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Owen Sholes

Living in the New England countryside

Goldenrod Leaf Beetles

Growing up in northern Illinois, I would sometimes walk or run through fields of weeds, fields where many, most, or all of the weeds were goldenrods.  After some of those forays, I noticed a few blue-black insect larvae on my jeans.  I had no idea what they were, and it was immediately obvious that they were just accidental passengers, easily removed and nothing for me to worry about.

I later learned that they were the larvae of Trirhabda beetles, a genus of leaf beetles (Chrysomelidae) whose species each feed on a small range of host plants.  At least two species, T. canadensis and T. virgata, specialize on goldenrods, both as larvae and as adults.  

The beetles cling to the surface of the plant and chew up the leaf tissue.  Each beetle can eat a fair amount of leaf material in its lifetime, and sometimes, there are a large number of beetles, each roughly a centimeter long.  After most of season of feeding on goldenrods, sometimes there isn’t much leaf tissue remaining (see photo above).

One might think that these beetles, exposed on the plant, would be easy pickings for predators, and some predators and parasites do attack them.  But the beetles smell like goldenrods, a distinctive odor that results from an array of chemicals in the plant tissues, chemicals other than the content that provides nutrition for the beetles.  When the late Robert Whittaker was a professor at Cornell, he tried to feed a pet lizard some Trirhabda beetles.  Though the lizard was quick to devour mealworm larvae, it wanted nothing to do with the goldenrod beetles.  Their chemical content, taken from their goldenrod food, seemed to provide some degree of defense (more on this in another post).

I don’t have a picture of Trirhabda beetles, but you can easily find some on line (for example: https://en.wikipedia.org/wiki/Trirhabda_canadensis).

Ann Herzig performed some elegant experiments with Trirhabda beetles and goldenrods in and around Ithaca, New York.  She planted goldenrods on the roofs of some buildings on the Cornell campus because the buildings were a long way from the fields of goldernrods around the city.  In various plots, she produced different amounts of beetle-like damage to see whether the beetles responded to the leaf damage they encountered.  The beetles found these plants (clearly, their dispersal abilities were considerable) and they were more likely to stay on plants with low levels of damage.  At least some of the females that arrived on her test plots had already mated, and they laid more eggs on plots with low damage than on plots with high damage.

When she set up other plots around Ithaca with different levels of damage, beetles tended to leave plots with high damage, and they tended to stay on plots with low damage.  If she put a dab of paint on the back of the beetle, gluing the forewings (elytra) together, they couldn’t fly (this was a brilliant experimental manipulation).  The painted beetles stuck around longer than other beetles, which might seem like an obvious result, but she couldn’t be sure of their main mode of dispersal until she prevented them from flying.  

So she showed (among other things) that the beetles can find goldenrods that are far away, can detect levels of damage on the goldenrods, and will fly elsewhere if the plants are already heavily consumed.  These beetles are well adapted for the exploitation of goldenrods.

But for various reasons (most of which I don’t know), the numbers of beetles fluctuate from year to year, so the goldenrods are not always slammed by these herbivores.  The plants and insects manage to coexist over a fairly large portion of North America.  Is their coexistence the result of coevolution?  Seems like a reasonable hypothesis.

One last thought: goldenrod ball galls can reduce the weight of seeds and the number of rhizomes produced by an infested goldenrod plant.  I suspect that Trirhabda, when they are abundant, can have an even greater effect on goldenrod growth.  Seems like another reasonable hypothesis.

Further reading

Ann L. Herzig 1995 Effects of Population Density on Long-Distance Dispersal in the Goldenrod Beetle Trirhabda virgata. Ecology 76: 2044-2054

Ann L. Herzig 1996 Colonization of host patches following long-distance dispersal by a goldenrod beetle, Trirhabda virgata. Ecological Entomology 21: 344-351

Owen D.V. Sholes. 1981 Herbivory by species of Trirhabda (Coleoptera: Chrysomelidae) on Solidago altissima(Asteraceae): variation between years. Proceedings of the Entomological Society of Washington (D.C.) 83: 274–282

David C Hartnett and Warren G Abrahamson 1979 The Effects of Stem Gall Insects on Life History Patterns in Solidago canadensis. Ecology 60: 910-917

Goldenrod Galls II

I wrote about goldenrod ball galls back in the spring when the gall flies were dormant pupae inside gray/brown galls from the previous year.  Now, adult flies have emerged from last year’s galls, mated, and laid eggs on fresh green stems.  There, the larvae have induced the growing stem tissue to swell into a gall surrounding them.  

