Data I’ll never publish II: Salinity and herbivory

I spent a lot of my second year of grad school thinking about salinity and insect herbivory. Generally, insects don’t like very much salt (i.e. how many marine insects have you seen?). Salt is a fairly effective herbivore deterrent – an observation seemingly first made in 1980 by D. Newbery in an Oecologia paper on mangrove herbivory. I made the same observation, and tested it experimentally, in chenopods in a 2014 paper (also in Oecologia – they’ve seemingly cornered the salinity/insect herbivory market).

Coconut palms might be the most widespread and useful (to human) halophytic plant. They were useful for that hammock, at least.  Abaco Island, Bahamas, 2011.

Plants are also affected by salt and have myriad ways to deal with it, basically all variations on either excluding it, sequestering it, or excreting it. Obviously some plants are much better at dealing with salt than others (see mangroves, Zostera, etc.) – we call plants that are adapted to saline environments “halophytes” (i.e. salt plant in Greek). I happened upon a little, weedy, nonnative, and pretty much unremarkable chenopod – Oxybasis glauca – growing at the edge of a building in Davis and somehow I decided it was a pretty cool plant. Given all the other cool halophytes available, I’m not sure why I chose this plant to do a bunch of experiments on, but I did.

This is Oxybasis glauca growing in volcanic sand on the edge of Mono Lake, Mono, CA. I was with a group of about 30 people when I found this and was very excited. I couldn’t really even articulate a single cool thing about the plant – it is salt tolerant, but every plant in that area is salt tolerant. Maybe the coolest thing is that Oxybasis species have really small seeds compared to Chenopodium or Atriplex… maybe there is nothing special about it?

Like most Atriplex and Chenopodium (the genus which Oxybasis was split from) species, Oxybasis glauca has salt bladders – little bubble like trichomes which the plant shunts salt to and then they burst, an odd but effective form of salt excretion. This leaves a layer of salt on the outside of the plant. This protects the plant from herbivory somewhat.

 

Pre- (above) and post- (below) bladder burst O. glauca leaves (lab-grown).

 

Because O. glauca is salinity-tolerant and the primary herbivore of most weedy chenopods in the valley, the spotted cucumber beetle (Diabrotica undecimpunctata), doesn’t like salt (see my 2014 paper), I wondered if there might be a refuge from herbivory effect at higher salinities and maybe there would be an intermediate salinity where the plant would still grow well, but herbivores would be deterred. So I did an experiment – I grew plants in three salinities* and then exposed half of them to a week* of cucumber beetle herbivory. I expected herbivore pressure would be most intense at low salinities, but also growth would be retarded at higher salinities.
So the hypothesis looks something like this – if plant “performance” is on the y-axis and the green line is effect of herbivory and grey the effect with solely salinity, if there is some overlap, the plant might do best at that overlap point (or it might not). (note: this is not a particularly good graphical representation for a number of reasons).
What did I find?
Plant response to salinity (w/o herbivores):
Salinity increasing left-right. Standard deviation plotted.
Plants did worse as salinity increased (as expected).
Herbivory:
Salinities increasing in treatments 1-4. Standard deviation plotted.
Total leaves damaged by the herbivores decreased with increasing salinity (as expected, as they are less palatable), but because the plants had fewer leaves, the proportion damaged increased.
THE INTERACTION
Biomass of plants. Dark green: with herbivores, light green: without herbivores. Salinity increasing left to right. Standard deviation plotted.
Sadly, there wasn’t. Beetles didn’t really have an effect on biomass (or any other metric). Maybe I didn’t have them in there for long enough? Maybe they really don’t have a fitness effect (I can certainly believe this).
Maybe this data will be useful to someone. Email me for the sheets.
*Note: the exact procedures are in one of about 40 notebooks in my office, so I don’t actually know exactly the salinities or number of days right now. If anyone is interested for any reason, I can easily dig this up.

Mark and recapture project for students!

I’ve had the pleasure of teaching many groups children from preschool to high school age during the last decade or so in a variety of settings: camps, classrooms, field trips and informal natural history discoveries on the sidewalk (just recently jumping galls in the Central Valley here).

