Marin Master Gardeners
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What's bugging your roses?

June 08, 2013
Nanette Londeree

This time of year our national flower puts on a dazzling show; roses strut their stuff with exuberant blossoms spilling onto walkways, tumbling over fences and fragrantly bursting out of planting beds.

Are your plants bedecked with pristine foliage and perfectly formed flowers? Or, like me, do you find blossoms riddled with holes, shriveled buds, ragged and tattered leaves?

Bugs that wreak havoc with our roses are at their most active now. Effective management of rose-loving pests starts with knowing the good insects from the bad ones, along with those that are just a nuisance. Some bad guys
Aphids are prolific pests that can envelop rose buds making them look pretty bad. They favor tender new growth and populations can explode on plants with a high nitrogen diet. The pear-shaped adults are green, yellow, brown, red or black and, on top of looking gross, they produce a sweet honeydew that attracts ants and supports the growth of sooty mold. Control is often as simple as knocking them to the ground with a shot of water or spraying with an insecticidal soap.

If left uncontrolled, rose curculios can decimate your springtime roses, leaving behind ragged blossoms, deformed flower buds that fail to open and rose stems with "bent necks." The beastly µ-inch long weevil is a dull lacquer red with a characteristic narrow, black, curved snout. The most effective means of eliminating them is to hand-pick and destroy them.

Looking similar to a butterfly or moth, caterpillar rose slugs are neither; they're the immature larvae of the sawfly, a type of primitive wasp. Leaves with a "window pane" look are the work of these critters, as are large holes in leaves. Remove affected leaves and destroy them. Spraying plants with neem oil or insecticidal soaps can help to control them — be sure to spray the top and undersides of the leaf.

Thrips are tiny, thin yellow-brown or black winged insects most commonly found among the petals of flowers. Brown or silver spots or brown edges on petals, buds that refuse to open and deformed buds are trademarks of thrips. Controlling them is difficult since they hide deep within the flower bud. Promptly remove spent blooms from infested roses and clean up garden litter from around your roses, especially over winter. Encourage beneficial predators such as lady beetles, minute pirate bugs and big-eyed bugs. Horticultural oil and insecticidal soaps may have some benefit if you spray as early in the bud stage as you can.

Nuisance insects
Cucumber beetles look like shiny, greenish-yellow ladybugs. While destructive to edibles, they don't do too much damage to roses. You'll most often see them early in the season consuming pollen and some petals of roses until their favorite food sources are available.

Earwigs are not particularly discriminating eaters; they eat mites, aphids and insect eggs, but also may gobble up beneficial insects. The damage they cause can mimic that of caterpillars — producing numerous irregular holes or chewed leaf margins. If you can tolerate the bit of damage they do in exchange for their potentially beneficial role, do nothing. If you're bothered by them, control by reducing their hiding places, lowering surface moisture levels by using drip irrigation and trapping.

Semicircular holes in the margins of leaves are likely the work of leafcutter bees. They cut and carry leaf material back to use in lining their nests. Their damage is simply cosmetic, and they are active pollinators.

The good guys
Ethereal-looking adult green lacewings have slender bodies, delicate gossamer wings and immense golden eyes. Their ferocious spiny, alligator-shaped larvae are the voracious hunters. Also called aphid lions, they are natural enemies of several species of aphids, spider mites, mealybugs, thrips, whiteflies, small caterpillars, beetle larvae and insect eggs.

Soldier beetles are predators to a host of bad bugs including aphids, spider mites, various caterpillars, beetle larvae and other insects and their eggs. Adults are brownish, straight-sided beetles about a half inch long with a striking red, orange or yellow head and abdomen.

If you see what looks like a small bee hovering helicopter-style over a flower, it's likely a syrphid fly. Again, it's the larvae that are the predators going after aphids, small caterpillars, thrips and other small insects and consuming as many as 400 aphids in their very short lifetime. The adults feast on pollen and nectar and their movement from flower to flower makes them an important pollinator.

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