The galls are large and conspicuous.  At least on the hillside where I found them, the galled plants are close together in a few groups, with plants in between those groups having no galls. This distribution pattern could be the result of females distributing their eggs on one plant after another as they encounter them, moving as little as possible between plants, until they run out of eggs.  Or perhaps some plants are more attractive and/or less resistant to the galls.  Because goldenrods spread vegetatively, each patch of goldenrod contains genetically identical stems.  If one of those stems resists gall flies, they all will.  If one of those stems is vulnerable to gall flies, they will all be vulnerable.  

There also is some level of interaction between plant and fly, resulting in variation in gall size.  Warren Abrahamson has explored goldenrod gall flies in detail.

The galled plants have one gall each (though sometimes there can be more).  The flies spend most of their lives inside their galls, feeding and developing from midsummer through winter and spring into the next summer.  The adult flies serve to produce another generation of galls. And the production of galls is always essential for the production of a new generation of flies.  Year after year, at least some manage to succeed.

Goldenrod Flowers

Goldenrods are in the daisy family (Asteraceae) even though they don’t look much like daisies.  The old family name was Compositae, a clue that the flowers occur in clusters, and that what looks like a single flower (a daisy) is actually a composite (!) of several to many flowers.  The family is huge and worth getting to know because it includes a vast array of species with a vast array of characteristics.  We eat them (sunflowers), poison them (dandelions), get poisons from them (pyrethrins), use them as medicines (Echinacea), use them as décor (marigolds), curse them (ragweed), and can hardly get away from them (daisies).  

Back to the flowers.  Goldenrods produce an array of flower heads, each called a capitulum, and each containing several actual flowers.  Those flowers around the edge of the capitulum are ray flowers (with elongated petals), and those in the middle are disk flowers (with tiny petals).  In a daisy, the distinction is obvious: the ray flowers have big white petals and the disk flowers are bright yellow with hardly any petal tissue at all. 

In goldenrods, you have to look closely to see the difference.  The flowers are all the same yellow color (in most species), and the rays are not large.  But they are there.

Each flower is tiny, but the collective display – in each head, and with all the many heads combined – is substantial.

According to Gray’s Manual of Botany, the ray flowers in each head are female only (pistillate), while the disk flowers are both male and female (staminate and pistillate).  Early in blooming, the disk flowers don’t appear to be open, but the ray flowers are.  Of course, the disk flowers have essentially no petals that it’s hard to tell, at first, what “open” means.  But later, it becomes obvious (again, you have to look closely).  Then the plants provide both nectar and pollen to the insects that visit.  And the pollen gets distributed widely.  Never doubt the power of group advertising.

New Flower Buds

Early goldenrod (Solidago juncea) gets its name from blooming early.  In central Massachusetts where I live, their buds are already well formed before other species have even begun to develop their inflorescences.

Inside these buds, pollen and ovules are developing, producing haploid sperm and egg cells for fertilization.  The stamens and ovaries are providing diploid cells for support.  It’s a productive time.

The growing tissues need a supply of materials, and the phloem provides it by translocating amino acids, sugars, and more to where they can be assembled into new cells.  Some photosynthesis takes place in the buds themselves, some occurs in the green parts of the inflorescence, and some occurs in the lower leaves.  The green parts of the plant are sources of raw materials for the process of reproduction.

In southern parts of the North America, some goldenrod species bloom year round, but in northern areas, they wait until summer.  Everything happens in a small portion of the year, and some things happen so quickly that they are truly fleeting.  In this window of opportunity, seeds and their encompassing fruits (achenes) will come to be.  

The public part of the process – blooming – is not far off.  The stages are being set.  It’s going to get exciting.  At the cellular level, it already is.

A Life Cycle Tied to Early Goldenrod

You might have heard of jumping plant lice (family Psyllidae) because they pierce plant tissues with needle-like mouthparts and feed on what’s inside.  Some of them transmit diseases of plants while they feed, like citrus greening, and are therefore of great economic importance.  Others just feed.

