A Great Summer for Bugs

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August 29, 2001

Sphinx moths

Have you been lucky enough to see the large insects that act and look like hummingbirds? I haven't seen so many since I was a kid. I also didn't have the heart to catch and kill one to only determine the species, so I'll give you some general facts about sphinx moths.

The Big Poplar Sphinx: 3.5 to 5.5 inches wide
  • The "sphinx" moniker is derived from the posture that the caterpillar takes when it is disturbed.
  • Many of the sphinx moths have about the same body size as hummingbirds. They hover near flowers, beating their wings so fast that you can not see them and use their long proboscis to reach the nectaries inside the flowers.
  • In many species, the proboscis is as long as the moth body.
  • Most moths are observed feeding during twilight.
  • They are some of the fastest fliers of the Lepidoptera.
  • Caterpillars are called "hornworms" after the fleshy protuberance on the 8th abdominal segment.
  • Hornworms are huge by insect standards, most growing to five inches in length and 1/2 to 3/4 of an inch in diameter.
  • Most hornworms feed on agricultural crops, such as, tomatoes and tobacco.
  • A few species feed on trees, but they rarely cause much problem.

Hornworm on leaf

Here's a sample of species and their hosts:

Hornworms that consume leaves of woody plants
Sphinx chersis ash, lilac
S. poecila many trees, including conifers
S. drupiferarum cherry, plum, apple
Lapara bobbycoides pines
Smerinthus jamaicensis willow, poplar
Cressonia juglandis walnut
Pacysphinx modesta poplar
Hemaris thysbe many trees and honeysuckle
Sphecodina abbotti grape and Virginia creeper

Larch casebearer

During the last two years, the larch casebearer, an exotic needle miner, has caused extensive yellowing of larch needles in northeastern and north central Minnesota. This year it has spread to several counties in northwestern Minnesota. Fortunately, there are two introduced parasites that help control the larch casebearer and other bio-controls including over twenty-five native parasites and predation on eggs or larvae by beetles, true bugs, lacewings, mites, and daddy long-legs. This forest pest was first found on this continent in the l880's, and it now occurs across much of Canada and the adjacent United States. The two imported wasp parasites, Agathis pumila and Chyrsocharis laricinellae, were first introduced in the l930's in Canada and Michigan.

In a collection of larch from the Aitkin Area, we found 90% of the parasites were Agathis. In a June collection of larch from Beltrami County, the parasites were mostly Chrysocharis.. During past surveys these two imported parasites were sometimes prominent, while in other surveys the native species were more prevalent.

This may be the peak year for damage to larch in northeastern and north central Minnesota by the casebearer because bio-controls tend to build rapidly. Since widespread mining of larch needles was first observed in northwestern counties this year, it will probably occur there again in 2002.

Yellow-headed spruce sawfly

Scattered pockets of white spruce up to fifteen feet in height were heavily defoliated by the yellow-headed spruce sawfly larvae by mid-July. YHSS caused significant damage this year in some young spruce plantations in the northern two thirds of the state. Old and new needles were consumed so extensively that some spruces had a brown appearance from the bare twigs and branches. YHSS have yellowish brown heads and olive green bodies with paired longitudinally grayish green dorsal and subdorsal lines. In late June through July, they drop do the ground, spin overwintering cocoons which become encrusted with soil. In late-May to early-June, adult saw flies emerge from their cocoons to mate and lay eggs.

If the larvae are not chemically controlled, successive years of needle defoliation can prevent bud formation and result in branch dieback and seedling mortality. Labeled insecticides should be applied in early- or mid-June when the young sawfly larvae are observed on the needles and before any have dropped to the ground to pupate.

Jack pine budworm

Jack pine budworm egg mass surveys are still being carried out it Region 1 and 3. While most of the Jack pine stands sampled so far indicate the population continues to be almost nil for 2002, there were two plots in Eckles township in Beltrami County that did have a few egg masses. So it's possible that we may have the beginnings of a defoliating population in that county.

