Here There Be Dangers
Today’s human-induced threats to bees and other pollinators are grave conservation concerns. Those recent threats, however, only add to the risks flower visitors have always faced: An array of voracious predators lies in ambush wherever flowers bloom, ready to exploit a consistent and predictable supply of prey. Like crocodiles in shrinking African watering holes, those predators know their prey has to come to them eventually. Bees and other pollinators need to feed at numerous flowers each day to survive, but each blossom they approach could be their last.
If you’re a bee or butterfly, there’s clearly nothing good about having predators hanging around flowers. However, if you’re a plant, the story is much more complex. Predators such as crab spiders, ambush bugs and praying mantids can scare away or kill off important pollinators, but they may also capture and feed on insects that damage flowers.
It can be tempting to think of predators as mean or evil, but they play crucial roles in ecosystems. Countless studies have shown the chaotic and negative impacts that occur when predator populations are artificially removed or reduced in size. Predators, even those who eat pretty butterflies, are essential components of a healthy natural world. Furthermore, those predators live intriguing lives and can be exquisitely beautiful.
In many wildflower patches, the most abundant predator hanging around blossoms is the crab spider or, more specifically, the flower crab spider (Misumena spp.). Crab spiders are so named because they resemble crabs in several ways: They have wide, flattened bodies; their front two sets of legs are extra long; and they often scuttle quickly backwards or sideways when threatened.
The hunting strategy of flower crab spiders is to find a likely spot, spread their front legs wide, and sit perfectly still until unwary prey comes within range. When a fly, bee, butterfly or other likely meal gets too close, the crab spider’s legs snap shut on it. The spider then bites its prey, injecting venom that both paralyzes the creature and begins liquefying its insides. Once its prey stops struggling, the crab spider typically carries it beneath the flower or to some other sheltered location. There, it sucks out the liquefied insides and discards the empty carcass.
Flower-based crab spiders can be very colorful, in contrast to the drab browns and greys of most other spiders. Many crab spiders are white or bright yellow, and some can change between those two colors over the course of a few days. Coloration that matches many flowers hides them from their prey, but may also help conceal them from birds and other predators. That’s particularly important for a creature that often sits in the open as it hunts.
Research has shown that crab spiders can find flowers by following the same scents that attract bees. In addition, flowers under attack by florivores (insects and other small creatures that feed on flowers) may produce extra strong scents as an attempt to attract crab spiders to defend them. Sure, the presence of crab spiders cuts down on the number of visiting pollinators, which might seem like a bad thing offhand, but limiting damage from beetles and other creatures feeding on flower parts might make up for that.
Crab spiders are not the only spiders that catch prey on or near flowers, though they are usually the most abundant. Others, including jumping spiders, will also take advantage of the plentiful prey found on blossoms. But for most of those spiders, flowers are just one of many potential hunting spots.
Assassin and Ambush Bugs
True bugs (those in the order Hemiptera) are different than beetles and other insects in a few ways. They have long piercing and sucking mouthparts as well as wings that are transparent at the tips and hardened at the base. Within the true bugs, however, there is tremendous variation in form and function. Some of the stranger-looking Hemiptera are assassin bugs — predatory insects that have raptorial front legs (like a praying mantis) and use their long beaks to inject prey with paralyzing poison.
Assassin bugs tend to be long and slender with lengthy legs and antennae. They can be found hunting on all parts of plants, though some species tend toward flowers. Most are earth-toned: brown, grey, black or some combination thereof. Their long front legs have sticky hairs that help hold prey while they insert their sharp beaks and inject a paralyzing toxin that also liquefies the entrails of their captured food. Aptly named assassin bugs are very effective predators and can kill animals twice their size.
Ambush bugs are a subgroup of assassin bugs with a few distinct characteristics. Imagine a short, stout assassin bug — like a champion weightlifter — and you’re in the ballpark. Besides having more compact bodies, their raptorial front legs are also enormously thickened, adding to their menacing appearance. Ambush bugs tend to be brighter colored and more strongly textured than assassin bugs, which provides excellent camouflage for hunting on flowers.
While assassin bugs hunt in a variety of locations, ambush bugs specialize in waylaying other insects on flowers, but both attack and kill prey with the same tactics. Like mantids and spiders, assassin and ambush bugs will kill pollinating insects if they get the chance, but they’re just as likely to capture and eat other tiny creatures that come within range.
Most assassin bugs are harmless to people, but some can inflict a painful bite when grabbed or handled roughly. For the most part, those bites just cause temporary pain, but a small group of assassin bugs called “kissing bugs” can cause bigger problems.
Kissing bugs, or triatomines, are nocturnal insects that feed on the blood of mammals (including humans). They use their sharp beaks to puncture skin and suck small amounts of blood, which can cause pain and sometimes an allergic reaction. Kissing bugs can also carry a parasite that causes Chagas’ disease, which may be transferred to people and cause significant health issues.
