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Extension > Yard and Garden News > May 2015

Friday, May 22, 2015

Watch out for sawflies on pines and roses

Jeffrey Hahn, Extension Entomologist

There are two kinds of sawflies that are active right now, European pine sawfly and roseslug.  European pine sawfly loves pine, especially mugo, red, and Austrian pines.  It has a black head with dark grayish green and light grayish green stripes.  They grow as large as one inch long before they are done feeding.  They chew the old needles of trees and shrubs.  While in many cases, defoliation is not severe, they do have the potential to do extensive damage.
European  pine sawflies. Photo: Jeffrey Hahn, UMNExt

Roseslug is a slug-like translucent greenish larva that grows no more than 1/2 inch long.  They windowpane feed the leaves of roses, i.e. they feed on one layer of leaf tissue between the veins.  Damaged areas are opaque at first but eventually turn brown.  Damage can range from minor to severe.
Roseslug sawflies. Photo: Jeffrey Hahn, UMNExt

Both of these sawflies only feed in the spring and are done by June.  If you find these insects on your plants, the first consideration is how large are they.  The closer they are to full grown size, then the closer they are to finishing their feeding.  When they are at that stage, treating does little to protect the plants.  If you find them at this stage, your best bet is to just ignore them for this year.  If they are a size where they are worth treating, usually about 1/2 full grown size or less, there are several options.  You can crush them (wearing vinyl gloves of course).  If you are interested in a low impact insecticide, try insecticidal soap or spinosad.  You can also use a residual product, such as permethrin or lambda cyhalothrin.

For more information, see Sawflies of trees and shrubs.

Thursday, May 21, 2015

Wasp Watchers: Biosurveillance of EAB

Jennifer Schultz, Wasp Watchers Program Coordinator

The University of Minnesota Extension has started a new program called Wasp Watchers that engages citizen scientist volunteers to help detect Emerald Ash Borer infestations throughout Minnesota. With the help of a harmless, native wasp, Cerceris fumipennis, volunteers can conduct biosurveillance for the Emerald Ash (EAB) and help with early detection of this invasive beetle that has destroyed tens of millions of ash trees in over 20 states.

The smoky winged beetle bandit, Cerceris fumipennis is a native, solitary, ground-nesting wasp that specifically hunts beetles like EAB. This harmless wasp is not known to sting people, even when handled. Females forage for beetles in trees and bring them back to underground nests as food for their offspring. This wasp has been used in over a dozen states as a biosurveillance tool for EAB detection. Biosurveillance uses one species to help detect the presence of another species. In this case, the beetle bandit wasp helps us locate areas where EAB is found. Early detection is critical for the management of this invasive beetle.
This small wasp preys on EAB and related beetles. By monitoring
their nests and watching what beetles they capture can help
determine whether EAB is in the area. Photo: Jeff Hahn, UMNExt

The process is simple: we search for beetle bandit nesting sites and monitor the type of beetle prey the wasp are capturing. Conveniently, these wasps prefer nesting in sandy, compact soil often associated with human disturbance. We’ve found that one of their favored habitats is along the infield edges of baseball or softball fields—especially fields that are not used frequently with some encroaching vegetation. We need volunteers to search their neighborhood ball fields to find Cerceris fumipennis sites. Once these sites are located, volunteers can help monitor the wood boring beetles that the wasps capture.
Infrequently used ball fields are a great place to look for
Cerceris fumipennis nests.  Photo: Jeff Hahn, UMNExt

An observer can easily watch a wasp colony (remember, they don’t sting people) and “steal” beetles from the returning wasp by simply using an aerial net to capture the wasp carrying its prey. When startled, the wasp drops its beetle prey and the beetle can be retrieved and collected by volunteers. If EAB is present, the wasp will find it for us!