Spruce budworm

A "new" spruce budworm infestation of balsam fir has been observed in Itasca State Park near Elk Lake. Although most of the defoliation occurred in 2001, inspection of some branches indicate that minor defoliation may have taken place last year. A casual survey on August 13th produced a significant number of egg masses. Formal egg mass surveys will be carried out later and more information will be available for the final newsletter of the season.

Introduced pine sawfly

Diprion similis

Introduced pine sawfly larvae, Diprion similis, were feeding on Scotch pine near Grand Rapids. As the name implies, this is an introduced pest. There are two to three generations per year. The larvae present in August and September are from the second generation. The larvae eat the old and new needles of pine trees. Eastern white pine is the preferred host but they will feed on Scotch, jack and red pine as well. Heavily infested trees can be completely defoliated. Presence of the larvae on lightly infested trees is most easily detected by having a picnic under the trees. The green pellets of frass on your plate and floating in your glass of milk are much easier to see than the larvae in the crown of the tree.

Bark beetles go for it (and get it)

If you live anywhere on the Anoka Sand Plain (Wright, Anoka, Sherburne and Isanti Counties) , you witnessed the red pines change color over winter and, in some cases, become worse this spring and summer. Normally, winter injury symptoms disappear as the new, green shoots and needles grow. Some of the red pines have followed the typical winter-injury scenario and are fine, others are struggling, and a number of pines have already died. But not from winter injury.

2001 is the beginning of the fourth year of drought on portions of the Anoka Sand Plain, as measured by subsoil moisture. In some Sherburne County locations, soil moisture and water table levels have met or are lower than those during the drought in 1988. Some lake levels have dropped six feet below their normal levels. As a result, tree root systems were left high and dry and this puts all trees under stress. For the last two years, red pines have been indicating their level of drought stress in the form of winter injury. You can see it in pine plantations, windbreaks, roadside plantings and backyard trees. Even the small understory pines in pine plantations died from a lack of soil moisture.

With the deepening drought stress, red pines became vulnerable to insects and diseases which took advantage of their weakened state, causing the foliage and branches to die. In early June, a few pines with discolored foliage were felled in three red pine plantations in the Sand Dunes State Forest. Pine bark beetles (primarily Ips pini); Diplodia shoot blight (Sphaeropsis sapinea); and red turpentine beetles (Dendroctonus valens), were attacking and killing the trees. Apparently, a recent hail storm had induced the Diplodia epidemic in the shoot tips and, in some cases, Diplodia was causing branch mortality.

An aerial survey the week of June 23rd found nearly fifty red pine plantations with discolored, dead and dying tree crowns in Sherburne and Isanti Counties. Most of the affected trees are located in plantation interiors, indicating a serious drought stress and likely bark beetle buildup. Pines in backyards, along roadsides and in windbreaks were also affected.

Three stands in the Sand Dunes S.F. were targeted for bark beetle management with trap trees. In mid-June, the bark beetles were active in the upper boles, 50 to 60 feet above the ground. Trap trees are usually felled in April and are most attractive to beetles that are emerging from their overwintering sites in the soil. This was late June and the bark beetles were already in the upper crowns. So, instead of dropping the trap trees to the ground, trap trees were girdled and left standing. Girdling was accomplished by hand with a chain saw blade at approximately four feet above the ground. The girdled trees were arrayed in rows because the infestation was general throughout the stand and because logging and cleanup would be easier. A logger was contracted prior to any girdling work, but this delayed the girdling by two weeks. In one stand, the girdling technique was compared to three other treatments; girdled and felled, girdled and baited with bark beetle pheromone and not girdled ( totaling fifty trees). Trees and bark beetle populations were monitored in early, mid and late July.

On July 25th and 26th, the experimental trees were felled so that treatments could be compared and so that the logger could come in and remove all the girdled trees before the bark beetles emerged and moved to nearby trees. On each of the 50 experimental trees, crown discoloration was estimated, bark was removed to measure the extent and condition of the bark beetle infestation, the presence of Diplodia blight, Dendroctonus, Armillaria or other agent was noted and the stump was inspected for evidence of active sap flow.