One activity that I have done a few times, and particularly enjoyed, was doing a mark and recapture study on dragonflies with elementary/middle school students. In my opinion, it is a pretty perfect project – you get to teach the scientific method, a little bit of math, and a good bit of natural history. I didn’t come up with this project (I think Taylor Yeager, of Mass Audubon, suggested doing it with grasshoppers, initially – but that was the summer of 2006 or 2007, so my memory is a bit hazy) but I’ve run it a few times with kids from ~9 years old to high school age.

Hetaerina americana, the American rubyspot, my favorite odonate in California. A damselfly, these are just as suitable for the study described here, though a little more fragile.

The goal of the project is simply to estimate the number of dragonflies in a given area such as a large field or a pond. You could easily adapt this to grasshoppers, milkweed beetles or any other larger invertebrate that can be easily handled and marked (bumblebees or butterflies might not be as good). Mark and recapture is a standard technique used in wildlife studies and the basic idea of it is very simple – you mark a known number of animals, then you go back and capture a bunch and see what proportion of that sample was marked. Obviously, in real-world applications, the math is much more complicated, but for our purposes, if we mark ten bugs the first day and capture 10 the second day, two of which are marked, we have a population size of 50.

 

You’ll almost certainly see Pantala flavescens, the world’s most widespread dragonfly. Catching them is a bit harder – they fly high and fast! This is a female.

Dragonflies are supremely suited to this activity however. They are often abundant, easy to handle and mark, children generally have no aversion to them and they are just challenging enough to catch to occupy students for hours (and to get lots of energy out while running around the field with nets!).

Rhionaeschna sp. Chiloe Island, Chile. WHO DOESN’T LOVE DRAGONFLIES?!?

Of course, the first thing you should do is to get all the students to guess the number of dragonflies in that area. They generally have no idea; guesses vary by orders of magnitude (from 10 to 1 million!). Then it is just a matter of giving everyone nets, teaching them to safely handle dragonflies and going out and catching ’em. We’ve used normal sharpies and put a dark band on both forewings of the individuals we captured as a mark. For easier record-keeping, we set up a station in the center with the sharpies. I found with younger students, it was easier (and safer for the insect) if I took it out of the net and marked it (those being the two steps where wings are easily shredded or broken), then let the students identify and measure it. Taking dragonflies out of nets isn’t hard – put your pointer and middle fingers on opposite sides of their body and gently move their wings up so that you have all four together and remove from net. Even 12 year-olds can remove and mark with proper instruction. Most dragons will need a few seconds to pump haemolymph back into their wings after this process; you can place the dragonfly on the catcher’s nose – this is especially entertaining for all others involved!

An Aeshna/Rhionaeschna sp. This is the proper way to hold dragonflies; using two fingers, pinch the four wings together gently. To mark it with sharpie, it helps to put the wings flat on a clipboard and gently put a small mark. This is a female – note the lack of a bulge on the bottom of the first couple abdominal segements (compare to photos below).

 

The other way to hold dragonflies is to firmly grip the upper segments of their legs (I usually try to hold two – though I am holding only one in this photo) between thumb and forefinger. This allows viewing of the wing pattern and veination, but is trickier and requires some practice to not rip off legs and let the dragonfly get away. They can also bite you in this grip, not a problem for little ones, but big Aeshnids can draw blood!

With high school groups, I’ve taught them how to sex the dragonflies and then made comparisons of male and female sizes and sex ratios. This is an interesting activity, as in upland areas, most caught are females and near water bodies, most are males (you can count on this result with all but a few uncommon species). The reason is that males of most species patrol territories near prime egg-laying spots and catch the females and mate with them immediately prior to egg laying. Females, being harassed constantly near water, generally forage in areas farther away. This is especially pronounced in Enallagma damselflies – the bright blue males may be found by the hundreds at any pond, but its really hard to find the duller females nearby – sex rations on a local scale may be 100:1 or more!

Blue dasher, Pachydiplax longipennis, one of the most abundant dragonflies in the US. Note the water mites on its abdomen – these have really interesting natural histories (too long to describe here, but look them up). Also, note the bulge on the lower side of the first couple abdominal segments – this is a male (compare above).

The next day, we go back out and catch them again – to avoid double counting individuals, we use a second color sharpie on these. Then we conclude by doing the calculation of total population size, figuring out who was closest (the most exciting part for the students) and discussing the drawbacks. The students have always come up with good hypotheses for why the estimate might not be accurate (there were too many high-flying dragonflies, one day was cloudy, etc.) and it generally provides good fodder for a short and informative discussion. With older students, summary statistics on sex ratio, the body size measurements and population sizes of each species can be done and discussed.

