Month: January 2020

Upside and downside of a warming world for the despoiler of hemlocks: Hemlock woolly adelgid, Adelges tsugae

  White bundles of fluffy wax are a sure sign of a woolly adelgid infestation on hemlocks.

White bundles of fluffy wax are a sure sign of a woolly adelgid infestation on hemlocks.

  Eastern hemlocks are beautiful native trees well suited for landscapes.

Eastern hemlocks are beautiful native trees well suited for landscapes.

Eastern hemlock is a wonderful tree native to North America, found in the wild stretching from the shores of the Chesapeake to the Blue Ridge Mountains and beyond. In neighborhoods it graces landscapes as an evergreen specimen or screen along property lines. More than six decades ago, the hemlock woolly adelgid, a dastardly sucking insect akin to an aphid, appeared near Richmond, Virginia. It likely entered this country on infested nursery stock from Japan. For many years this pest made its presence known mostly in home landscapes and parks, where it often disfigured and sometimes killed hemlocks. As it spread to the Appalachian Mountains, it killed thousands of eastern hemlocks in the Shenandoah, Blue Ridge, and Smokey mountains. A recent study estimated government and household expenditures to control hemlock woolly adelgid and lost property values when hemlocks died exceed $214 million annually in the United States. From Maine to Georgia this pest threatens eastern hemlock in the north and its rarer cousin, the Carolina hemlock, in the south.

The hemlock woolly adelgid has killed thousands of eastern hemlocks throughout natural and managed landscapes in eastern North America.

The hemlock woolly adelgid has killed thousands of eastern hemlocks throughout natural and managed landscapes in eastern North America.

This mini-monster spends most of the summer and early autumn hunkered down as an inconspicuous immature stage, called a nymph, on the bark of the hemlock near the base of needles. When the cold winds of winter blow in late October and November, the nymphs resume development and mature in mid-winter. Currently in my neighborhood this process is underway, with adelgids producing large amounts of white, woolly wax from which the woolly adelgid takes its name. The waxy cover provides protection for the adelgid and for eggs she will lay in the woolly sac in late winter. Between March and June, a second generation of adelgids will hatch out and mature, and then the cycle begins again with females producing the next batch of nymphs that will summer on hemlock twigs. Excepting the summer season of dormancy, developing nymphs and egg-laying females feed by inserting hypodermic-like mouthparts through the bark of twigs. The long, sucking mouthparts search along the tree’s vascular system and eventually find specialized tissues called parenchyma cells of the xylem rays. The mouthparts are inserted into parenchyma cells and the adelgid greedily robs the tree of its stored nutrients. Heavily infested trees decline in vigor, turn a sickly grayish – green color, lose their needles, and may die in five to ten years, if adelgids are not controlled.

What lies beneath the wax? Wax on reveals little, but wax off reveals wiggling legs and rows of wax-producing pores lining the underside of the bizarre looking immature adelgid.

We have painted a picture of the adelgid as a despicable pest very much accustomed to thriving in winter’s cold in many parts of the eastern United States. As the world warms, scientists fear the range of the adelgid will expand dramatically to many northern reaches in the United States and southern Canada. In these chilly lands, the adelgids’ hemlock hosts are currently protected by frigid winter temperatures lethal to the overwintering stages of the adelgid. As the world warms, this thermal refuge for hemlocks is in jeopardy. But there seems to be an upside related to the warming of our world with respect to the current geographic range of the hemlock woolly adelgid. Recently, scientists have discovered that in the southernmost lands invaded by the adelgid, the hemlock forests of Georgia, summer temperatures have become hot enough to significantly reduce the survival of the adelgid on imperiled hemlocks, particularly those at warmer low elevations. While a warmer world may open the door for the adelgid’s continued northward march, perhaps the same phenomenon will close the door to its persistence in the south. Only time will tell.

