Forest Health Monitoring Review: Spring 2003

• Weather
• Bark Beetles
• Gypsy Moth
• Other Defoliators
• Hemlock Woolly Adelgid
• Voles
• Study
• Shorts
• Sentinel Pines
• Shortleaf Advantage

Health Watch
Health Watch

Health Primer
Induced Resistance
Tree Resistance, Southern Pine Beetles and More
A Note of Communications

Health Update

WEATHER - Complaints of drought are losing their force. Since October, we've been treated to plenty of precipitation; too much in some places. Every climatic division was considerably wetter than normal during the months of October, November, December and February. In January, by contrast, precipitation ranged from 20-55% drier than normal in every division. The net overall effect was precipitation amounting to 171% of normal for the period October-February. Record maximum daily precipitation occurred in Blacksburg, Lynchburg and Roanoke on February 22. March, too, has been generally wet so far.

October temperatures ranged from 1.5 degrees below normal in the northern division to 2.5 degrees above normal in the southwestern division. From November on, it was cool to cold everywhere. January ranged from 2.9 to 5.6 degrees colder among all 6 divisions. Record lows were set in Blacksburg and Lynchburg during early December.

We also received a few special weather events this winter. You might have noticed. Ice brought hazardous conditions to the central piedmont and mountains in mid December and across most of the Commonwealth in mid February. Snow, sleet, freezing rain, hail, floods and wind damage combined to make February particularly memorable. Floods caused evacuations in Salem and resulted to two fatalities in the Roanoke area. The governor declared a state of emergency for 19 counties and 4 cities. DOF personnel assisted with the flood responses in most of these locations. Suffolk sustained a tornado on February 22; and not to be outdone, Norfolk recorded a record high temperature of 77 degrees on the same day. Ice damage affected southern Halifax County in mid March according to Alex Williamson, who spent most of one night helping to clear roads.

Bark Beetles - Last year was a good one for bark beetles generally, but overall it was not extraordinary. Engraver beetle activity was apparent in almost every county, but it was not responsible for the early liquidation of many stands. Black turpentine beetles remained a component of most pine mortality and were major factors in a few instances. Southern pine beetles continued to kill trees in the southwest mountains, but apparently at a decreasing rate. They also appeared at alarming densities in a few central Piedmont locations and prompted emergency salvage operations. Surprisingly, however, these local outbreaks did not extend to surrounding areas.

Currently, there is still evidence of lingering activity; but no apparent cause for alarm. Very few infestations involve just one bark beetle species. A break in the drought will help trees begin to rebuild resistance. Our annual spring trapping survey should give us a good idea of what to expect later this year.

Gypsy Moth - This spring threatens to be a bad one for gypsy moth impact and population expansion. Trees are under great stress from drought, and secondary organisms have had plenty of opportunity to gain momentum. Only the toughest trees are likely to survive defoliation. In addition, detectable levels of infestation span a much greater portion of the Commonwealth than ever before. The cooperative suppression program has expanded considerably over last year. If the gypsy moth does well this spring then 2003 could be a record breaker for Virginia. If natural enemies fair well and the weather is adverse for gypsy moth development then the outcome could be very different.

Slow The Spread Program treatments have increased greatly for this year owing in part to a surprise local outbreak in Wythe County last spring. Treatments with the growth regulator, Dimilin, will be applied in Wythe and Pittsylvania Counties on about 4,000 acres. Eleven additional treatments with the mating disruption pheromone, Disrupt II, will cover nearly 160,000 acres in portions of Bland, Carroll, Charlotte, Floyd, Franklin, Giles, Greensville, Halifax, Mecklenburg, Patrick, Pittsylvania, Pulaski and Wythe Counties.

Federal research to improve gypsy moth pheromone application will be conducted again on the Cumberland and Appomattox-Buckingham State Forests this spring. Great progress has been made in using the pheromone against low-density populations. It is not effective once populations reach the nuisance level.

Defoliation surveys will be a particular challenge this June because we no longer have an official pilot, our most experienced sketch-mapper has retired and we'll need to cover about twice the usual acreage. If the weather is unfavorable we will have to omit areas where defoliation is not expected to be extensive.

Quarantine regulations have become somewhat more stringent over time and when regulated goods now cross from infested to uninfested areas required documentation must accompany them. For example, a signed "Accurate Statement" must travel with loads of logs, posts, pulpwood, wood chips, and bark whenever the quarantine line is crossed Nursery stock and Christmas trees require other documentation. Questions should be addressed to the nearest Department of Agriculture and Consumer Services regional office. Alleghany, Botetourt, Bedford and Halifax Counties and points north and east are considered infested.

Other Defoliators - Aerial surveys will be conducted in late May or early June to map defoliation by fall cankerworm, buck moth and other early spring defoliators. These surveys are limited to areas where historical experience or recent survey results indicate a reasonable probability of finding something.

