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This is why a common beetle isn't harmful to local flowers

"Fielding Questions" columnist Don Kinzler also answers questions about leftover garden seeds and if cardboard can be used to smother weeds.

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A reader wonders if this beetle is harmful to their zinnias.
Contributed / Special to The Forum
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Q: The photo is from this past summer, and shows a type of insect on our flowers. I wondered at the time if they were beneficial or harmful. Can you identify them? — Lori K.

A: To identify the insect perched on the yellow zinnia, I turned to co-worker Janet Knodel, North Dakota State University’s Extension entomologist, who provided a great summary:

“The insect is the goldenrod soldier beetle, which has the scientific name Chauliognathus pensylvanicus. Soldier beetles are common on flowers and nicknamed leatherwings because of their soft, clothlike wing covers, which when brightly colored are reminiscent of uniforms.

“The beetles are elongate, soft-bodied and about one-half-inch long. Colors vary from yellow to red with brown or black wings or trim. Soldier beetles resemble lightning bugs but do not have light-producing organs. Both adults and larvae are predacious and feed on other insects. The adults eat caterpillars, aphids, and other soft-bodied insects and can be important beneficial predators.

“As they lie in wait for prey on flowers such as goldenrod they may feed on nectar and pollen, but they do no damage to the plants. Since soldier beetles are beneficial and harmless it is unnecessary to control them.”

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Q: I’ve got some leftover packs of garden seeds from last year. I hate to throw them out, with the current price of seeds. Do you think they’ll come up OK this year if I plant them? — Jim H.

A: The seeds will most likely grow. Seed longevity depends greatly, though, on how they’re stored. If leftover seeds are left all summer in a hot garage, germination ability can quickly diminish. If seeds are stored in a cool, dark place, such as a closed container in the refrigerator, they’ll remain viable longer.

If you’d like, you can do an easy germination test. Place 10 or 20 seeds between two sheets of moist paper towels, roll up and place in a plastic bag. Loosely tie the bag and place in a warm area. Open and check for growth after 10 days, noting the percentage of seeds that sprouted. If germination is reduced, seed proportionately thicker this spring. Small seeds like carrot, lettuce and radish generally last longer than large seeds like corn, beans and peas.

Q: I’ve read in gardening magazines that cardboard can be used to smother weeds in areas you don’t want to use weed killers. Does it work? — Jenny M.

A: Yes, flattened cardboard can be very effective at controlling weeds, and I’ve used it myself, even on tough perennial weeds like Canada thistle. When deprived of light, plants can’t do their normal photosynthesis, and energy stored within plants is depleted. Eventually the plants die, roots and all.

How long the process takes depends on the type of weed. Canada thistle and quackgrass, with their deep network of thick roots, have a great deal of stored energy and potential to resurrect. Hard-to-kill weeds might require an entire growing season before smothering is effective.

When using cardboard for weed control, it must be overlapped generously, because weeds will easily find their way out through even small spaces in cardboard, or joints that aren’t well-lapped. If weeds sneak through, and begin growing, they quickly recharge their stored energy.

Cardboard makes an effective weed control underlayment in perennial flower beds, and it can be covered with shredded bark or other mulch to make it attractive, and to prevent the wind from blowing the cardboard away. Cardboard, a wood-based product, will eventually decay, adding organic material to the soil below.

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If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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