Friday Phymatorama: Not as simple as it looks

Ambush Bug eating Cabbage White on Canada Thistle

This is the kind of picture any biological control entomologist or food-web modeller can instantly understand. The biocontrol person may see a natural enemy (the American Ambush Bug Phymata americana americana Melin, 1930) feeding on a pest (the Cabbage White Butterfly Pieris rapae (L., 1758)). If they are particularly perceptive, they also see that the pest is a potential pollinator of another pest, the misnamed Canada Thistle  Cirsium arvense (actually a native of Eurasia introduced here). So, the ambush bug is doing double duty. The food-web modeller might see it a little different: predator (bug), herbivore (butterfly), and primary producer (thistle), the typical three trophic-level food chain.

Ambush Bugs transformed into pests

Let's change the set-up slightly and look at another thistle. Here the female ambush bug (guarded by a smaller, darker male) is feeding on a Honey Bee (Apis mellifera L., 1758). The food web modeller doesn't see much different here, although they may wonder if the bugs would be better modelled as parasites of the plant (they are eating its pollinators). But, Hey! That's one of our bees! That bug is a pest, not a biocontrol agent/ natural enemy!

A slight change of focus turns good bugs bad

Let's change focus slightly again and look at another thistle plant. Here another pair of ghoulish bugs have a flower fly (Family Syrphidae: Helophilus sp.) by the tongue and the female bug is draining out its life's blood. Many flower flies are our friends, their maggots eat aphids, but this one's larvae feed on rotting vegetation. So, this is a more neutral interaction for our biocontrol person: the use of an alternative prey by a natural enemy/ pest of bees, depending on their perspective. This does complicate our food web model, though, the adult fly is an herbivore (feeding on pollen and nectar), but the larva is a detritivore (feeding on microbes and detritus), so a fourth trophic level needs to be added to the model as well as some omnivory (defined as feeding at more than one trophic level). The bugs are getting some of their energy directly from the detritivore level (as well as the herbivore level).

Flower fly that should've watched where it put its tongue

Let's shift focus slightly again, to another unfortunate fly. Here another pair of ambush bugs are feeding on a thick-headed fly (Family Conopidae), in this case Physocephala furcillata (Williston, 1882). Well, really the female bug is feeding and the male looking out for any other males that might be interested in his perch (but males aren't above sneaking a bite or two).

Ambush bugs feasting on a thick-headed fly

Our biocontrol specialist is likely to see this as more alternative prey behaviour. Good for keeping a predator around to feed on Cabbage Whites or bad if they are feeding on Honey Bees. But for the food-web modeller, this is just another complication.

Thick-headed flies mimic wasps, but are parasites of bumble bees

For our thick-headed fly is not a detritivore, but a fast-flying, wasp-mimcking parasite of bumble bees. They lay eggs on the bumble bees while they are in flight and the parasitic maggots eat out the bumble bees insides over a matter of a few weeks. This means we have to add another trophic "level" to our model - number 5 (detritivore, plant, herbivore, predator, parasite), at least as a concept, and because a predator feeding on a parasite is taking energy to one trophic level above the parasite (which in this case is functionally equivalent to our predator). 

Helophilus mimics a bee, but functions as both bee and detritivore (and dinner)

All of this is very destabilizing to our model food web: the more links (especially those between more than two levels), the more likely the model is to crash. And all of this comes from just looking at what one species of predator is doing on one afternoon in one field and on one plant. Imagine trying to model all of the interactions that must be going on in that one field with its dozens of plant and many hundreds of insect species. No wonder most modellers prefer to sit at their computers and play with their models and leave the real food webs to the bugs.

Not seeing eye-to-eye, but looking for the same thing: dinner