For The Union-Tribune
It’s early spring and our roses have taken the stage. They are adorning our gardens with their big, breathtaking blooms. Spoiler alert … anxiety will soon challenge our peaceful bliss, because where there are blooms and fresh new foliage, there will be pests.
But we are not powerless. With attentiveness, knowledge and action, we can prevent pests from engulfing our joy and doing too much damage.
Our first course of action is to spot and identify the pest and pest damage early. The main pests in the spring garden are aphids, hoplia beetles, caterpillars, rose slugs and thrips.
Gardeners who use a “one with nature” approach have the same strong desire to protect our roses from pests as do the gardeners who use pesticides. But because pesticide interventions, both chemical and organic, carry risks to beneficials, we leave this control as a rarely used last resort. We utilize mechanical and cultural controls — and our beneficial heroes and heroines — to manage these spring pests.
Aphids
These tiny, pear-shaped, soft-bodied insects are about 1/8 to 1/4 inch long and suck sap from the tender new growth of roses with needlelike mouthparts that penetrate plant tissue. They multiply very rapidly. Usually green, they can also be pink, gray and brown.
Aphid damage: This includes damaged stem tips, deformed flower buds, weakened flower bud peduncles and curled, distorted and crinkled leaves.
Mechanical control: Aphids are the easiest pest to dispense with mechanically. We can annihilate hundreds of aphids on a daily walk-through of our garden, simply by squishing them with the skillful use of our thumb and finger. The remainder are easy to dislodge with the use of strong high-pressure blasts of water from a hose. Dislodged aphids rarely find their way back up and are often devoured by spiders and ground beetles.
Biological control: Aphids entice beneficial insects such as lady beetles, soldier beetles, lacewings and syrphid flies into our garden. Both the adult lady beetle and its crocodile-shape larvae feed on aphids, as do the larvae of lacewings and syrphid flies. Tiny parasitic wasps also control aphids with adults laying their eggs within the aphids and the developing wasp larvae feeding on them.
Cultural tip: Avoid overfertilizing roses with water-soluble, nitrogen-based fertilizers, which prompts overgrowth of aphid-favorite soft, succulent growth. Sweet alyssum interplanted among roses provides shelter to many aphid predators.
Chemical control: Pesticides are not warranted for such an easy-to-control pest.
Hoplia beetles
These beetles measure about 1/4 inch long and are dark brownish-gray in color, making them very visible as they feast on their preferred, light-colored blooms. In San Diego, hoplia beetles’ heavy onslaught on our roses has a short time span, lasting only from late March to early May. After feeding for several weeks, they fly back to their egg-laying sites. Fortunately, there is only one generation each year.
Hoplia damage: Adult hoplia beetles can destroy our beautiful blooms and stamens with their chewing. They do not feed on leaves.
Mechanical control: Hoplia beetles caught between our fingers can be crushed underfoot, squished between two petals, clipped with our pruners or dropped into a bucket of soapy water. Check also the funnels of calla lilies, which are a favored hoplia resting site.
Biological control: Birds and spiders in our garden can be counted on to take care of the remaining beetles we miss.
Cultural tip: Since these beetles prefer light-colored blooms, plant darker-colored roses.
Chemical control: Spray must come in direct with the beetles. Chemical control of the larvae isn’t possible, because they live in the soil outside the garden or in surrounding landscapes.
Caterpillars
There are many varieties of caterpillars, with a huge variation in size, color and appearance. Mating female butterflies and moths lay eggs from late spring through to the fall on their preferred host plants. The hatched caterpillars (larvae) continually eat and can devour many times their weight. It is easy to spot holes in buds, damaged blooms and chewed, ragged and skeletonized leaves, but the caterpillars themselves can be hard to find. Black droppings (frass) deposited on foliage is a tipoff to search close by for actively feeding caterpillars. Some caterpillars fold or roll leaves together with silk to form shelters.