Hemlock Woolly Adelgid - Monitoring of two isolated hemlock stands in Campbell and Lunenburg counties remains interesting (a third site was lost to harvest). Two transects of 25 trees each were established in 1992 at each location and have been examined annually since. Both areas were infested when established, but neither had sustained significant impact. The Campbell trees have remained generally infested at moderate densities, except for 1996 and 1997 following adelgid mortality from cold, and still look quite thrifty. There has been no tree mortality attributable to adelgids. The Lunenburg trees have sustained somewhat lower rates of infestation and 33% mortality. The surviving trees are not particularly thrifty, but there has been no obvious sign of infestation for nearly three years now. These trends are different from those in the mountains where mortality has generally come more quickly and severely.

Study - Replicates of an eastern white pine survival study have been established in six counties. These plantings are intended to evaluate the relative importance on first year survival of seedling handling, root treatment and planting care. Details and initial results will be reported in the next issue.

Voles - The fall and winter of 2000 was a period of extraordinary damage to young pines from vole feeding (see 4/01 issue for info re voles). Local losses have continued wherever open fields have been converted to pines. Without sod control, pine planting in fields is a real gamble.

Shorts - The European hornet sometimes girdles Rhododendron and other stems as illustrated on Plate 11 F in your Tree and Forest Health Guide for Virginia Foresters. A few cases of this strange phenomenon have been reported this winter.

• The notorious "Spanish fly" is actually a blister beetle - an insect family of global distribution. These beetles produce the toxin, cantharidin, which has been responsible for many human fatalities. Its function as an aphrodisiac remains unsubstantiated. Various cultures around the world have employed at least 25 different kinds of insects for a variety of sexual enhancements.
• Thousands of fly species hear with their antennae and some apparently can use echoes to navigate in total darkness.
• Researchers at NC State University have developed a grafting procedure that joins Fraser fir to root stocks that are resistant to Phytophtophora root rot.
• Many predaceous insects liquify their prey externally by injecting or flooding them with digestive juices before imbibing the resulting nutritious slurry - a phenomenon called extra-oral digestion.
• New high-definition X-ray videography has revealed that insect respiration is not strictly passive, as long believed. Apparently, certain muscles contract to make the tiny breathing tubes 'exhale,' and 'inhalation' results passively when these muscles relax.
• People who talk a lot and say little are called blatherskites. Philography is the practice of collecting autographs. Don't say armpit, say oxter.
• Investigation of elm mortality in Illinois revealed that the causal agent was not the familiar phytoplasma that produces elm yellows disease, but a different kind.
• A swarm of Rocky Mountain locusts was estimated in 1875 to be 1,800 miles long and at least 110 miles wide. The last known living specimen of this species was collected in 1902. This inadvertent extirpation is attributed to habitat destruction associated with development of the transcontinental railroad.
• Periodical cicadas should be emerging over much of R-5 and R-6 this summer. References to these insects as "locusts" presumably originated from metaphorical comparisons to biblical locust plagues. The earliest record of periodical cicadas was written by Plymouth Colony Governor, William Bradford, in 1633.
• We don't know when honey bees were first introduced into North America, but one European species was already present in Virginia in 1620. The common Italian honey bee was first imported in the latter half of the nineteenth century after regular steamship service made bee transport cheaper and more reliable.
• Remontant means blooming more than once a season.

Sentinel Pines - Another great tale from Virginia Folk Legends as recounted by Mrs. Robert Edwards in Northumberland County sometime during the Virginia Writers' Project 1937-42. I'm sure this one's true.

On the tip of the peninsula where Fleeton is now located stood two large pines where the local children used to play. One day while George Edwards and some other children were playing under the pines they saw a big black ship arrive and anchor off the point. In those days ships of this type were seldom seen, so this ship attracted everyone's attention. As night came on the children forgot about the ship and hurried home to supper. The next day, when they went back to play, they found their play house scattered and a deep hole in the ground. Upon close examination, rust was found all around the hole and a chest of gold was believed to have been removed.

Shortleaf Advantage - Native American Ethnobotany points out that shortleaf pine is useful for more than making canoes and carving decorations as the Cherokee did. The Choctaw used a cold infusion of buds to treat worms and the Nanticoke found that pellets of tar were "beneficial for soreness of the back." The Rappahannock treated swellings with a decoction from top branches, and a compound with grated dried bark was used to induce vomiting - I can't imagine that not working.

Health Watch

Health Primer

Induced Resistance

Plants have various characteristics that can help protect them against pests. Hairs, thorns, waxy cuticles, resins, tissue toughness and thickness, color, odor, growth habit, growth rate and chemical make-up are familiar ones. But plants are not limited to such pre-existing (constitutive) defenses. They can also resist pests actively in response to attack.

Insect feeding, microbe infection and mechanical injury can induce changes in plant chemistry that affect pests adversely. Some of these changes are generalized and common to most plants; others are unique to a particular combination of plant and pest. They can be confined to the point of attack or occur throughout the plant. They are complex and incompletely understood.

