How to Catch a Bee

During my days as a summer camp unit director, one of my campers brought me a flower. When I looked closer, I realized this 11-year-old was not interested in the flower, but the bee and spider on top of the flower, head to head. I assumed they were fighting, but they were both pretty still, and it dawned on me that the bee was a goner, locked in the “jaws” of the spider, whom I figured had won the fight. I was still wrong, and the reality of this interaction is even cooler than I had imagined:

The spider hunts by hiding in flowers that attract their prey, the bees.

I posted my spider-bee-flower pic on Instagram recently, which received more interest than I had expected. I even had comments asking about the evolution of this flower-sitting ambush spider, so I looked it up, and I stumbled upon a cool chemical ecology story.

The first thing I learned is that, when given a choice, both the bees and this group of spiders, crab spiders, pick the same flowers. By covering the flower with saran-wrap and watching the spiders’ flower choice change, scientists were able to figure out that spiders are choosing their flowers by following chemical signals in the air, aka smells. This means these spiders are adapted to smelling out flowers that are more likely to attract their dinner.

Do bees fall for this trap? Bees not only fall for this ambush, but are more attracted to flowers with spiders than to safe, spider-free flowers.  Why would a bee fly closer to a purple flower that has a white, hungry spider in the middle?

Clearly, bees see differently from us. As I was perfecting my Instagram post, I was struck by my black and white Instagram filter, which showed the flower and spider as same shade of white (Figure 1). Even without extensive training in bee sensory biology, I figured there must be some sort of visual trick at play.

Figure 1. Screenshot of my Instagram post with 100% of the color saturation removed.

Too bad there is no UV filter on insta, because bees can see ultraviolet. Flowers take advantage of the bees’ visible spectrum in UV and often attract bees with target-like patterns, using dark UV spots in the middle. Darker UV target patterns can mean more pollination, so this trait is selected for in both flowers and bees.

So why are bees attracted to the crab spider flowers? Crab spiders have a layer of transparent cells covering spider skin cells that can change color! There are a few types of this pigment- the ommochrome pigment, which either allows spiders to or yellowish to red, or allows white spiders to have UV fluorescent patterns. With a UV pattern on their backs, flowers with spiders look like an extra dark flower target and attract bees more effectively than flowers without spiders.

Of course, natural selection goes both ways. Native bees in Australia fly close to, but can recognize and veer away from native crab spiders, whereas introduced honeybees have not adapted to recognize this danger.

Plants interact with insects and their predators. Scientists use the term “tritrophic interactions” to describe three trophic levels, or links in the food chain, interacting and affecting one another. As my thrilled camper and curious Instagram “fans” could pick up on, tritrophic interactions are fascinating! From an applied science perspective, knowing the intricacies of tritrophic interactions is essential to fully understand the side effects of potential global solutions in food security, and pest management, and conserving biodiversity.

-A

Note: All the papers I cite in this post are by Dr. Astrid M. Heiling, who has many other fantastic papers. Check her work out!

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Armed and Delicious.

Go to your spice rack, make a kale smoothie, have a cup of coffee. Just about everything we use to add flavor to our lives comes from a co-evolutionary battle between an herbivore and a plant protecting its leaves.

That bite on your tongue from an arugula salad? The sulfur-containing cyanide molecules you taste are the result of glucosinolates, a characteristic defense of the mustard plant family. Mustard plants—e.g. horseradish, wasabi, mustard—all use this metabolic pathway because the burning sensation, which many people enjoy with oysters, actually works as an effective anti-herbivore toxic defense. When bugs break open cells, the enzyme myrosinase cuts a precursor to release nitriles, isothiocyanates, and other various bioactive toxic compounds. Plants in a population that are slightly more toxic survive the constant herbivore attack better and can pass on their genes to the next generation. Ah, bittersweet natural selection.

How do we know it is the bugs that put the pressure on? This paper (also summarized in a great article here) swapped mustard plants from Colorado and Montana, and found that not only did the unique spice of each plant stay consistent, but bugs preferred the visiting treat—plants that did not adapt to the local suite of herbivores. The difference in plant survival in this case is an example of local adaptation, all starting from bugs preferring the new mustard spice.

Just like these bugs and everything else in nature, we choose what we eat based on the flavors we like and what won’t kill us.

So why do we intentionally eat so many compounds plants use to make feeding difficult? Often these same toxins are essential for nutrition. The darkest green vegetables, pungent garlic, soothing mint—all play a health benefit role because of the energy plants put into making defense compounds. Bioactive toxins in low doses continue to do their toxic, bad-self thing: the alkaloid caffeine in your coffee stimulates the nervous system, the indole-3-carbinole in your kale salad degrades excess hormones that can lead to cancer, and the terpenes in oak barrel-aged wine rich in phenolic tannins can prevent carcinogens from binding to DNA and reduce the risk of harmful blood clots. The underlying theme here is that many toxins are reactive, for better or for worse.

Disclaimer: Some plant defenses are toxic to humans in high concentrations, and some plants are just plain poisonous at any dose. Don’t start eating everything toxic. Instead, appreciate the nutrients plants invest in creating highly reactive compounds in order to protect themselves, as well as the coevolutionary arms race that made plants with these exciting products succeed.

What is plant defense?

Let’s start with plant defense in general. To follow this blog, here are some key things to know:

Plants can’t run away from their attackers. Instead, over evolutionary time, plants have developed secondary metabolism (aka, not photosynthesis- that is primary metabolism) to make toxic, tough, unpalatable, or otherwise unpleasant experiences for the bugs that try to eat the plants (I use the term bug here loosely to include all herbivorous arthropods, fully knowing that technically a true bug belongs to the phylogenetic order Hemiptera). Plant defense is when plants resist being eaten by bugs.

Plant defense has many flavors:

Chemical defense: the toxic and bitter stuff. We like to consume many of these things that plants designed in order to kill their opponents. (Small apology- I will only minimally anthropomorphize plants throughout my blog.)

Mechanical defense: the tough stuff. Usually this comes in the form of thicker and rougher leaves, or leaves covered in trichomes (fancy word for hair, and very effective: picture walking on velcro as an aphid… difficult right?).

Direct defense: the plant makes a toxic or tough compound that deters bugs.

Indirect defense: the plant relies on predators to provide the defensive service. (This is my personal favorite!) Here is how it works: plants under attack send out a cry for help either as a sugar reward or a signal in the air to attract predator bugs (ants, wasps, or spiders) that fill a hit-man role and kill or evict the herbivore from the plant. Badass! Can you attract ants to be your bodyguards? Didn’t think so.

Inducible defense: This is the on/off switch for plant defense. Usually it behooves the plant to use energy defending themselves when herbivores are around, and to save that energy when they are not under attack.

Constitutive defense: Leaving the secondary metabolite lights on all the time, even when there is nobody home (aka no herbivore attack)

That’s all for vocabulary today. I know, plant biology can get pretty crazy exciting. Get ready for the underlying evolutionary tradeoffs and hypotheses explaining why, how, and when plants defend themselves.