The only individuals we don’t mark are tenerals – these are just emerged and they have not fully dried their wings and marking would almost certainly hurt them). Note the glistening wings and really pale body. In another day or two, this Sympetrum sp. will be cherry red!

 

No dragonfly post would be complete without this monster. Arguably the world’s largest dragonfly, Phenes raptor lives in bogs in Patagonian Chile and Argentina and has somewhat terrestrial nymphs, an oddity for an odonate. Males also have the coolest set of abdominal claspers (those projections at the tip of the abdomen) of any of the hundreds of species I’ve seen!

 

The eyes of emeralds, family Corduliidae, lend them that common name.

 

What you’ll need (not very much!):

1) Nets – 1 per student is ideal, but partners are fine, too. Wooden-handled aerial nets are not expensive (<$10) and will last a long time and take a good bit of abuse.
2) Sharpies
3) A good field guide. I use Dennis Paulson’s excellent guides for the US, though there are really good regional ones, like Blair Nikula’s Massachusetts guides and others. Identifying dragonflies and damselflies in all but a few genera (Sympetrum, Enallagma) is really simple and can be done by most high school age children with pretty good accuracy.
4) Clipboard, data sheets.
5) Two days of predicted sunny weather!

You could have students make nets. During a trip to Peru, my net was stolen within a week. I bought some mosquito netting, bailing wire and made this net for <$1. It lasted me the whole season without issue – several of the dragonflies on this page were caught with it.

Do give this a try next year if you have students for a couple days! Let me know if you do, I’d love to hear how it goes.

Another interesting thing to note – and could be measured by the students – is the size of the wings (length, width). This dragonfly, Pantala flavescens, has HUGE wings for its size. Unsurprisingly this species is probably the most migratory and best dispersing insect – of any group – on earth. You can find this species near you – pretty much nomatter where you live!

 

A meadowhawk (Sympetrum sp.) like above. This is a male – told by the bulge in the abdominal segments as well as its red color (females of this genus are yellowish).

 

Sticky plant attraction, a new paper

I could not possibly do as good a job as the summary of this paper written by Elizabeth Preston, here, so I’ll first tell the backstory – and when it comes out, I’ll detail another cool part of it (which is not in the preprint version – instead buried in the appendices).

Heliothis phloxiphaga eating a flower bud of Aquilegia eximia, the serpentine or sticky columbine. Lake County, CA. 

Last summer, I spent most of my time studying Trichostema laxum, which I’ve written about in a previous post. I was trying to test my hypothesis that external chemical defenses are easily washed away by rain (which may be driving the pattern that few are found in wet areas) and spent countless hours doing various pieces of this – observing pollinators (the volatiles washed off might affect pollination positively or negatively), counting insects, leaves, buds, flowers and seeds (which can be done in situ!) and by July, the drought and some jackrabbits made this experiment look rather grim. I still haven’t brought myself to analyze these data, as it ended so depressingly…

As the floral color polymorphism was not in the experimental population, I didn’t notice it until the end of the summer, and gathered some, but not enough, data on it. Lake County, CA. 

I was getting a little frustrated and I wandered around, naturalizing, which is always a remedy for frustration (to me at least). I came upon a columbine – Aquilegia eximia – which I instantly knew held some potential for cool experiments. The first thing I noticed was that it was extremely sticky and covered in dead insects and the second was that it had a bunch of predators on it. I immediately thought to Billy Krimmel and Ian Pearse’s cool paper on tarweeds (doi:10.1111/ele.12032) , in which they demonstrated that the dead insects provided food for predators, which protected the plant from herbivores. I figured initially that I would simply test this in another system.

A Hoplinus nymph (probably the most important predator in the system) approaches an
entrapped fly.