Plant response to injury generally follows a series of steps. Something in the plant or attacking organism (e.g., insect saliva) produces a molecular signal that sends a message calling for particular types of local or systemic responses. From one to many genes are activated. This results in the synthesis and transport of compounds to the affected site or throughout the plant's vascular system. Often the response involves multiple chemicals and pathways. It can occur in a matter of hours or extend over a period of weeks, and will sometimes have effects that persist from one growing season to the next.

Induced resistance can take many forms. Only sometimes do plants react by producing toxins or barriers that affect pests quickly and directly. The nutritive value of foliage might be reduced in response to feeding, for example, and this can have several adverse effects on defoliators. Weevil feeding and oviposition will cause some eastern white pines to reduce the viscosity and increase the flow of resin that can drown weevil eggs and young larvae. Larval survival and adult fecundity of some pine sawflies are reduced when the tree responds to feeding by increasing resin acid concentrations. Spruce has been found to increase bark thickness and the diameter of bark resin canals in response to wounding. Sometimes mite or insect feeding will cause plants to produce volatile compounds that attract the parasites and predators of these pests. Such reactions can be amazingly specific. One study showed that the volatile compounds released by a plant varied according to the species of insect feeding on it and these different signals were used by different parasites to locate their appropriate hosts.

Induced resistance is also affected by growing conditions. Expression of resistance often comes at the expense of normal growth and differentiation because it requires considerable energy. Trees under stress to begin with might not have sufficient reserve energy to respond to pest invasions effectively. This is one reason that thinning is so important. A full crown is necessary if trees are to produce the extra carbohydrates necessary for defensive responses.

Increasing knowledge about induced resistance has allowed researchers to begin manipulating plant defenses. Certain treatments can elicit plant defensive responses prior to pest attack. One such commercial "plant activator," called ActigardTM is registered in Virginia for control of downy mildew on spinach.

Resistance is not a one-way phenomenon, of course. Pests react to plant defenses and have evolved many strategies to counteract them or take advantage of them. This is a subject for another time.

Tree Resistance, Southern Pine Beetles and More

Site-host-pest interactions have been subjects of investigation for a very long time. Anyone who begins to feel well informed about the workings of nature should do a little reading in this area. It's a quick way to turn hubris into humility. A cursory review of pine tree and southern pine beetle (SPB) interactions should make the point.

Researchers have yet to determine exactly how SPB finds its host tree. Visual cues appear to be involved, but other factors probably play a role too. Once SPB arrives and begins to enter the bark, a lot happens. First of all, a beetle is not alone when it lands on the tree. Riding on the beetle are mites, many kinds of fungi, bacteria and sometimes other organisms. These can include parasites, predators, competitors, commensals (organisms that benefit from, but do not harm SPB) and mutualists (organisms that benefit SPB, and benefit from SPB). There are more than a dozen species of mites that sometimes travel on SPB and can interact with the beetle or each other or fungi or the tree or all of these in any combination. Certain fungal spores are carried by SPB in special body structures; others are picked up on the body surface or on the hitchhiking mites. Some of these fungi enhance beetle brood survival and development, perhaps in more than one way; but the mechanisms are not fully understood. Other introduced fungi are antagonistic to the beetle, the tree or other fungi, or all three. Some cause blue stain, some don't. The role of blue stain is not clear.

Early arriving beetles release pheromones to recruit other beetles. Males and females release different chemicals that can serve several purposes depending on circumstances. As beetles penetrate the bark to the cambium they inoculate the tree with fungi and introduce mites and rupture resin ducts, which release volatile compounds that interact with the pheromones. This volatile mix of chemicals varies with stage of attack, beetle sex ratio, tree condition, fungal development, time of year and weather, among other things. Parasites, predators and other insects also respond to the complex signals released by recently attacked trees.

Trees have to take attacks standing still, but they don't have to take them without a fight. They respond not just to beetles, but also to the other invaders. Cells surrounding the attack point die, others lose starch content and increase cell wall thickness. Resin flow increases and toxic compounds accumulate that can affect both beetles and their associated fungi. Some toxins already exist in the tree's resin ducts, others are synthesized in response to attack. Sometimes the tree wins and both beetles and fungi fail to become established. In any case, when bark beetles attack a pine tree, the ensuing process could easily involve two dozen organisms all interacting among themselves and the host tree and the physical environment. We don't know enough to predict the outcome.

A Note on Communications

Acoustical, chemical and visual signals are familiar forms of communication among insects. Another less-studied, but common form is low frequency, substrate-borne vibration. Vibrations are produced by percussion, trembling, membrane movements, rubbing or a combination of these. Signals range from primitive volleys of evenly spaced vibrations to complex and rhythmic patterns involving various substrates.

Generally, males initiate vibrations to communicate with potential mates. Females respond and then sometimes move to avoid vibration-detecting predators. Mating-related vibrational communication is known among grasshoppers, crickets, roaches, stoneflies, booklice, leafhoppers, beetles, flies and others. Additional functions include colony alarm in termites, attack and danger signals in ants and egg-laying stimulation in wasps.

Last modified 2006-10-10