At least one species, Craspedolepta veaziei, feeds on goldenrods, especially early goldenrod, Solidago juncea.  In June, adults move up the stem of the plant on to the branches of the inflorescence, feeding wherever they sit.  And they are mating.  And after they mate, the females has to lay eggs.  

But where?

Inside the flower heads of early goldenrod.  By the time the flowers bloom, the adults are nearly all gone (as shown in the graph; Craspedolepta veaziei used to be named Aphalara veaziei).  But the eggs are just getting started.  They hatch among the flowers inside the flower head where they are hard for enemies to find, and where they can feed as the flowers develop from bud through blooming into fruit.

Goldenrod species that develop later are not good hosts for these psyllids.  They need new flower buds when they are ready to lay their eggs.  Early goldenrod is the plant that has buds at the right time.

But by the time the flowers have become fruit, the psyllids are still immature.  Where do they go?  I think they spend the winter in the soil, perhaps associated with the underground parts of goldenrods, or perhaps just waiting, in a dormant state called diapause (it’s an entomological term – there are so many).  

The next growing season, large nymphs that have begun to resemble the adults, except for the absence of wings, feed on the stems of early goldenrod.  They seem to be well adapted for feeding and developing in conjunction with this host plant.  As far as I can tell, the plant is not significantly harmed by the psyllids.  Perhaps the late nymphs and adults, being exposed on the surface of the stems, are discovered by enemies often enough to keep these bugs from becoming too destructive.  

Further reading:

I apologize for not having a picture of this small insect, but you can get a sense of their appearance from photos posted on the web: https://inaturalist.ca/taxa/207542-Craspedolepta

Owen D. V. Sholes. 1984. Responses of arthropods to the development of goldenrod inflorescences (Solidago: Asteraceae). American Midland Naturalist 112: 1-14. http://www.jstor.org/stable/2425450

Mining Gold(enrod)

Some of the leaf is brown

(And the sky is not always gray)

Are there brown or gray patches on the leaves of your plants?  Does the leaf appear intact, but no longer green?  And is there no external sign of any insects?

Your plants might have leaf miners.  These are insects that have small and/or flattened larvae that live between the upper and lower epidermis of the leaf, mining (eating) the tissue in between.  It is an impressive adaptation, and there are several kinds of insects that do it (some flies, moths, beetles, and sawflies).

If the miners are still living in the leaf, the larvae might be obvious between the translucent layers of cells, or you can pry the leaf open and find them (what you do with them is up to you).  But it is quite likely that the miners have left the mine.  They might have pupated in the mine and emerged as adults.  Or they might have emerged as larvae and sought a place to pupate elsewhere.  

There are several species of leaf miners that feed on goldenrods, but one of the most common is the beetle Microrhopala vittata, the goldenrod leaf miner (in the family Chrysomelidae, the leaf beetles).  I found some of them in my examination of goldenrods back in the 1970s.  Naomi Cappuccino, studying goldenrods at multiple sites, including some near where I worked, often (but not always) found many of these beetles.  They were spotty, and their numbers fluctuated widely from year to year, for reasons that were hard to track down.  But sometimes, they were really abundant!

Walter Carson experimentally reduced beetle abundance in places where there were a lot of beetles.  When he did so, the goldenrods grew much better.  Where he left the beetles in place, the goldenrods grew poorly.  So these beetles matter, at least sometimes and in some places, to goldenrods.  

These mines on Solidago juncea (early goldenrod) certainly resemble those of the goldenrod leaf miner, though I’m not sure how distinctive the mines of the different species are.  I found them only on this tiny patch of stems, even though there are quite a few others within about 100 meters (though down a shady road – does shade affect the movement of the adults?).

I haven’t noticed any adults of the leaf miners, but it’s only June.  Stay tuned.