Because I was out in the field without access to a genetics lab to get dead flies, I couldn’t replicate their design – where they added dead fruit flies to plants to supplement carrion – so instead I removed all the carrion from half of my 50 plants, hypothesizing that I would get a decrease in predators and an increase in herbivory (which we did!). I also thought hard about what else to test to add to Billy and Ian’s work. I thought that, perhaps, it would be interesting to test whether the plants attracted the various entrapped insects (mostly small flies, wasps and beetles) somehow. Lots of plants attract insects – pollinators are the most obvious, but volatile signals attract predators, other herbivores and even birds (doi: 10.1111/ele.12177). Having petri dishes, plastic mesh and tanglefoot in my field kit – I made little sticky traps, with a sticky mesh top and a petri dish bottom and I put either columbine stems and leaves (a very small amount) or nothing in them. Collecting them 24 hours later, I found that the dishes with columbine had higher insects than the empty ones (which would demonstrate the ambient rate of insects landing on these traps). The trapped insects were also little flies, wasps and beetles, just like on the plants themselves.

Dead Hymenoptera on columbine (I may be giving up entomologist credentials, but I am not sure whether it is a wasp or an ant alate).

So this became a story – perhaps logically – that the plants were somehow attracting insects to kill and feed to the beneficial predators on their surfaces (retaining their services). I presented this work in a talk at a little student conference at Davis during recruitment weekend and played with several ways to frame the story. The first was to be rather dry – columbines attract insects and control a tritrophic defense (or something along those lines). Instead, I thought long and hard about trying to make a metaphor (socialism – a worker’s paradise for the predatory bugs or a Roman bread and circus type thing, but they didn’t really work) and while I don’t remember how I came up with this – I settled on the sirens, figures of classical mythology who lured sailors to their deaths. I therefore framed it as these poor insects – innocent sailors of the California air – are somehow drawn to their deaths on the columbines. Of course, the columbines put the insects to good use in their defense, leaving open the question – which I am sure classical mythologists lose much sleep over – what did the sirens do with their collection of dead sailors?

Serpentine columbines in flower. Lake County, CA. 

Read more here!

Eric F. LoPresti, Ian Seth Pearse, and Grace K. Charles In press. The siren song of a sticky plant: columbines provision mutualist arthropods by attracting and killing passerby insects. Ecology. http://dx.doi.org/10.1890/15-0342.1

External Chemical Defenses; a natural history view

After a long hiatus (due to qualifying exams, a dissertation proposal, hundreds of Trichostema in the greenhouse and a couple manuscripts in progress), here is the long-awaited (by likely my parents, grandparents, and maybe 3 other people) blog post that outlines THE TOPIC OF MY DISSERTATION (in a rather digressive way). I took a long and circuitous route to find this topic, but its very exciting to me and when I’ve presented it publicly (at Ecological Society of America meeting last August and informally to many colleagues), it has been greeted with interest. This post will mostly deal with how I got to this topic and why it is a great thing for insect-plant biology, a later one will detail the implications (a paper on which was recently accepted!).

The stem of Trichostema laxum (Lamiaceae). The little bubbles on the tips of the glandular trichomes are full of an oily, wonderfully vinegary-smelling fluid, of unknown utility.

Coming from the northeast and botanizing and entomologizing in Massachusetts and Rhode Island, I knew my fair share of plants – herbs and trees, shrubs and aquatic plants – and many of their associated insects. Coming to California, I was exposed to more. A whole lot more! I met a great deal of new taxa – from aromatic plains of sagebrush, to strange long-horned moths (Adela) to smooth barked manzanitas and madrones to the most spectacular diversity of wildflowers I’ve ever seen to albatrosses wheeling offshore. As a natural historian, I had a lot to learn and I continue to learn new plants, insects and even occasionally birds, each time I venture out to a new spot, and often when I revisit old spots.

A Mormon cricket, Anabrus simplex. White Mountain, CA, Sept. 2012. A mind-blowingly large katydid.

In the first year, I bounced from project to project, trying to raise parasitoids from caterpillars, looking at galls and leaf miners for inspiration and tractable systems to look at the effects of plants and their chemistry on predators and parasitoids of the herbivores feeding on those plants. None of which really panned out individually, but I spent a lot of time wandering around with a net, hand lens and notebook. And I learned a lot of natural history of California, the coast, the coast range, the valley, and the west side of the Sierras, in the process.Then I happened upon the chenopod Blitum californicum and ended up spending a summer playing with various chenopod species and getting a paper out of it. This, coupled with work done with Billy Krimmel and Ian Pearse (see their research here), got me thinking about plants really hard. And I realized a major difference between plants back home and here: plants back home are generally rather glabrous (smooth-surfaced) and here they have a myriad of glandular trichomes, general stickiness, oiliness, resins, etc. and they often have strong smells (the latter point is an oft-mentioned feature of plants of Mediterranean climates and why many of our kitchen spices – e.g. sage, rosemary, oregano – hail from these areas).

