Insect Egg Killers

© Copyright Walter Baxter and licensed under CC BY-SA 2.0

Plants and herbivores are engaged in an evolutionary arms race hundreds of millions of years in the making. As plants evolve mechanisms to avoid being eaten, herbivores evolve means of overcoming those defenses. Our understanding of these dynamics is vast but largely focused on the actual act of an organism consuming plant tissues. However, there is growing evidence that plants can take action against herbivores before they are even born.

Taking out herbivores before they even have a chance to munch on a plant seems like a pretty effective means of defense. Indeed, for a growing number of plant species, this starts with the ability to detect insect eggs deposited on or in leaves and stems. As Griese and colleagues put it in their 2020 paper, “Every insect egg being detected and killed, is one less herbivorous larva or adult insect feeding on the plant in the near future.” Amazingly, such early detection and destruction has been found in a variety of plant lineages from conifers to monocots and eudicots.

Gumosis in cherries is a form of defense. Photo by Rosser1954/Public Domain

There are a few different ways plants go about destroying the eggs of herbivores. For instance, upon detecting eggs on their leaves, some mustards will begin to produce volatile compounds that attract parasitoid wasps that lay their eggs on or in the herbivore’s eggs. For other plants, killing herbivore eggs involves the production of special egg-killing compounds. Research on cherry trees (Prunus spp.) has shown that as cicadas push their ovipositor into a twig, the damage induces the production of a sticky gum that floods the egg chamber and prevents the eggs from hatching. Similarly, resin ducts full of insect-killing compounds within the rinds of mangoes will rupture when female flies insert their ovipositor, killing any eggs that are deposited within.

One of the coolest and, dare I say, most badass ways of taking out herbivore eggs can be seen in a variety of plants including mustards, beans, potatoes, and even relatives of the milkweeds and involves a bit of sacrifice on the plant end of things. Upon detecting moth or butterfly eggs, leaf cells situated directly beneath the eggs initiate a defense mechanism called the “hypersensitive response.” Though normally induced by pathogenic microbes, the hypersensitive response appears to work quite well at killing off any eggs that are laid.

“Leaves from B. nigra treated with egg wash of different butterfly species and controls inducing or not a HR-like necrosis. Pieris brassicae (P. b.), P. mannii, (P. m.), P. napi (P. n.), and P. rapae (P. r.) and Anthocharis cardamines (A. c.) induce a strong HR-like necrosis. Egg wash of G. rhamni (G. r.) and Colias sp. (C. sp.) induces a very faint response resembling a chlorosis and does not fit into the established scoring system (faintness indicates 1, but showing up on both sides of the leaf indicates 2).” [SOURCE]

Once eggs are detected, a signalling pathway within the leaf ramps up the production of highly reactive molecules called reactive oxygen species. These compounds effectively kill all of the cells upon which the butterfly eggs sit. The death of those plant cells is thought to change the microclimate directly around the eggs, causing them to either dry up or fall off. These forms of plant defense don’t stop once the eggs have been killed either. There is evidence to suggest that the hypersensitive response to insect eggs also induces these plants to begin producing even more anti-feeding compounds, thus protecting the plants from any herbivores that result from any eggs that weren’t killed.

Plants may be sessile but they are certainly not helpless. Defense mechanisms like these just go to show you how good plants can be at protecting themselves. Certainly, the closer we look at interactions like these, the more we will discover about the amazing world of plant defenses.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2]

Why Plant Relationships Matter for Caterpillars

Photo by Judy Gallagher licensed under CC BY 2.0

When it comes to caterpillars, plant diversity matters. By studying nearly 30,000 plant-caterpillar interactions across three continents (Asia, North America, and Europe), scientists have uncovered important insights into lepidopteran biodiversity in temperate broadleaf forests.