Further reading:

Naomi Cappuccino 1991

Mortality of Microrhopala vittata (Coleoptera: Chrysomelidae) in Outbreak and Nonoutbreak Sites 

Environmental Entomology 20 (3): 865–871

Walter P. CarsonRichard B. Root 2000

Herbivory and Plant Species Coexistence: Community Regulation by an Outbreaking Phytophagous Insect

Ecological Monographs 70 (1): 73-99

Froth on Goldenrods

In some northeastern states, this is the time of year when white froth can be found on the stems of goldenrods and other plants.  The froth is a bunch of persistent bubbles hanging precariously on the plants, yet more firmly than we might think possible.  Within each mass is the nymph of a cercopid (family Cercopidae, Order Hemiptera), aka froghopper, aka (crudely, but also evocatively) spittlebug. 

The nymphs suck fluids from the vascular tissue of the plant, receiving far more water than they need.  Nitrogen and other nutrients are so dilute that the nymphs have to process a lot of water to meet their requirements.  While other types of sucking bugs dump the water with less fanfare, cercopid nymphs add something to it to generate the frothy, spittle-like globs.  Their waste ends up as defense against enemies, and against dehydration from living on the outside of plant stems.

Peter McEvoy examined and experimentally moved the nymphs of two species of spittlebugs in old fields near Ithaca, NY.  They tended to prefer to feed (and froth) in the leaf axils (where a leaf joins the stem) instead of on the plain stem between leaves, and preferred wider axils over narrow axils.  And on goldenrods, they preferred to be near the apex of the plant.  Their food is quite dilute, but perhaps more nutritious in some locations than others.  And their defensive spittle is more likely to be successful when supported by parts of the plant and not merely hanging insecurely from a bare stem. 

I am not sure how these nymphs are able to breathe from inside their masses of bubbles, but they obviously can.  The spittle is visually conspicuous on their host plants, but that doesn’t seem to be a problem.  The spittle is also quite gross to the touch, apparently a deterrent to an array of predators, large to small.  Thus defended, cercopids can be quite abundant on herbaceous plants, and goldenrods are among their favored hosts.  Yet there are still far more leaf axils than spittlebugs, so feeding locations do not appear to be restricting the abundance of nymphs.  Something else seems to be keeping their numbers in check.

This fly landed in a bad place, got stuck in the spittle, and died. No, the nymphs are not predators. Their defenses are just too much for some delicate neighbors.

The adults do not produce spittle, but instead depend on hopping and flying to escape danger.  Years ago, I found a fair number of adults feeding on goldenrods and other plants in the latter part of summer.  At some point, they mated, laid eggs, and (having initiated the next generation) died.  I will watch for the adults this year, and if I can get a photo, I will post it.  Don’t hold your breath.  The adults are not nearly as conspicuous as the bright white batches of bubbles they inhabit in their youth.

Peter B. McEvoy 1986 Niche partitioning in spittlebugs (Homoptera: Cercopidae) sharing shelters on host plants.  Ecology 67: 465-478

https://esajournals.onlinelibrary.wiley.com/doi/abs/10.2307/1938590

The Goldenrod Apex

Taking some off the top (sometimes)

On its way to flowering, a goldenrod plant grows upward.  Sometimes they grow really tall, two meters or more, and sometimes they are fairly short.  Height differs between species and between locations.

The apical meristem adds cells at the top of the growing plant.  These cells become stem, leaf, and eventually, flower, seeds, and fruit.  The apex also inhibits the growth of lateral meristems lower on the plant.  The ideal growth direction is up.  Only when it’s time to produce flowers might multiple stems be allowed to branch out and form the inflorescence (or not – many species have little or no branching to support their flowers).  

But things can go wrong as the stem grows.  If the apex is destroyed by a disease or herbivore or physical damage, then that avenue for growth is gone.  But all is not lost, far from it.  Those abundant lateral meristems, some not far below the apex, can take over once the inhibition of the apical meristem is gone.

For a species that has short branches in its inflorescence, as in Solidago puberula (downy goldenrod), a damaged apex leads to the production of branches far larger than normal, and the earlier the damage occurs, the longer those branches are.  I know this because, many years ago, I removed the apical meristems from downy goldenrod plants at various times during the growing season and watched what happened.  Apex-free plants looked quite different from intact plants.   But as much as they grew, they couldn’t quite produce as many flowers as intact plants.  They never caught up, though they certainly tried.

This June, there are already some stems that have lost apical dominance.  Here are two Solidago arguta stems, one that has an intact central stem growing taller, and one that has lost its apex and started to grow lateral branches.  This species normally produces a wide, branched inflorescence even when the apex remains intact.  We’ll see how different they look by the time they flower.  