Acknowledgements

The following articles where consulted in preparation of this episode: “Economic impacts of non-native forest insects in the continental United States” by J. E. Aukema and colleagues, “Biology and Control of Hemlock Woolly Adelgid” by N.P. Havill, L. C. Vieira, and S. M. Salom, “Increases in summer temperatures decrease the survival of an invasive forest insect” by Angela M. Mech, Patrick C. Tobin, Robert O. Teskey, and J. Rusty Rhea, and Kamal J. K. Gandhi, and “Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?” by Jeffrey S. Dukes, Jennifer Pontius, David Orwig, Jeffrey R. Garnas, Vikki L. Rodgers, Nicholas Brazee, Barry Cooke, Kathleen A. Theoharides, Erik E. Stange, Robin Harrington, Joan Ehrenfeld, Jessica Gurevitch, Manuel Lerdau, Kristina Stinson, Robert Wick, and Matthew Ayres.

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Scorpion glow, scorpion sting: Slenderbrown and other scorpions in Central America, Centruroides spp.

  Like psychedelic posters from the 60’s, rockin’ tropical scorpions glow beneath the beams of a blacklight.

Like psychedelic posters from the 60’s, rockin’ tropical scorpions glow beneath the beams of a blacklight.

 

One delightful adventure not to be missed is a nocturnal foray into the heart of a tropical rainforest. This escapade can produce memorable encounters with whip scorpions, large tarantulas, and giant katydids like those we met in previous episodes. During one such foray in Costa Rica, I was surprised to see a beautiful blue-green scorpion resting on the ground when a trail guide moved a fallen leaf and cast the beam of a blacklight near the path. Being a child of the 60’s, I was instinctively struck to understand what my blacklight posters shared with this stinging eight-legged rainforest predator. It turns out that blacklight posters contain compounds, phosphors, capable of capturing the high energy photons of UV light and releasing their energy in longer and less energetic forms of visible light, producing dazzling, glowing hues. Scorpion glow results when UV light is captured by two compounds, beta-carboline and 4-methyl-7-hydroxycoumarin, found in the scorpion’s exoskeleton. Once captured, UV’s energy is released in the form of eerie blue-green florescence.

A scorpion is revealed amongst leaves on the rainforest floor as it fluoresces bluish-green when illuminated by rays of a blacklight.

In a series of clever studies, Dr. Douglas Gaffin and his colleagues discovered that the scorpion’s entire body may act as a photoreceptor or universal “eye” used to detect different levels of light. Light in the UV range directed at scorpions produced bouts of rapid movement. These researchers suggested that the scorpion’s whole-body “eye” might help it move to places where light no longer illuminates its body, such as locations beneath vegetation where the searching eyes of larger predators were less likely to spot it. Whole-body photoreceptors might also be used by scorpions to detect the waning light levels of twilight, the signal to exit burrows and start their nocturnal hunt for prey.

Who’s that lurking in the corner of the bed frame? And is that another wedged in the crack above?

Who’s that lurking in the corner of the bed frame? And is that another wedged in the crack above?

On another tropical adventure in the rainforests of Belize I had the good fortune to encounter scorpions in a somewhat different context. After a long day of feeding mosquitoes and avoiding crocodiles with a group of students on a study abroad, the prospects of enjoying a little shut-eye in the bunkhouse was most appealing. Unfortunately, one student climbed into his lower bunk bed and was surprised to see a rather impressive scorpion beneath the mattress of the upper bed just a few inches above his head. In Belize, the solitude of bedtime is often punctuated by colorful and enthusiastic expletives describing the creatures found in cabins. The exciting Slenderbrown scorpion, like the one encountered by our student, is a regular visitor to cabins and outhouses at night, and goes by many names including the Slenderbrown, Brown Bark Scorpion, or Alacran azul. In the wild, I have seen it beneath loose bark and under logs. This scorpion is found in Central and South America, the Caribbean, and southern Florida.

Scary pinchers, or pedipalps, are used to capture prey.

Scary pinchers, or pedipalps, are used to capture prey.

If this sting gets you, you will be sending out an SOS to the world.

If this sting gets you, you will be sending out an SOS to the world.

Scorpions are not insects. They belong to another part of the arthropod clan called arachnids and are relatives of spiders and ticks. The scary pinchers on the front end of the scorpion are its pedipalps. They are used for grasping and dismembering insects and spiders that comprise most of the scorpion’s meals.