The glandular trichomes on Aquilegia eximia (serpentine columbine: Ranunculaceae)
are extremely sticky and entrap enormous quantities of small flies and wasps.

So I spent more time seeking out these plants and examining them closely. I also spent time reading the literature on secretory tissues in plants. And I began to think, abstractly, what might the differences between compounds put onto plant surfaces and those inside a plant be? I made lists, I woke up in the middle of the night with ideas and eventually, I distilled these many ideas into five broad differences between the potentially-defensive chemical secretions and those sequested inside plant tissues. These form the meat of the accepted paper, to be detailed later. Instead I’ll briefly touch on how I experiment with external chemicals and why this is important and exciting.

The first approach I took to playing with (e.g. experimenting) external chemicals was testing their efficacy at preventing damage to plants. I did, and continue to do, this in two ways (in the chenopod paper as well as many small unpublished tests). The first way is to remove or reduce the defense, gently, using a paintbrush or a sponge, leaving the leaf surface intact. I then run choice or no choice (palatability) assays on these plants, usually using a the wonderfully generalist spotted cucumber beetle, which rarely fails to eat at least a little bit of a plant (but also will eat a LOT of a plant it likes). The second is to take the external chemical (either from the plant or the known chemical) and place it onto another plant, either in natural concentrations, or varying the concentrations, looking for changes in herbivory.

Lab-grown, highly glandular Antirrhinum californicum.

I also do this in the field – something that is really hard to mimic with internal chemicals. Removal of exudates from plants can be difficult (e.g. Yerba Santa, with really tough leaf resins), but can also be really easy, as in the case of Antirrhinum californicum, the California snapdragon. In the field, removing the exudates of this snapdragon caused a really interesting response: insects – mostly a heliothine noctuid, caused more damage to the exudate-removed plants,as expected; but mammals (deer and/or black-tailed jackrabbits) ate preferentially the plants with exudates intact (highly significant result). This suggests that the exudates may be both a defense and a liability in nature, an interesting result that wouldn’t have been possible in a lab – and wouldn’t have been easy to find with an internal defense, as manipulation would have been more difficult.

Deer or jackrabbit eaten A. californicum from the experiment. 

Why is this exciting? Well, a manipulation like this wouldn’t really be possible with internal defenses. There are two general ways to look at the effect of internal chemical(s) on other organisms: (1) find or create lines that differ in concentrations of chemicals, or look comparatively across species that differ, or (2) create, via genetic techniques, knockout lines that lack a compound. The problem with both approaches is that there is pleitropy (one gene doing multiple things). If you have lines that differ in a compound, they also differ in other aspects. Similarly, if you have a gene knockout (or duplication), that will almost certainly have effects on other aspects of the plant. Of course, there are positives to both of those approaches: the comparative approach allows investigations on evolution of a trait and a genetic manipulation allows a level of integration and detail that no field study on a natural population will ever approach. In my systems, it seemed like a good way to get at defense mechanisms. But was I the first to do it? Absolutely not, in the marine world (Mark Hay’s lab) have been doing these sorts of exudate removals on seaweeds for decades. Yet terrestrial folks don’t cite these papers or think in quite the same way (the marine folks cite terrestrial chemical ecology all the time!). And even they weren’t the first! Thomas Hartmann pointed out in a 2007 paper on the history of plant-insect science, Ernst Stahl, in ~1900, removed the acid droplets secreted by evening primroses (Oenothera spp.) and found that the plants became far more palatable to herbivorous snails and slugs. However, despite the ease and history of these experiments, they are rare. And I’m not quite sure why.

The most satisfying part of these investigations so far (more to be detailed soon) has been combining a variety of approaches to think about a problem. I’ve made lots of natural history observations (examined lots of plants with secretions), thought about problems in creative ways (can I think of fundamental differences between internal and external chemicals, ecologically?), read the literature (from Darwin and earlier, to the present), planned and ran experiments, interpreted data (with such strange results as in the snapdragons) and am working on integrating it all into a dissertation, which I hope – in a few years – will be a cohesive body of work that other people will add to, build upon and apply to new systems and problems.