Girdled experimental trees

The findings:

  1. Unfortunately, most 'girdled only' trees were not colonized by bark beetles. Why not? We suspect that girdling trees on the 23rd of June was already too late for the generation emerging in June and that the girdled trees were not any more "stressed" than surrounding trees. So the bark beetle infestation remained a general infestation rather than being trapped out. The only girdled trees that were colonized by bark beetles were the ones with greater than 70% foliage discoloration and even then, they may have been infested earlier in June.
  2. 'Felled and girdled' trees attracted lots of bark beetles and still had active infestations in them in late July. So, even during the growing season, trap trees should always be felled, not left standing. On the ground they dry out quicker, become attractive and colonized much more quickly than standing girdled trees.
  3. The most surprising result is that 'pheromone baited and girdled' trees were not colonized by any bark beetles. This group of trees had little or no crown discoloration and all had abundant sap flow. Perhaps beetles were attracted to the trees but were unable to successfully colonize the trees because of the profusion of sap flow.
  4. Twenty standing trees received no treatment and were chosen to be the experimental controls. These trees were not attractive to bark beetles unless they had more that 70% crown discoloration, then they were completely colonized by the end of July.
  5. For all treatments, bark beetles were collected from the base, middle and top of the stem for identification and cataloging by Dr. Seybold, Entomology Dept, University of Minnesota.

From this data, rules were developed to aid the forester in choosing which trees to remove from the stand in addition to all the girdled trees.

  1. If crown discoloration was 0 - 40%, bark beetles did not colonize these trees. Do not cut.

  2. If crown discoloration was 80 - 100%, bark beetle colonization was 100%. Cut these trees.

  3. If crown discoloration was 41 - 79%, bark beetle colonization was variable.

    If two or more entire branches or the top of the tree was dead, then bark beetle colonization was 100%. Cut these trees. All others with less branch mortality should not be cut because bark beetle colonization was not predictable based on external symptoms.

  4. Trees with pre-existing problems at their bases had 50 to 100% bark beetle colonization. Cut these trees.

Problems included Dendroctonus infestation at base (pitch tubes), canker in lower bole, more than 40% of tree's circumference dead, or freshly wounded by machinery.

The most predictive symptom of bark beetle colonization is lack of sap flow on the cut stump. If the stump was dry, there was always 100% bark beetle colonization. If the stump was at least 50 - 60% wet, there was no bark beetle infestation anywhere on the bole.

Using the above rules, a salvage sale was set up during the first week in August and removed 270 cords in the three stands. The logger finished by the end of the first week in August. The stands will be continually monitored for bark beetles for the remainder of the summer. As other bark beetle pockets continue to develop, trap trees will be felled and the new pockets will be cleaned up by late September.

Disease and parasitism of forest tent caterpillars

In late June, cocoons of the forest tent caterpillar were collected at twelve locations in central and northeastern Minnesota to determine levels of parasitism and disease. Cocoons from each location were sealed in a paper grocery bag and examined in late July after moths had emerged and died. We found that parasitism and disease of pupae varied from 43% to 96%. Sarcophogid flies were the major parasite. (See table at end of article.) From egg to moth, all life stages of the forest tent caterpillar are subject to mortality from many different causes. Extremely cold temperatures sometimes kills eggs during the winter. Freezing temperatures just prior to, during and following hatching kill many young caterpillars. Starvation following complete leaf defoliation also takes a great toll. Predatory beetles, ants, true bugs, spiders, birds, small mammals and bears also consume many caterpillars. Viral, fungal and bacterial parasites also help control this pest.

This year many dead caterpillars were observed at most collections sites. These dead caterpillars may serve as reservoirs of diseases for next year's populations. Sarcophogid flies were the major parasite found in the collection. When forest caterpillars have formed their cocoons in tents of one or more leaves tied together with silk, the adult flies deposit living maggots on the cocoons. These maggots bore or chew through the silken cocoons and through the body walls of caterpillars or prepupae, consuming soft tissues and causing death.