Help us find more Cerceris wasp nesting sites. Currently we have around 25 confirmed sites, but we need many more. Each known nesting site is another location where we can monitor for the presence of EAB. So the more sites the better! Volunteers search your community’s ball fields for new Cerceris fumipennis sites. This takes very little time—approximately 15 minutes per ball field. Volunteers walk the border of the infield/outfield; where the grass meets the sand searching for nesting holes. After scouting out the ball fields, volunteers record their findings online on the Wasp Watchers website.
Two Cerceris fumipennis nests.  Photo: Jennifer Schultz, UMNExt

Once Cerceris nesting sites have been confirmed in your community, we need volunteers to adopt a site. They will then monitor their adopted site by visiting it 4-6 times in July and early August for 1-3 hour intervals. While at the Cerceris site, volunteers will intercept the foraging wasp to capture her beetle prey. Dropped or abandoned beetles are often found near nesting holes, too. These wood boring beetles (buprestids) will be collected and will be sent to the University of Minnesota for identification. Data on the beetle collection will be entered into our online database as well. However, if you suspect you have found Emerald Ash Borer, you should contact us immediately.

Wasp Watchers
“Whack a Wasp” video

Jennifer Schultz, Wasp Watchers Program Coordinator

Monday, May 18, 2015

High Risk for Oak Wilt

M. Grabowski, UMN Extension

Minnesota is now in the high risk period for new oak wilt infections. This means that the fungus that causes oak wilt, Ceratocystis fagacearum, and the beetles that transmit the pathogen are active. The good news is that oak trees have a natural defense mechanism that is very effective in stopping beetle transmission of the pathogen - bark!

Beetles in the Nitidulidae family are attracted to the sweet fermented smell produced by spore mats of the oak wilt fungus. As they crawl over the spore mat, spores cling to their bodies. Nitidulidae beetles are sap feeding beetles. They are attracted to the sap coming from wounds or pruning cuts on oak trees. As they feed from these fresh wounds, spores from the oak wilt fungus are introduced to the tree's vascular system and disease begins.

Red oak leaves with symptoms of oak wilt. M. Grabowski
A few simple steps will help to protect oaks from beetle transmission of oak wilt. Do not prune oaks during the months of April, May or June. The best time to prune oaks in Minnesota is November through March when neither the fungus nor the beetle are active. Avoid wounding oak trees during the high risk period. Mulch around the base of oak trees to help prevent wounds from weed whips or lawn mowers. Do not allow cars or other heavy equipment to come close to oak trees. Avoid doing construction that might damage the oak trees during the high risk period. Strategies to protect trees during construction can be found at the UMN Extension Garden webpage. Avoid removing oak trees during the high risk period. If an oak must be removed, contact an arborist to cut the root grafts between the tree and neighboring oaks, as oak wilt can move through root grafts from one tree to another. If an oak tree must be pruned during the high risk period, shellac or water based paint should be applied to the wound  within 15 minutes of making the cut as the beetles arrive quickly.

Oak wilt infects all species of oaks grown in Minnesota. Leaves on infected trees brown around the edges,wilt and fall off prematurely. Red oaks can be killed in several weeks, where as bur and white oaks may take several years to die. The oak wilt pathogen is not present in every county in Minnesota. To prevent introducing the pathogen to new areas, never move firewood or lumber with bark intact out of an oak wilt infected area.

If you suspect your oak may be infected with oak wilt, contact an arborist or send a sample to the UMN Plant Diagnostic clinic.

Friday, May 15, 2015

Asparagus beetles are active now

Jeffrey Hahn, Extension Entomologist

Asparagus beetles have been sighted in gardens recently.  They can damage asparagus by feeding on the spears and causing browning and scarring.  It can also cause spears to bend over into a shepherd's crook.  If you are growing asparagus, scout your garden now for their presence.  See the Minnesota Extension publication Asparagus beetle in home gardens for information on recognizing asparagus beetles and their best management.
Asparagus beetle are active now, check your garden to see
if  they are present.  Photo: Jeff Hahn, UMNExtension

Tuesday, May 5, 2015

Plant Seeds When the Time is Right

M. Grabowski, UMN Extension

Warm sunny days inspire gardeners to work outdoors. Although everyone is anxious for fresh vegetables from the garden, it is important to be patient and wait for the correct time to plant. Spring soils can be cold and wet. Some vegetables like peas and many leafy greens easily tolerate these conditions and can be planted in the garden as soon as the soil thaws and can be worked.