A Caterpillar Mystery in the Bahamas

I’ve been in the Bahamas for the last two weeks, studying the effect of resource pulses (hurricane-wrecked seaweed) on island communities with this project. In doing so, we kept coming upon these strange shelters on wild guava (Psidium longipes), locally called Bahama stopper, since the hard wooded bush/tree would apparently stop any progress you try to make into the thick coppice.

What on earth is this? ~  4 cm tall (pretty damn big by insect standards).
Inside some of these shelters (2/13), there was an odd larva, apparently a beetle larva, or so I initially thought. Because I am mostly a caterpillar person, I didn’t really pay it much mind. 
A really terrible picture, but notice the “antennae”. About 2-3 cm long. 
Louie, while processing insect samples one night, noticed that some things were not right about the apparent beetle larva – namely it had prolegs, the fleshy appendages that give caterpillars the appearance of having more than the six legs all insects have. I then looked at the “antennae” and found that they were not segmented, a dead giveaway that this was, in fact, a caterpillar and the “antennae” were actually tentacles (yes, that is the technical term for the fleshy projections that many caterpillars have – monarchs for instance).
Several of the shelters were torn like this, suggesting predation (by a bird [?]). This was the
only shelter with lines affixing it at the top – many had lower lines. 
I got much more interested after that, and sent along these pictures to Charley Eisemann, a good friend and probably the person on earth with the most knowledge about insect shelters. His blog – linked above – is simply phenomenal and if anyone was going to know the answer, he would. Very quickly (within a few minutes), he had correctly found the family of the moth – Mimallonidae. The amazing part here is that Charley has never seen a member of this family! Mimallonidae is an extremely small family by Lepidoptera standards, ~200 spp. – only 3 of which occur regularly in the US, a fourth is described from the US in Brownsville, TX, but is probably a tropical stray. He even dug pretty deep and found a very likely species identity, Ciccinus packardii – known from Cuba and known to feed on other Psidium species. While I do not know this for sure, it seems that is the most likely candidate as the larva matches very well the few images of Ciccinus online, and less so the other mimallonid genera. 

After a bit more searching, we came upon a young larvae feeding in a leaf press on P. longipes, which was not what I expected. This family is known as the “sack-bearers” and I was expecting something more along the lines of a bagworm (Psychidae), instead of a leaf presser.

A young (2nd, 3rd instar?) larva of this Ciccinus sp. ~ 8mm

Which brings us to the strange, pitcher plant-like shelters. The larva is oriented vertically inside the shelter, with a strange butt plate plugging up the bottom hole and the head just below the upper hole. What function the little hood forms is mysterious – perhaps shading the larva from the hot Bahamian sun or fierce rains? The better-known Ciccinus species of the US, C. melshiemeri, feeds on old oak leaves (too tannic for most caterpillars) and constructs a shelter, sort of like the pictured ones of frass pellets, silk and oak leaves in which it spends the winter as a larva, prior to pupation in the spring. This seems to be the case for this species as well – in two cases, I found spent pupal skins.

Spent pupal skin (successful emergence!) inside one of the shelters. You can also see the construction of silk and what
appears to be finely ground frass (caterpillar poop – a common building material for cats). 

Interestingly, I did find one that fit the description of the C. melshiemeri shelters well.

This was the only shelter anchored into leaves (it was vacant, unfortunately). You can see well the frass pellets forming the top of the shelter here.
The same shelter, with a Psidium leaf forming one side. 

These guys kept me occupied for quite awhile (I even dreamt about them!) and seem like a worthy avenue for future rearing efforts… there are a great deal of questions that remain about the shelters: Why the strange shape with a hood? Why build a free standing shelter, as opposed to anchoring it to a stem like most moths? Why wait around in a shelter instead of pupating right away? Do the shelters protect inhabitants from predators and parasitoids?

perhaps the prettiest of all found. I like the subtle banding.

Many thanks to Charley, Julia Blyth, John De Benedictus, Louie Yang, Jonah Piova-Scott and Jenn Mckenzie (who was the only one that could find occupied shelters) for help with the identification and finding of these guys.