Overall, life stage mortality must be very high before it causes a reduction in the next year's population. For example, if each female lays an average of 150 eggs and 98.7% of the progeny dies, the number of emerging adults will be equal to the parent population. In order to reduce the population by one half, 99.3% must die. This study only determined mortality of the pupal stage and therefore does not gives us an accurate prediction of next year's population. After two or three years of defoliation, the level of mortality is increasing and will eventually result in a population collapse. An egg mass survey would be required to more accurately predict population levels in 2002 and will be done over the winter. We can confidentially conclude that FTC is expected to remain at high population levels in much of Minnesota for 2002 based on this study and past experiences.

Disease and parasitism study of forest tent caterpillar in Minnesota - 2001
Location of Cocoon Collection Collection Date Cocoons Collected Diseased or Parasitized Emerged Moths Number Emerged Moths Percent Fly Maggots or Puparia Adult Wasps Dead or Diseased Caterpillar/Pupae Cocoon Abundance Refoliation Comments Other Comments
Lakewood (N. Duluth)
NWNW S23 T51 R12
6/26/01 91 71% 26 29 30 0 36 sparse starting many dead and few live caterpillars
SWNE S34 T63 R12
6/26/01 114 83% 19 17 69 0 4 abundant starting many dead and few live caterpillars
SESE S30 T65 R19
6/26/01 108 71% 31 29 21 0 1 sparse   many dead and few live caterpillars
International Falls
SENE S35 T71 R24
6/26/01 106 96% 4 4 66 0 10 sparse   many dead and few live caterpillars
SWNE S12 T58 R18
6/27/01 147 67% 48 33 52 5 9 abundant   many dead and few live caterpillars
SESE S23 T57 R21
6/27/01 93 78% 20 22 18 7 32 rare occurred many days previously many dead and few live caterpillars
NWNW S19 T57 R22
6/27/01 130 43% 74 57 41 0 25 abundant   many dead and few live caterpillars
Grand Rapids
SESE S32 T55 R25
6/27/01 124 77% 28 23 37 4 20 abundant   many dead and few live caterpillars
Rum River S.F.
NWNW S20 T40 R26
6/28/01 139 71% 40 29 42 2 4 abundant occurred many days previously many dead and few live caterpillars
Father Hennepin S.P.
SENW S3 T42 R25
6/28/01 110 71% 34 29 19 and many tiny maggots 2 4 rare occurred many days previously many dead and few live caterpillars
Birch Lake S.P.
NWNE S36 T127 R33
6/28/01 94 82% 7 8 62 3 23 sparse occurred many days previously many dead and few live caterpillars
Bay Lake
NE S3 T45 R28
6/28/01 140 71% 41 29 37 and many tiny maggots 0 6 abundant occurred many days previously many dead and few live caterpillars

Aspen leafblotch miner

Many quaking aspen in the northern half of our State have had their leaves mined by the aspen leafblotch miner, Phyllonorycter ontario. These aspens had also been defoliated by the forest tent caterpillar earlier in the year. Such removal of functioning leaf tissue by two insects considerably reduces energy reserves and may cause some dieback in the aspen..

Tiny caterpillars hatch from eggs laid on the underside of leaves, bore through the lower epidermis and feed on the spongy parenchyma cells just below the upper epidermis. The mines are whitish due to loss of the green parenchyma and they become oblong in shape, as caterpillar feeding continues and later turn brown. Mature caterpillars are about 1/4 inch long when they change into pupae. As pupae, they push their way through their silken cocoons and partially through the lower epidermis. Tiny moths emerge from these pupal skins and fly away to hibernate for the winter under bark scales of pine, spruce or fir trees. many dead and few live caterpillars decayed larvae were observed in their mines during early August. This may help reduce next year's population.

Elm leaf beetle

Elm leaf beetle

From a distance, you might suspect Dutch elm disease caused the discoloration and drooping of elm leaves. Up close, you can easily see the skeletonized, curled, drying and falling leaves. The elm leaf beetle, Xanthogaleruca luteola, as larvae and adults were responsible for this damage which was observed in central and north central counties in July and August. Larvae resemble ladybug larvae and are black when young and golden with black and white markings when mature. Adults are about l/4 inch long, elongate oval in shape and are yellow to olive green with a black stripe along the outer end of their wings. Adults overwinter in homes, buildings, wood piles, tree bark crevices, debris at the tree bases and other protected places. Egg laying begins in May to early June, with each female laying between 400 and 800 eggs over her life span. Eggs hatch in about 7 days. Several labeled insecticides provide satisfactory control.

