Thursday, July 3, 2014

Sand Digger Wasp Drama in Our Playgrounds

Sand Digger Wasp Bicyrtes quadrifasciatus flicking sand

     This is about the time of year I start receiving calls regarding wasps nesting in our sand volleyball and tot playgrounds. More often than not, what everyone is asking about are Sand Digger Wasps. These are solitary wasps (one female per hole) but which can aggregate in large colonies when the conditions are right. They prefer open, sunny locations with soft, clean, and often lighter colored sand. That unfortunately is just what we strive for in our sand lots.

Sand Wasp carrying a paralyzed stinkbug into its burrow

     The most common species we encounter locally is Bicyrtes quadrifasciatus, a stinkbug and leaf-footed bug specialist. In other parts of the country, they have species that go after flies. These are then actually beneficial insects. Each female digs her own tunnel (up to a foot deep) and then brings back stinkbugs she has stung and paralyzed. This she takes down her burrow and the paralyzed prey then acts as the living food for her larvae. 
     Though they can look intimidating flying over the sand, they are actually extremely unlikely to sting unless actually grabbed. In fact, most of the ones flying over the sand are actually males who cannot sting at all. In the world of bees, wasps, and ants, only females (of the few species who can sting at all) can sting, because the stinger is actually a modified ovipositor (egg-layer). Males of course do not lay eggs and so therefore cannot have a stinger. 
     Sand Digger Wasps are only are active for a little over a month, but this happens when the sand lots are in great demand for the summer. Every year, complaints come in and we need to educate people about their pollination and stinkbug control benefits.
     Ideally we would just close down the lots for the time they are nesting, but that is not practical. We are experimenting with various methods to deal with or discourage them. Unfortunately making the area more shady and changing the type of sand is not very well received by the public either.   
     Pesticides are also of course out of the question (more dangerous to kids/public than the critters themselves), as is covering the area up or flooding them (sand drains too well anyways). We are also tilling and sifting the areas they use once the season is over, since they show nest site fidelity (nest in the same area they were born). This hopefully means less of them the following year. We have many other locations for them and they are beneficial, but we need to have them not take up residence in the sand playgrounds.


Sand Wasp pupa next to its cocoon

Sand Wasp cocoons with paralyzed stinkbug prey

     I suspect that Sand Digger wasps are actually increasing in numbers not only because we have increased what once was an uncommon habitat (sunny sand lots with no plants growing), but because we have so many stinkbugs. I think they are benefiting because they are now preying on the invasive exotic Marmorated Stinkbugs that are not only a nuisance in our homes, but cause great agricultural harm. People may have noticed how much of a problem and how numerous these invasives were before, and how much fewer we have now in certain locales. As I have said, sand wasps are very beneficial if we can just get them to do their good work in other locations.
     More on them on the Capital Naturalist YouTube Channel: https://www.youtube.com/watch?v=cHhrQROrSvw
     Another interesting association with them is a neat little Satellite Fly that parasatizes these wasps. These can be seen following the sand wasps around and waiting for them to open up their burrows once they arrive to them. I have not identified the species yet, only that they are Miltogrammine flies. Once they get an opportunity, they lay their eggs on or next to the paralyzed stinkbug. The fly is a kleptoparasite, meaning its maggot steals the food and often kills the wasp larva in the process. The sand wasp goes through all that work to feed the fly. 

Satellite Fly, a Miltogrammine kleptoparasite fly of these wasps

     I'm often amazed at all the drama that goes on in miniature right under our noses. Here we have wasps who use our playgrounds to raise their young. They do this by providing a paralyzed living food source for their babies (perhaps controlling an invasive pest in the process). But waiting in the wings is a fly who steals the food and kills the wasps larvae. All of this drama taking place less than one foot underground where our children play. Its easy to see why people might get upset, but we need to realize this part of a natural cycle. These wasps are providing a valuable benefit in pollination and controlling stinkbug pests as well, and are harmless to us anyways. Please let others know so maybe we can all figure out a way to share the same space, if only for a month during summer. 




3 comments:

  1. There are currently sand wasps in many of the sand bunkers on DC area golf courses. Do you think they are Bicrytes or could they be Stictia carolina?

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  2. Sorry for the delay in responding, they are likely Bicyrtes, but check to see the prey. Bicyrtes go for stinkbugs. Should be wrapping up by now though. Thanks.

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  3. I picked up some sand encased eggs/cocoons. They're identical to the ones pictured above. I've left them in a jar with mild window access. I believe they've been exposed to a great deal of water. I'm curious to see if they are still viable. How porous would these cocoons be? They seem awfully sturdy.

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