The business end of the scorpion is the sting, an enlarged segment at the end of the scorpion’s tail that contains a venom gland and a needle-like poker to deliver the poison. The sting is used to immobilize and kill prey and also as a means of defense against larger animals. When scurrying across a floor or ceiling, the scorpion’s sting is often curled up and over its back. Scorpions move surprisingly fast. The venom of the Slenderbrown scorpion carries a punch similar in pain to the sting of a honeybee or yellow jacket and is not generally life threatening. However, some relatives of the Slenderbrown scorpion, including those in the genus Tityus, are very dangerous and their venom can be fatal to humans. One courageous and somewhat impulsive student tested the potency of the Slenderbrown’s sting when she grabbed a scorpion lurking over her bunk and was stung. She summarily hurled said scorpion out the door of her cabin. Her assessment of the experience: “It only hurt a little and that thing was really annoying me”. You go girl!

Acknowledgements

Many thanks to Dr. Jeff Shultz for an enlightening discussion about scorpion glow. The fascinating article “Scorpion fluorescence and reaction to light” by Douglas D. Gaffin, Lloyd A. Bumm, Matthew S. Taylor, Nataliya V. Popokina, and Shivani Manna provided much background information for this episode. Thanks also to Matt Tabisz for wrangling the scorpion in this Bug of the Week.

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Wintry appearance of a strange moth: Fall cankerworm, Alsophila pometaria

  Wingless, flightless, non-feeding, winter-active, what a strange moth is this female fall cankerworm.

Wingless, flightless, non-feeding, winter-active, what a strange moth is this female fall cankerworm.

  Flight-capable male fall cankerworms are often seen on mild winter nights resting on trees or near porch lights.

Flight-capable male fall cankerworms are often seen on mild winter nights resting on trees or near porch lights.

Last year was spectacular for moths and butterflies. We visited many beautiful butterflies including brush-foots and swallowtails, and several marvelous moths such as silk moths and webworms. One of the more curious members of the moth clan, fall cankerworm, made its presence known on a blustery day last week. This enigmatic creature defies several “norms” found in the rest of the moth coterie. As you know, most moths are winged creatures that frequent the skies on summer nights as they search for mates and suitable plants on which to lay their eggs. However, female fall cankerworms are wingless. They have forgone their ability to fly. Is this some unfortunate twist of fate or the curse of a malevolent sylvan fairy? Perhaps, but many entomologists believe that wingless cankerworm moths have found a clever way to leave behind more offspring. By shifting precious bodily resources from equipment needed for flight, such as wings and muscles to flap them, and redirecting these resources to the production of eggs, female cankerworms may be able to bring more little caterpillars into the world and enhance their lineage’s odds for survival.

High in the treetops fall cankerworms deposit eggs on the bark of branches and twigs.

High in the treetops fall cankerworms deposit eggs on the bark of branches and twigs.

Regardless of the reason that underlies the mystery of the wingless moth, they are a wonder to see. Beginning in late autumn, adult fall cankerworms emerge from pupal cases in the soil. Females move from the soil and climb vertical structures such as trees and buildings. Shortly after sunset, on milder winter nights, female moths release a chemical signal called a sex pheromone that attracts male moths. Fall cankerworm males have functional wings and are good fliers. Each male tracks the pheromone to its source and the chilly couples mate. After this interlude, females climb high into the tree and place their eggs on the bark of small branches and twigs. Females do not live to see their offspring. Unlike other species of moths that have tubular mouthparts used to sip nectar, the female fall cankerworm lacks functional mouthparts. She cannot feed and shortly after depositing her eggs she dies.

Shredded leaves left behind by hungry caterpillars give these pests their common moniker, cankerworm.

Shredded leaves left behind by hungry caterpillars give these pests their common moniker, cankerworm.

