underground wasp nests - Ask Extension
Wasps have nested underground on my front lawn, at the site of an old tree that was cut down some years ago. The area, about 6 feet in diameter, is s...
Knowledgebase
underground wasp nests #617411
Asked March 17, 2020, 11:46 AM EDT
Wasps have nested underground on my front lawn, at the site of an old tree that was cut down some years ago. The area, about 6 feet in diameter, is subsiding unevenly. (See attached photos, which don't give a good indication of the extent of the area.)
I have a life-threatening allergy to wasp venom, which became apparent when I was stung while walking across my lawn. Last summer an exterminator put poison down the holes, and the wasps disappeared. But recently I saw a wasp near one of the holes. We started to put pebbles on the area but aren't sure if that's the best solution.
I don't want to use poison again. The Takoma Park arborist suggested shooting water down the holes to drown the wasps. Do you agree, or do you have other suggestions?
Montgomery County Maryland
Expert Response
We have social-living wasps in our area as well as solitary wasps. Species of both can nest in the ground, so without a picture of one to ID, it could be either investigating the holes in the lawn. (Did the exterminator ID the type of wasp? Perhaps a Yellowjacket?) https://extension.umd.edu/hgic/topics/predatory-wasps
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7450.html
Social wasps do not re-use nests from previous years. A young queen, ready to start a new colony, may be investigating the area if it has appeal as the site of a new nest. If a solitary wasp, there will be no nest queen to defend, and stings and defensive behavior are much less likely. Something about the area is suitable for a nest site if one was there before; perhaps the open ground or soil texture is ideal.
Flooding the holes is not the best idea, simply because if there were a population of wasps already present, it will aggravate them, and if they are out foraging, it won't help at all. Workers spend the night back in the nest, but it's riskier to attempt any sort of spraying at night when you can't see flying insects easily. Insects are naturally quite water-repellent, and the water will probably drain through the soil before they run out of oxygen. If you have spare garden soil, you can toss some onto the holes to fill them in the meantime. If they re-appear, you know that either wasps or digging animals (moles, voles, squirrels, chipmunks, etc) are active in the area.
Keep observing the spot and see if you notice more than one wasp near a hole at the same time. New colonies are so small that it is likely you may only see one wasp at a time approaching or leaving a nest. We have not seen scientific evidence for any sort of repellent treatments or trapping that would force the wasps to search elsewhere, so an exterminator may again be needed if this develops into a new colony.
If you are ready to replant the area, now is probably a good time, not only because we've entered good perennial/shrub/tree-planting season, but to make this location less suitable for wasp nests. Given the risks to you in case a colony is present, it is reasonable to ask someone else to plant for you. Alternately, planting on a cold day - which won't harm any hardy plants - will prevent wasps from being able to fly. Since the ground has settled a bit from the decay of the old tree remains, you might need to add some topsoil or compost to the planting area to level it out.
Miri
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7450.html
Social wasps do not re-use nests from previous years. A young queen, ready to start a new colony, may be investigating the area if it has appeal as the site of a new nest. If a solitary wasp, there will be no nest queen to defend, and stings and defensive behavior are much less likely. Something about the area is suitable for a nest site if one was there before; perhaps the open ground or soil texture is ideal.
Flooding the holes is not the best idea, simply because if there were a population of wasps already present, it will aggravate them, and if they are out foraging, it won't help at all. Workers spend the night back in the nest, but it's riskier to attempt any sort of spraying at night when you can't see flying insects easily. Insects are naturally quite water-repellent, and the water will probably drain through the soil before they run out of oxygen. If you have spare garden soil, you can toss some onto the holes to fill them in the meantime. If they re-appear, you know that either wasps or digging animals (moles, voles, squirrels, chipmunks, etc) are active in the area.
Keep observing the spot and see if you notice more than one wasp near a hole at the same time. New colonies are so small that it is likely you may only see one wasp at a time approaching or leaving a nest. We have not seen scientific evidence for any sort of repellent treatments or trapping that would force the wasps to search elsewhere, so an exterminator may again be needed if this develops into a new colony.
If you are ready to replant the area, now is probably a good time, not only because we've entered good perennial/shrub/tree-planting season, but to make this location less suitable for wasp nests. Given the risks to you in case a colony is present, it is reasonable to ask someone else to plant for you. Alternately, planting on a cold day - which won't harm any hardy plants - will prevent wasps from being able to fly. Since the ground has settled a bit from the decay of the old tree remains, you might need to add some topsoil or compost to the planting area to level it out.
Miri
Thanks for your quick reply to my question about underground wasps. I believe the wasps were yellow jackets, but they could have been white-face or yellow hornets, to which I'm somewhat less allergic.
I tried planting saplings at the site of the old tree before; two died within a year of planting.
What about re-grading the entire area? It's so uneven now that it's hazardous to walk on.
You're welcome. Yellowjackets are a commonly-encountered ground-nesting social wasp, so that makes sense.
A lot of factors can be at play regarding planting failures. Moisture levels (too much, too little), improper planting depth, health of the new plant, injury from transplanting, and more can cause problems and loss. The site being the location of a removed tree is not likely the problem, though if sinking soil buried the root flare too deeply, this may be a factor. Some tree species are more fickle than others; which saplings did you try?
Grading soil takes experience and skill to understand how it may affect water drainage, especially if it directs water towards a home. Depending on how large of a job you feel this would be, hiring a landscape company with grading experience may make sense. If the stump and major roots are still decomposing, future sinking may still occur. (A landscaping company might have tools to help dig out more of what's left.) Otherwise, simply adding soil as needed and leveling-out what's there with a stiff-tined rake (not a leaf rake) is the simplest approach.
Miri
A lot of factors can be at play regarding planting failures. Moisture levels (too much, too little), improper planting depth, health of the new plant, injury from transplanting, and more can cause problems and loss. The site being the location of a removed tree is not likely the problem, though if sinking soil buried the root flare too deeply, this may be a factor. Some tree species are more fickle than others; which saplings did you try?
Grading soil takes experience and skill to understand how it may affect water drainage, especially if it directs water towards a home. Depending on how large of a job you feel this would be, hiring a landscape company with grading experience may make sense. If the stump and major roots are still decomposing, future sinking may still occur. (A landscaping company might have tools to help dig out more of what's left.) Otherwise, simply adding soil as needed and leveling-out what's there with a stiff-tined rake (not a leaf rake) is the simplest approach.
Miri
