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Garden Q&A: Are robber flies good to have around?

Q: I saw this interesting critter sitting on my window screen and it looks like a giant, oddly-shaped bee. Is it good to have around the garden?

A: Yes. This is a robber fly, and they come in a range of sizes from almost 2 inches to quite tiny. They are general predators, catching insects while in flight, including stink bugs, wasps, grasshoppers, and beetles. Although they may occasionally nab the passing pollinator, they’re considered beneficial due to their consumption of pests. Their terrestrial larvae are also predators. It can be fun to watch adults perch on a prominent sunlit hunting spot and see what they come back with after each foray.

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Robber fly, waiting for take-out to fly by.
Robber fly, waiting for take-out to fly by. (Baltimore Sun staff/Baltimore Sun)

Q: The leaves on a few of my perennials and shrubs are becoming pale, like their color has been drained. I don’t see any insects. Do they just need fertilizer to green them up again?

A: It’s likely the work of spider mites, and interestingly, the leaf’s color literally has been drained. At their tiny scale - around the size of a period in your question - the mites feed by jabbing plant cells and sucking up the contents, which includes chlorophyll. It’s impressive that something so tiny can create such prominent symptoms. Though a few species prefer the cooler days of spring and autumn, most thrive in summer’s heat; all do well in dry conditions and on plants that are drought-stressed. Though they are named for their spider-like use of silk, their webbing might not be obvious until populations are high enough to practically carry the plant away.

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Plants sited next to a wall that reflects heat and reduces air circulation are more likely to experience mite outbreaks due to chronic stress. Avoid fertilizing excessively, since the nitrogen boost can actually benefit the mites more than the plant. Similarly, avoid the general use of broad-spectrum pesticides that don’t target a specific pest at the ideal time or in the right manner, as these can kill beneficial insects that would otherwise keep mite numbers low. (Mites are not insects, and few insecticides also kill mites.)

Instead, monitor plants for irrigation needs and use a strong blast of water to knock off the mites. If you can, hold or rub the leaves in one hand while you spray their undersides with a hose so the mites get the full force of the water. Repeat this a few times about a week apart and you’ll probably get good results. Leaves can’t heal the damage, but look for new growth that remains unharmed as a sign your efforts are working.

If needed, you can resort to either horticultural oil or insecticidal soap treatments, as their short persistence is more likely to spare mite predators. Follow label instructions about when to repeat, and do not apply while temperatures are above 85 degrees. You can search “spider mites” on our website for more information.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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