Plants and the caterpillars they host are engaged in an evolutionary arms race. As plants evolve different defenses, caterpillars evolve new ways overcoming them. As you can imagine, studying these intricate relationships can be as fascinating as it is challenging. One could easily spend a lifetime trying to understand the relationships among only a handful of species. However, by taking a step back and asking bigger questions related to evolution and herbivory, scientists have found some interesting patterns than help describe the diversity of plant-caterpillar relationships.

As one might expect, they found that as plant diversity increases, so too does the diversity of caterpillars an ecosystem can support. Many caterpillars specialize on one or only a few different host plants and these are often (though not always) within the same plant family. The reason for this has to do with plant defenses. The more closely related plants are, the more likely they are to share similar defense strategies. For instance, most milkweeds (Asclepias spp.) produce toxic compounds called cardiac glycosides and many different members of the nightshade family (Solanaceae) produce similar suites of toxic alkaloids. As a result, insects that munch on their tissues have similar hurdles to overcome in an evolutionary sense.

The more closely related plants there are in an environment, the more likely it is that the caterpillars they host can jump from one plant species to another. As a result, ecosystems that boast relatively few plant lineages support relatively few caterpillar species in part because the caterpillars they do host can more easily jump from plant species to another. The same logic applies in the opposite direction as well. Ecosystems comprised of a diversity of plant lineages limit the likelihood that any given species of caterpillar can find multiple different hosts. Because each clade of plants produces their own brand of herbivore defenses, the caterpillars hosted by each are also more likely to be different. Thus, as plant diversity goes up, so too do the numbers of caterpillar species an ecosystem can support.

Though not tested by this research, this also provides yet another example of why invasive plants harm biodiversity. Plants from other areas of the world are more likely to present novel defenses to native herbivores. If the caterpillars do not have what it takes to overcome these defenses or simply don’t recognize the plant as food, the fewer caterpillars that ecosystem can support.

Of course, none of this should come as a surprise to those interesting in native plants and gardening. The more indigenous plants you grow in and around your landscape, the more insects you can support. I also firmly believe that the results of this research are not limited to caterpillars. The same pattern likely applies to any number of plant eaters, from microbes to mammals, no matter where you look. What this research gives us are some answers to questions like “why does biodiversity matter?”

Photo Credit: [1]

Further Reading: [1]

Fossils Shine Light On the History of Gall-Making Wasps

M J Richardson / Common spangle galls / CC BY-SA 2.0

We can learn a lot about life on Earth from the fossil record. I am always amazed by the degree of scrutiny involved in collecting data from these preserved remains. Take, for instance, the case of gall-making wasp fossils found in western North America. A small collection of fossilized oak leaves is giving researchers insights into the evolutionary history of oaks and the gall-making wasps they host.

Oaks interact with a bewildering array of insects. Many of these are gall-making wasps in the family Cynipidae. Dozens of different wasp species can be found on a single oak tree. Female wasps lay their eggs inside developing oak tissues and the larvae release hormones and other chemicals that cause galls to form. Galls are essentially edible nursery chambers. Other than their bizarre shapes and colors, the compounds released by the wasp larvae reduce the chemical defenses of the oak and increase the relative nutrition of the tissues themselves. Often, these relationships are precise, with specific wasp species preferring specific oak species. But when did these relationships arise? Why are oaks so popular? What can fossil evidence tell us about this incredible relationship?

Photo by Beentree licensed under CC BY-SA 4.0

Though scant, the little fossil evidence of oak galls can tell us a lot. For starters, we know that gall-making wasps whose larvae produce structures similar to that of the Cynipids have been around since at least the late Cretaceous, some 100 million years ago. However, it is hard to say for sure exactly who made these galls and exactly what taxonomic affinity the host plant belongs to. More conclusive Cynipid gall fossils appear again in the Eocene and continue to pop up in the fossil record throughout the Oligocene and well into the Miocene (33 - 23 million years ago).

Miocene aged fossils are where things get a little bit more conclusive. Fossil beds located in the western United States have turned up fossilized oak leaves complete with Cynipid galls. The similarity of these galls to those of some present day species is incredible. It demonstrates that these relationships arose early on and have continued to diversify ever since. What's more, thanks to the degree of preservation in these fossil beds, researchers are able to make some greater conclusions about why gall-making wasps and oaks seem to be so intertwined.