Cucumber seedlings killed by damping off. M. Grabowski, UMN
Members of the cucurbit family, including cucumber, zucchini, summer squash, melon, winter squash, and pumpkin need warmer soils to grow. These plants originate from tropical climates. Other tropical plants like tomato, eggplant and pepper are often started indoors and transplanted to the garden when weather conditions are right. Unfortunately, although cucurbits can be transplanted with care, they have delicate root systems and often do better when direct seeded into the garden. Winter squash and pumpkin seeds can be planted when soils reach 65 F at a 2 inch depth. Zucchini, summer squash, and cucumber need soils that have warmed to 70 F at 2 inch depth. Melons prefer to wait until soils are between 70 and 90 F. Soil thermometers are available at many garden centers.

If planted too early, seeds will not grow or will grow very slowly. Sitting in cold wet soil, seeds are very susceptible to several root rotting pathogens that cause a disease known as damping off. Damping off is rarely a problem for vigorously growing seedlings in warm soil that is moist but not soggy. Under cool wet conditions, however, damping off can kill every seedling.

To learn more about how to grow vegetables in Minnesota, visit the UMN Extension Garden web page for vegetables in the home garden.

Check Spruce Trees for Rhizosphaera Needle Cast

Spruce buds opening. M. Grabowski, UMN Extension
M. Grabowski, UMN Extension

Spring is here and spruce trees are opening their buds and developing young new needles. Now is a critical time to inspect spruce trees for Rhizosphaera needle cast, a common disease of spruce trees in Minnesota.  Rhizosphaera needle cast is caused by the fungus Rhizosphaera kalkhoffii. This fungus infects the young growing needles of Colorado blue spruce, white spruce and Norway spruce. New infections do not have obvious symptoms and many gardeners do not recognize the disease until it is too late to prevent new infections this year.

What to look for
Discolored needles with black spore producing structures. USDAFS
Examine the fully grown needles at the branch tips. These needles were new needles last year and if infected, will be developing symptoms now. Look for discoloration of any kind. Needles should be dark green to blueish green. Needles that look pale yellow green, brown or purplish brown are unhealthy. Use a hand lens to examine the needles. Tiny raised black dots on the needles are suspicious and may be fungal spore producing structures. Spore producing structures may be found on green needles or discolored needles. If needle discoloration or potential spore producing structures are observed, contact an arborist or send a sample to the UMN Plant Diagnostic Clinic to determine if Rhizosphaera is the cause of the problem.

Help for sick trees
Spruce Needles at half their mature length. M. Grabowski
If a spruce tree is infected with Rhizosphaera Needle Cast it can be helped, but gardeners will need to act quickly to protect this year's needles. The fungal pathogen overwinters on previously infected needles. Spores are spread from last year's infected needles to new developing needles by splashing rain or irrigation. A fungicide with the active ingredient Chlorothalonil can be applied to protect needles but timing is critical. The first spray should be applied when new needles are half the length of mature needles. A second application should be made 3 to 4 weeks later. This will protect the needles from infection. Fungicide sprays should be repeated next year, but beyond that trees will just need to be inspected to determine if sprays are needed. Many trees recover from Rhizosphaera needle cast after 2 years of treatment.

If left untreated, infected needles will die and fall off after 1 year. If a branch looses its needles 3-4 years in a row, the branch dies. Untreated trees often have dead branches on the lower part of the tree and infected branches higher up.

Preventing future problems
Needle loss from Rhizosphaera needle cast, M. Grabowski UMN
Gardeners can help keep spruce trees healthy with several simple practices. Rhizosphaera needle cast is most problematic on stressed trees, so reducing stress will reduce disease problems. Remove all weeds and turf grass from around the base of the tree. Add a level 2-4 inch layer of woodchip mulch around the base of the tree to keep moisture in the root zone, reduce competition with weeds, and prevent accidental damage from lawn mowers and weed whips. Water the tree during periods of drought with a soaker hose or garden hose directed at the base of the tree. Redirect lawn sprinklers to keep needles dry. Spores of Rhizosphaera need splashing water and moisture to start new infections.

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