistance

There are some warning signs that may signify when resistance is present. With postemergence applications, look for areas where some plants die, while adjacent plants of the same species survive. In other situations, look for surviving weeds of a species that should be controlled by the herbicide used. Also take note if the same herbicide has been applied to the same field for several years or if the weed species of concern has already developed resistance in other areas. Certain weeds are more likely to develop resistance than other weeds due to factors including population numbers, genetic plasticity, and prolific seed production.

Resistant weeds of note

Some important herbicide resistant weeds that have been documented in Michigan or other parts of the U.S. include:

§ Marestail: triazines, glyphosate (Roundup, Touchdown, others), ALS inhibitors (Manage, others), other modes of action

§ Common ragweed: triazines, ALS inhibitors

§ Common lambsquarters: triazines, ALS inhibitors

§ Common purslane: triazines

§ Pigweed or Amaranth species: triazines, ALS inhibitors

§ Goosegrass: Seedling root inhibitors (Surflan, Treflan, Pendulum, Barracade, etc.)

§ Large crabgrass: ACCase inhibitors (Fusilade, Envoy, etc.)

§ Wild carrot: Synthetic auxins (2,4-D, etc.)

Watch for grub damage in drought-stressed turf

Dave Smitley
Entomology

The unusually hot summer and recent drought is a perfect recipe for grub damage in turf. The grubs have developed faster than usual this summer, meaning they are already large enough to cause to turf injury. With the drought conditions, turf will be unable to produce new roots to make up for the roots consumed by grubs. Also, if most of the roots are consumed, turf will die if there is no soil moisture. Think of grub-infested turf as “new sod.” It just won't survive unless the soil is kept moist.

Grub-infested turf should be treated with Dylox or Sevin and watered frequently to keep it moist. More than five grubs per square foot can cause injury to drought-stressed turf. The best way to check for grubs is to cut a square of turf that is 6 inches on each side. Pull up the square of turf and roots to a depth of 2 inches and sift through the soil roots looking for C-shaped white grubs. This sample is 1/4 of a square foot, so if you find more than one grub per sample, it is enough grubs to injure drought-stressed turf. However, if you water frequently (about 1inch of water per week) and keep the soil moist, turf can tolerate 15 grubs per square foot without any visible damage.

Turf remains idle

Kevin Frank
Crop & Soil Sciences

While some areas of Michigan, particularly northern Michigan, have had some timely rains in the last week, most of the Lower Peninsula is bone dry. Normally this is the time of year that the turf recovers from a tough summer, but at least at this point in time that’s not the case.

Unless you are irrigating the turf, there’s really not a whole lot to do right now except hope that you don’t have grubs decimating whatever turf you still have left. The turf is in a resting state until rains return. Continue to expect rust to be a problem with the turf dry and dormant. Fertilizer applications won’t provide much benefit right now unless the turf is irrigated. Consider applying the fall fertilizer application a little later in autumn when hopefully we have some moisture.

So what’s a turf nut to do right now? I would suggest that you take the opportunity in this management lull to sharpen the blades on your mower. Mower blades are likely to be a little dull after the long season of mowing and now would be the time to sharpen them up before you start mulching those leaves that will begin falling soon. Another task for those planning on killing some weeds with an October broadleaf herbicide application would be to head to the store soon before the products are displaced by Christmas lights.

Spider mites might increase again

Jill O’Donnell
Christmas Tree ICM Agent

We will sometimes see mite populations build again this time of the year. Pay particular attention to the trees you plan to harvest this year. Check for mite activity by closely inspecting the older needles near the tree stem or by sharply rapping shoots over white cardboard or a notebook with white paper. Immature mites will be pinkish in color while adults are dark green or brownish.

Douglas fir needle midge in Michigan

Jill O’Donnell
Christmas Tree ICM Agent

In 2003 we found Douglas fir needle midge in two locations in Michigan. This fall I have received calls on two more plantations with severe midge damage. The top third of the trees were yellow and unsalable. The damaged area is initially pale yellow bands, resembling Rhabdocline needlecast symptoms but as the season progresses, will darken and eventually turn brown. In late summer, larvae emerge from the undersides of the needle and drop to the soil.

For further information and pictures http://ctrees.cas.psu.edu/pest_alerts/0408.htm