Oak tatters: What's in a name?

Again, oak tatters did strike portions of southern Minnesota this year. As of this report, all the affected trees in several counties did recover new foliage. Its is interesting to note that as far as we know all of the new foliage was free of symptoms.

The most striking aspect of this season's tatters was that it could be more appropriately named oak/ hackberry tatters as the vast majority of the affected trees were hackberry. There was also a difference however between the southeastern and the southwestern parts of the state. In the southeast, primarily bur oak was affected. The defoliation was widely scattered and usually only small acreages were tattered. The defoliation was seen in Wabasha, Fillmore, Olmsted, Dodge and Mower Counties. In the southwest, hackberry defoliation was very high and seemed to affect every hackberry in at least the seven southwest counties of Brown, Watawan, Murray, Martin, Jackson, Cottonwood and Redwood.

This was the first time we have seen a large outbreak on hackberry. The oak tatters in southeastern Minnesota has been diminishing in extent for the last three years. In talks with Iowa officials, they have seen similar outbreaks in hackberry in past years.

Oak tatters is a relatively new condition that affects the first emerging oak leaves in the spring. It has been observed throughout several mid-western states for years. The first reports were from Iowa in the 1980's. More recently, it has been observed in Minnesota and Wisconsin. Oak tatters affects primarily the bur oaks. It has also been observed on a few other species. The newly emerged leaves of affected trees have a reduced leaf tissue between the veins which gives the leaves a lacy or tattered appearance, hence the name oak tatters. The injury appears at the time of leaf emergence, late May. Generally large portions of the landscape have been affected. However, within the affected area, a few unaffected trees can be found. Within a few weeks, a new flush of leaves will appear and be free of the symptoms.

The cause remains unknown. The pattern of symptoms suggests internal injury to developing tissues, before or at the time of leaf development and inside the overwintering buds. Oak tatters can be confused with early season outbreaks of oak anthracnose and the effects of late spring frosts.

Willow leaf beetles

Imported willow leaf beetle
Imported willow leaf beetle

Imported willow leaf beetle larva

Chrysomela species adult
Chrysomela species adult

Chrysomela larvae
Chrysomela larvae

Many willows in central and northern Minnesota have had their leaves so heavily skeletonized that only a few major veins remain. This damage is caused by the larvae and adults of two species of beetles, Chrysomela interrupta and Plagiodera versicolora, which are respectively known as the cottonwood leaf beetle and the imported willow leaf beetle. Both beetles have two generations each year and the cotton wood leaf beetle also feeds on poplars. Black or greenish blue adults and black larvae identify the imported species, while light yellow and black stripes or bars on adults, and whitish larvae with black spots distinguish the cottonwood beetle. Chemical control of these beetles on yard willows will prevent their disfiguring defoliation.

In northwestern Minnesota, defoliation by the imported willow leaf beetle is considerably reduced compared to last season. However, defoliation of willow brush and trees is widespread in northeastern Minnesota this summer. The adults and larvae are skeletonizing, as well as, eating small holes in leaves of willow and poplar. Fields of willow brush are now brown due to feeding by this insect. Imported willow leaf beetle is native to Europe and was first reported in North America between 1910 and 1915. In Canada outbreaks have been reported in Quebec and southeastern Ontario but it is reported to generally be held at low levels by an imported pupal parasite.

Walking sticks

Walking stick on leaf.

By August 7th , walking sticks had heavily defoliated oaks in northeastern Stearns County just south of the Birch Lake State Forest Campground. On this date, they averaged three inches long and were mating. Their eggs drop to the ground and will lie dormant until the spring of 2003. This is the only known location in Minnesota where large numbers of this interesting insect persist.