The praying mantis ranks as one of the largest and most impressive predators among invertebrates, some measuring over 4 inches in length. Yet it still manages to blend in with surrounding vegetation well enough to seize a large number of flower visitors. Mantids (insects in the Mantidae family) don’t hunt exclusively on flowers, but it’s common to find them lurking just below the blossom, waiting for a butterfly, moth or other large insect to pay a visit.
While spiders and predatory bugs use toxic injections to paralyze and kill their prey, mantids rely on their long legs to catch and hold prey while they start eating it alive — often starting with the head. Those front legs can extend outward and snap shut more quickly than the human eye can register, and the formidable spikes extending from them help prevent prey from escaping.
The head of a praying mantis can swivel nearly 180 degrees, so it can scan for prey while its body remains completely still. Once it spots a potential meal, the mantis will either move slowly closer or simply wait for the unlucky creature to stray within range. Often, a mantis will sway slowly back and forth, which, combined with its stereo vision, helps it precisely locate its prey before striking.
The Chinese mantis and European mantis are two of the biggest and most common praying mantis species; both are often sold as pest control agents. While they certainly do catch and kill insect pests, their benefits are likely outweighed by the number of pollinators and other beneficial insects they take. Scientists are also concerned that those introduced species are outcompeting native species such as the Carolina mantis. Regardless, mantids are imposing predators of insects on flowers and elsewhere.
Other Remarkable Predators
Crab spiders, mantids and assassin bugs make up a large percentage of the creatures lying in wait for pollinating insects to visit flowers, but they aren’t the only ones. Two other fascinating examples include robber flies and blister beetle larvae. Both are less common than the species discussed prior, but they’re definitely worth mentioning.
Flies are the most diverse group of insects in North America with over 61,000 species, and they play a variety of roles as decomposers, predators and pollinators. Robber flies are a relatively small group of predatory flies that rely on speed to capture prey. They often perch on a plant and wait for an unwary insect to coast through the air nearby. When a robber fly spots movement, it takes off like a (much more agile) guided missile and knocks its prey from the air. Like some of the other predators mentioned, it then injects its prey with a paralyzing toxin before sucking out the liquefied innards.
It may be hard to imagine how robber flies could become even more menacing, but some have found a way to improve on that already-sinister template: A few species have evolved an appearance that bears an uncanny resemblance to a bee. This allows the fly to spend time on or around flowers they know insects will visit, as opposed to sitting on a random perch and hoping something will happen along.
While humans might be apprehensive about approaching a flower with a big bee-looking creature on it, pollinators may interpret the presence of a bee as an indication of safety. That assumption could prove fatal and has probably been the last thought of many pollinators.
Blister beetles are also common visitors to flowers, where they feed on parts of blossoms. A particular group called oil beetles has a truly incredible strategy for attacking bees. Oil beetles have unique kind of larvae called triungulin, which are very mobile immediately upon hatching. They form themselves into large clusters and emit a chemical that mimics the pheromone of a female bee. Male bees find themselves irresistibly attracted to the scent and land on the cluster, expecting to mate with a female bee. Instead, the triungulin quickly clamber aboard the male bee, which flies off frustrated.
When that male bee later meets an actual female, the triungulin hop onto the female while the bees are mating; they then hitchhike back to her nest. Once at the nest, the oil beetle larvae attack and consume the bee’s eggs and larvae, along with the pollen and nectar she had collected for her offspring. As a result, instead of producing a batch of bees, the female’s nest becomes a nursery for oil beetles. Even the most creative Hollywood screenwriter might struggle to imagine such an ironic twist.
A Scary World for Pollinators
For bees, butterflies and other pollinating insects, life is harrowing. Pesticides and habitat degradation can severely reduce population sizes, but individual insects don’t recognize those threats.
Instead, any anxiety dwelling in their tiny brains likely comes from the possibility of predation: Research shows that flowers with predators on them are visited less frequently than those without, so pollinators must be at least somewhat aware of the enemies that might await them at each flower. However, they’re also desperate for food, and it takes a lot of flowers to meet the energy needs of a moth or butterfly — or to supply sufficient pollen for a female bee to feed her brood.
Given the number and effectiveness of predators lying in wait for pollinators, it might seem surprising that we still have any bees or butterflies left. As with all predator-prey relationships, though, each side of the equation is regulated by the other. There are still enough flowers that aren’t harboring a crab spider or ambush bug to furnish most pollinators’ needs.
Predators are a necessary part of any ecosystem. As such, we should admire and be grateful for them. They certainly employ a variety of fascinating strategies worthy of our respect. At the same time, it’s appropriate to feel some sympathy for the creatures who spend their lives trying to avoid the danger that lurks among the flowers.
Chris Helzer is the Nature Conservancy’s director of science in Nebraska. He writes The Prairie Ecologist blog and is the author of “The Ecology and Management of Prairies in the Central United States.”