Holotype of Antronoides cyanomontanus galls on fossilized leaves of Quercus simulata. 1) Impression of the abaxial surface of the leaf, showing the galls extending into the matrix. 2) Galls showing close association with secondary veins. 3) Gall showing the impression of rim-like base partially straddling the secondary vein. 4) Close-up of gall attached at margin extending down into the matrix. 5) Gall uncovered revealing spindle-shaped morphology.

1) Xanthoteras clavuloides galls on fossilized Quercus lobata, showing gall attached to secondary vein. Specimen in California Academy of Sciences Entomology collection, San Francisco. 2) Two galls of attached to a secondary vein showing overlap of their bases. Specimen in California Academy of Sciences Entomology Collection, San Francisco. 3) Three galls collected from leaf of California Quercus lobata showing clavate shape and expanded, ring-like base. 4) Gall showing the annulate or ribbed aspect of the base, which is similar to bases of Antronoides cyanomontanus and A. polygonalis. 5) Galls showing clavate shape, pilose and nonpilose surfaces, and bases.

Gall-making wasps seem to diversify at a much faster rate in xeric climates. The fossil records during this time show that mesic tree speciess were gradually being replaced by more xeric species like oaks. Wasps seem to prefer these drier environments and the thought is that such preferences have to do with disease and parasite loads.

Again, galls a large collections of nutrient-rich tissues that are low in defense compounds. Coupled with the juicy grub at their center, it stands to reason that galls make excellent sites of infection for fungi and other parasites. By living in drier habitats, it is believed that gall-making wasps are able to escape these environmental pressures that would otherwise plague them in wetter habitats. The fossil evidence appears to support this hypothesis and today we see similar patterns. White oaks are especially drought tolerant and its this group of oaks that host the highest diversity of gall-making wasps.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]

The Trumpet Creeper

Photo by beautifulcataya licensed under CC BY-NC-ND 2.0

With its impressive bulk and those stunning tubular red flowers, one would be excused for thinking that the trumpet creeper (Campsis radicans) was a tropical vine. Indeed, the family to which it belongs, Bignoniaceae, is largely tropical in its distribution. There are a handful of temperate representatives, however, and the trumpet creeper is one of the most popular. Its beauty aside, this plant is absolutely fascinating.

As many of you probably know, the trumpet creeper can reach massive proportions. In the garden, this can often result in collapsed structures as its weight and speed of growth is something few adequately prepare for. In the wild, I most often see this vine in somewhat disturbed forests, usually near a floodplain. As such, it is supremely adapted to take a hit and keep on growing year after year.

Photo by Maja Dumat licensed under CC BY 2.0

One of the many reasons this plant performs so well both where it is native and where it is not is that it recruits body guards. This is easy to witness in a garden setting as the branches and especially the flowers are frequently crawling with ants. Trumpet creepers trade food for protection via specialized organs called extrafloral nectaries. These structures secrete sugary nectar that is readily sucked up by tenacious ants. When a worker ant finds a vine, more workers are soon to follow. 

Amazingly for a temperate plant, trumpet creepers produce more extrafloral nectaries of all four categories - petiole, calyx, corolla, and fruit. What this means is that all of the important organs are covered in insects that viciously attack anything that might threaten this sugary food supply. Hassle one of these vines at your own peril. With its photosynthetic and reproductive structures protected, trumpet creepers make a nice living once established.

Photo by Salicyna licensed under CC BY-SA 4.0

Reproduction is another fascinating aspect of trumpet creeper biology. A closer inspection of the floral anatomy will reveal a bilobed stigma. Amazingly, this stigma has the ability to open and close as potential pollinators visit the flowers. Stigmatic movement in the trumpet creeper has attracted a bit of attention from researchers over the years. What is its function?