From Brown To Green

Spring happens every year, and every year it is amazing.  This is what happened to one small oak tree in 2021.

MARCESCENT LEAVES

Why do things hang on when there is no hope?  In the lines of Robert Frost: “The leaves are all dead on the ground/ Save those the oak is keeping” (the poem is “Reluctance”).

Usually, seasonally deciduous trees drop their leaves before the harsh season arrives – cold and/or dry – when the leaves cannot survive.  The process of leaf fall is abscission, the thinning and breaking of cell walls where the leaf meets the stem.

But some leaves are marcescent, remaining on the tree after death.  They have an abscission layer right were it is supposed to be, but it remains strong enough to keep the leaves on the tree.  These are the leaves “the oak is keeping.”

The leaves leave eventually.  They don’t stay on among the new green leaves.  When do they fall?  One source claims that the initiation of new leaves (the swelling of leaf buds) is the signal that sets the old leaves free.  The authors suggest that some early-season hormone (probably from the growing buds) travels through the tree and stimulates the completion of abscission.  Poof, no more brown leaves.

Their claim became the hypothesis for my observations in 2021.  I expected the dead leaves to remain fast until the buds started to expand.  Then the transition from brown to green would be quick, brown down, green up.

What actually happened?

There were quite a few leaves on a small red oak on February 28.  I photographed the tree nearly every day until May 27 to see what happened.  I also photographed some buds high on the tree (too high to measure directly), and measured the lengths of some buds on low branches that had no marcescent leaves.  

By March 15 or so, half the leaves present at the end of February had fallen.  In another two weeks, roughly a quarter of them remained. By April 10, it was down to about ten percent.  

ABSCISSION

But it’s not quite that simple.  Early on, the leaves broke off at the petiole (leaf stalk), not the abscission point where the petiole meets the stem.  The petiole had weakened more than the abscission layer.  On March 27, there were petiole stubs still on this stem, even though a large majority of the leaf blades had fallen.  Technically, the leaves had not yet abscised.  

But abscission was coming.  Two days later, one petiole from the stem had cleanly abscised.  By April 6, another was gone (the little wasp had nothing to do with it).  Once the abscission layer had weakened sufficiently, the stub of the petiole could break free, leaving the typical leaf scar. 

The weakening of the abscission layer is obvious in this April 14 photo

On April 16, we got several inches of wet snow.  The next day, all the petioles were gone. 

The late snow had accomplished what all the snows of winter had not.  Only when the tree had weakened its abscission layers could the petioles fall off.  

Several hormones affect abscission, some by inhibiting it, some by stimulating it.  Petioles were breaking before the hormonal pathway had kicked in.  But eventually, it did, and then the once stubborn stubs became stubborn no more.

BUDS

What were the buds doing as the leaves were falling?

Buds low on the tree, where there were no marcescent leaves, remained essentially the same length through May.  The fluctuations were mostly (entirely?) from my inconsistency in measurement, though there might have been some actual swelling and shrinking with humidity and internal moisture (if internal moisture changes, I don’t know).  By the end of May, there was no sign that these buds were going to do anything at all.  Was the tree sacrificing its lower branches?  They had to go sometime, maybe this was it.

Buds higher on the tree, where the marcescent leaves were present, changed little through March, but began to swell in April.  (I used a distinctive lenticel on the branch as a unit of measurement because the buds were out of my reach.)  

But not all buds changed at the same rate.  Some were only swelling while others had already burst with new tissues showing.  These tissues turned out to be stems and leaves, no flowers.  The buds that swelled only a little stopped swelling and remained intact by the end of May.  Will they burst later?  We’ll see.

Most marcescent leaf blades were gone before the buds began to swell.  But true abscission happened later.  The buds still hadn’t begun to lengthen when the first petioles fell, but the first sign of enlargement was evident shortly before the snow pulled off the last of the leaves.  I suspect that hormones were stirring well before the buds started to expand, initiating the formation and growth of tissues at essentially the same time as the marcescent leaves were being set free.

NEW LEAVES

The new leaves and stems were tinged with red, the brief pulse of color before they were fully expanded (see Robert Frost: “Nothing Gold Can Stay”).  And they expanded really fast.