The larvae of fall cankerworms hatch early in the spring soon after the buds of trees open and young leaves appear. Caterpillars of fall cankerworms and other members of their clan are also known as inchworms. They have multiple legs on their front and rear ends. By alternating their grasp between front and rear legs and bending their body upward into a loop, they move along twigs and leaves as if measuring the world an eighth of an inch at a time. The name cankerworm derives from the shredded, cankered mess caterpillars make as they consume foliage of trees. Their larvae reach phenomenal numbers in some locations and years, and may devastate many shade trees such as oaks, maples, elms, and lindens. We learned more about the ravages caused by this native pest in a previous episode of Bug of the Week. In addition to the fall cankerworm, other members of their clan such as spring cankerworm and linden looper, are active in the winter and have flightless females. A close relative of the fall cankerworm called the winter moth has recently appeared in cities and suburbs in New England, where it has become a perennial pest wreaking havoc on several species of shade trees in those areas.

On a bright winter day an adult fall cankerworm tries to ignore the annoying thumb of a bug geek. Can you guess why cankerworm caterpillars go by the name inchworms?

On a bright but chilly winter day visit a maple, elm, or oak and try to catch a glimpse of these strange ladies as they escape their earthly confines and slowly ascend trees in search of suitable repositories for their eggs. 

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A surprise visit from my ants: Citronella ants, Lasius sp.

  A cluster of female (larger ant) and male (smaller ant) smaller citronella ants seem ready to set forth to found new colonies on a warm afternoon in December.

A cluster of female (larger ant) and male (smaller ant) smaller citronella ants seem ready to set forth to found new colonies on a warm afternoon in December.

 

The appearance of winged citronella ants emerging from beneath my front sidewalk was a welcome surprise in this somewhat dismal season for observing insects and their kin outdoors in the DMV.  Swarms of winged, a.k.a. alate, ants are often seen emerging from the earth to mate and found new colonies in warmer months in temperate regions. However, ants in the genus Lasius have been observed emerging from the earth in Maryland as late as November, so maybe December 27 is not such an odd date to witness this remarkable event. While not an ant expert, these little rascals looked like the smaller yellow ant, Lasius claviger, to me.

What a holiday surprise! Watch as large winged
females, smaller winged males, and wingless yellow worker citronella ants emerge from the earth and swarm on my front sidewalk.

When disturbed, bright yellow worker ants release alarm pheromones to recruit nest-mates to assist in defending the colony.

When disturbed, bright yellow worker ants release alarm pheromones to recruit nest-mates to assist in defending the colony.

Lasius ants go by many colorful names including citronella ant, yellow ant, and lemon ant. The lemony scent we know as citronella owes to a compound found in many plants including lemon, lemon grass, and several species of eucalyptus. Special glands near the jaws of Lasius ants produce aromatic lemon scented citronellal. When threatened by an intruder, citronella ants send out an alarm by releasing citronellal. This pheromone brings nest mates running to assist in the defense of the colony. In addition to the well-known insect repellency of citronella, citronella ants have one more chemical trick up their sleeve, or should we say, in their rear end. Glands near the tip of the abdomen produce irritating formic acid. Formic acid contacting the eyes or face of an attacking predator serves as a powerful deterrent. This type of chemical warfare is an important strategy used by many species of ants.

Large yellow ants will sometimes swarm inside homes much to the dismay of unsuspecting homeowners.

Large yellow ants will sometimes swarm inside homes much to the dismay of unsuspecting homeowners.

In a previous episode we met large citronella ants that sometimes making a surprising appearance when their subterranean colony vents inside a home. Fortunately, Lasius ants are not wood destroyers like their cousins the carpenter ants. In addition to hunting soft-bodied prey, Lasius are herders. Yes, that’s right, they actually shepherd root feeding aphids, moving them from the roots of one plant to another to optimize the production of nutrient rich honeydew excreted by the aphids. As you may know, honeydew is an important and much sought-after food for many species of predatory and parasitic insects.

Citronella ants are relatively common in urban landscapes. I frequently encounter them while working in my flower beds and garden. You will too. Just take a moment to notice the pleasant fragrance of lemon as you turn the soil and then have a close look in the soil for these tiny and fascinating chemical warriors. 

Acknowledgements

The wonderful references “The Ants” by Bert Hölldobler and Ed Wilson, and “Secret Weapons” by Tom Eisner, Maria Eisner, and Melody Singer were used in preparation of this episode. Jeff and Linda shared the swarm of large yellow ants that emerged inside their home. Thanks also to Dr. Paula Shrewsbury for assistance with photography.

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