Evidence suggests that the opening and closing of the lobed stigma is way of increasing the chances of pollination. Touch alone is not enough to trigger the movement. However, when researchers dusted pollen onto the stigma, then it began to close. What's more, this action happens within 15 to 60 seconds. Amazingly, there appears to be a threshold to whether the stigma stays closed or reopens after 3 hours or so.

Photo by Jim Conrad (Public Domain)

It turns out, the threshold seems to depend on the amount of pollen being deposited. Only after 350 grains found their way onto the stigma did it close permanently. Experts feel that this a means by which the plant ensured ample seed set. If too few pollen grains end up on the stigma, the plant risks not having all of its ovules fertilized. By permanently closing after enough pollen grains are present, the plant can ensure that the pollen grains can germinate and fertilize the ovules without being brushed off.

It is interesting to note that the flowers frequently remain on the plant after they have been fertilized. This likely serves to maintain a largely floral display that continues to attract pollinators until most of the flowers have been pollinated. Speaking of pollinators, observations have revealed that the trumpet creeper is pollinated primarily by ruby-throated hummingbirds. Although insects like bumblebees frequently visit these blooms, bringing pollen with them in the process, hummingbirds, on average, bring and deposit 10 times as much pollen as any other visitor. And, considering the threshold on pollen mentioned above, trumpet creeper appears to have evolved a pollination syndrome with these lovely little birds.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]

Snuffing the Fire

Photo by Peilun Hsu licensed under CC BY-NC 2.0

Few childhood memories are more fond to me than catching fireflies on summer evenings. These little beetles are famous the world over for their dazzling light displays. Using chemical means, they are some of the most efficient light producers ever discovered. Their displays are for the purpose of mating and there are as many variations on the theme as there are species. Sadly, like so many natural wonders that we take for granted, fireflies are disappearing from our wild places. Future generations may never know the joys of these natural fireworks. 

Exactly why we are seeing a decline in fireflies is not certain. Researchers are only just beginning to uncover the secret world of the firefly. The answer is undoubtedly complex, however, evidence is beginning to pour in that we should look no farther than ourselves for the cause.

Fireflies require a few things to get by. The first is some sort of standing water. They seem to love ponds, creeks, rivers, and vernal pools. Second is tall grass and a lot of forest litter. Their larvae live and hunt in and amongst fallen logs and plant litter. Though we aren't entirely sure what their larvae eat, they are certainly hunting things like snails, slugs, and small insects, which also require moist areas with a lot of debris. Fireflies also need taller plants like grasses. They will climb up the stems to begin their aerial light displays. Finally, fireflies need darkness. They communicate by light and any surplus light sources are likely to mess them up. 

With increasing human development, former firefly habitat is giving way to paved roads and chemical laden lawns. Mowers run endlessly during the summer, eliminating fireflies and their habitat. People are needlessly clearing land of brush piles and fallen logs, which their larvae as well as their prey need. Light pollution is only getting worse too. As with many other insects, the wanton use of insecticides are undoubtedly taking their toll as well. Areas that once harbored huge populations of fireflies are quickly becoming overrun with human traffic as new housing, commercial and other forms of development garble up what free land remains. 

At this point you may be wondering what you can do to help. If you are a land owner, please consider the following:

- Turn off outside lights at night when they aren't needed
- Let logs and other plant debris accumulate in places around your property
- Consider creating a water feature of some sort
- Avoid the use of pesticides and fertilizers on your lawns
- Plant native plants
- Don't over-mow your lawn and leave some areas un-mowed

The best part about these solutions is that they benefit so much more than just fireflies. Native wildlife will be all the better if you take these steps to making your property more ecologically friendly. We are lucky to be aware of this issue but we must take matters into our own hands. Get out there and enjoy nature and try to be a bit of steward at the same time. 

Photo Credit: Peilun Hsu (http://bit.ly/1rJkufG)

Further Reading:
http://www.firefly.org/why-are-fireflies-disappearing.html