The new stem elongated impressively in ten days. The new leaves took on their distinctive oak shape and became bright green.

Some buds among the new leaves had still not burst. They had begun to swell with the others, but then stopped. What is their fate?

Buds low on the tree showed no sign of growth, and the branch they were on – a significant branch on this small tree – had drooped about 25 centimeters from March to May. What is going to happen to the buds and this branch?

There remain many things to observe.

A FEW MORE WORDS

Marcescent leaves are remnants of a bygone year, a season of productivity that went dormant for the winter. They have no reason to linger when the new season bursts forth, which is when they finally fall off. But perhaps we shouldn’t be surprised that there are a few that don’t play by what we think are the rules. Here are two brown leaves that were hanging on well into May.

Many arthropods (insects, arachnids, and others) live and hide among the tissues of trees. Some blend in so well that we miss them. I almost missed this spider among the buds in April. It was among these buds for three successive days, then was gone. Perhaps it is still somewhere on the tree. I don’t expect I’ll find it.

One more thought: This red oak tree, though small, has a lot going on: lingering leaves, buds that burst, buds that swell, buds that remain small, branches that burst forth with green, and branches that droop and remain gray. Such small trees might be useful – and accessible – as subjects for explorations of developmental physiology. If that is your field of interest and expertise (it is not mine), go for it.

Further reading

Earl Berkley 1931. Marcescent leaves of certain species of Quercus. Botanical Gazette 92: 85-93.

Robert Hoshaw and Arthur Guard 1949. Abscission of marcescent leaves of Quercus palustris and Q. coccinia. Botanical Gazette 110: 587-593

https://www.jstor.org/stable/2472663?seq=1#metadata_info_tab_contents

https://www.woodlands.co.uk/blog/woodland-activities/leaves-in-winter-marcescence/

Goldenrod Growth

To elongate or not to elongate

The goldenrods are sending up their stems in spring.  Or not.

This is a growing stem of early goldenrod, Solidago juncea, and it might get to be a meter tall.

This is also early goldenrod, but it won’t get tall.  It will remain a rosette, a cluster of leaves close to the ground.  The leaves are large, sometimes quite large, but the plant remains short.

This is another species, probably S. arguta, sharp-leaved goldenrod (I’ll check my identification later in the season).  Sometimes they grow even taller than early goldenrod.

But sometimes they remain a rosette.

Goldenrods are perennials, but even so, a plant that produces a tall stem in one year might be a rosette the next year, and vice versa.  Or the plant will be the same form this year as last.  I don’t know what triggers a switch, or triggers a repeat, but the growth form is one or the other each year, not in between.  

Other goldenrods nearly always produce all tall stems, almost never any rosettes.  These clusters of goldenrods (probably S. rugosa and S. canadensis) are all growing tall.  Next year, they will grow tall again.  Some plants might not grow tall enough to flower, but they will have an elongated stem.

In the key for the identification of goldenrods in Gray’s Manual of Botany, Merritt Fernald split major groups of goldenrods according to how their leaves and stems grow.  On some, the basal leaves are the largest, and the leaves decrease in size dramatically going up the stem – if there is an elongated stem – or remain as a rosette.  

In contrast, the leaves on other goldenrods are similar in size up the stem, and the plants hardly ever form rosettes of leaves.  

What’s going on in each growth form?  Defining a few terms will help:

What’s a node?  That’s a location on a stem where one or more leaves are attached.

What’s an internode?  The stem between nodes.

What’s a rosette?  A stem that didn’t elongate its internodes. 

What makes a stem tall?  One or more elongated internodes.

Somewhere in the ancestry of goldenrods, one or more lineages went all in on elongated internodes, and others retained internode plasticity, able to elongate them or not.  I don’t know the underlying mechanisms of internode development, but I am pretty sure hormones are involved, probably gibberellins.  In some, they are always on (expressed).  In others, they can be turned on or off, apparently early in the growth of any one stem.

I think this difference is impressive, one more aspect of the genus Solidago that intrigues me.

This is the only website I’ve yet found that points out this developmental contrast.  From my perspective, its inclusion speaks well of the authors of the website.

https://michiganflora.net/genus.aspx?id=solidago

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