Caterpillar damage: Orange tortrix, tussock moth, fruit tree leafroller, tent caterpillar and omnivorous looper feed on rose leaves. Some caterpillars bore into buds and ruin and disfigure blooms.
Mechanical control: Caterpillars are very quick and nimble. Before opening leaves, hold them over a bucket. Cut out and dispose of damaged buds, which may still harbor caterpillars. Look for eggs that are usually laid on the underside of the leaves.
Biological control: Tachinid fly larvae and ichneumonid wasps parasitize the caterpillars. Birds, lacewing larvae, assassin bugs, spiders and certain beetles also are common predators.
Cultural tip: Cut out suspected caterpillar hideouts.
Chemical control: Bacillus thuringiensis subspecies kurstaki (Btk) is a microbial insecticide that controls caterpillars, but to be effective, caterpillars must feed on treated leaves. Btk will not get rid of caterpillars within the rosebud.
Bristly rose slug
The bristly rose slug is the most common rose slug in California and is the larva of a sawfly. Larvae are pale green or yellowish green with a brown head and tiny, hairlike bristles. They look like (but are not) green caterpillars. When newly hatched, they are difficult to spot as they are tiny, green and on the underside of the leaf. They grow to about 3/4 inch long. There are five or six generations each year.
Rose slug damage: When you spot lacy, skeletonized rose leaves, these pests are the culprit.
Mechanical control: Early detection is key, and gardeners should start monitoring their roses in April and keep monitoring for the remainder of the year. Infested leaves can be cut out and larvae can be handpicked off leaves and squished between the fingers. When sprayed with a strong burst of water from a hose, the larvae are incapable of climbing back up to the leaves to resume feeding.
Biological control: Parasitic wasps, predaceous beetles and birds assist in lowering sawfly populations.
Cultural tip: When you spot lacy leaves, squish the rose slugs immediately to interrupt the life cycle.
Chemical control: Bacillus thuringiensis (Bt) is not effective against sawfly larvae because they are wasp larvae and not caterpillars.
Western flower thrips
These thrips are minuscule yellow-brown or black, fast-moving specks that are only about 1/20-inch in length and have fringed or feathery wings.
Thrips damage: Thrips are rasping pests. Both immature and adult thrips feed by scraping surface cells to suck plant sap. Heavy infestations can cause buds to bend over, ball up or open partially. When blooms do open, petals are blemished, with silvery-white or brown edges. These pests prefer white and light-colored yellow and pastel pink roses.
Mechanical control: Infestations can be prevented by cutting out damaged buds and blooms as soon as they are noticed. Sprays of water may dislodge them and force them to the ground, although this can mar the blooms.
Biological control: Plant a diversity of plants among your roses to encourage thrips-eating beneficials such as lacewings and minute pirate bugs to come to your garden.
Cultural tip: Select darker-colored rose varieties. Because the thrips can drift on the wind, it is important to space plants. Keep gardens free of debris and weeds, which are a favorite environment for thrips’ development.
Chemical control: Pesticides must make direct with the adult insect to be effective, and thrips are protected within the buds and plant tissues.
Caveats of chemical intervention
Focus: Identify and target just the pest. Follow label directions
Unintended consequences: Secondary pest outbreak. Pesticides kill pests, but many can also harm the beneficial insects that keep other garden pests in check. For instance, if you use a pesticide to manage aphids — a primary insect pest — you might increase the population of spider mites, a secondary pest. Spider mites then become the primary pest, and you would require a miticide to get rid of the spider mite infestation.
Target pest resurgence: The initial target pest can reach higher numbers than before applying the pesticide because the beneficial insects — which were also killed with the application — take longer than the pest to reach numbers that will manage the offender.
Natural or organic pesticides: These are not necessarily “safe” and can still be toxic to beneficial insects. Neem oil can smother insects on . Spinosad is a microbial-based insecticide that has a negative impact on some beneficial insects, and it is toxic to bees when wet. Products used for caterpillar control also affect butterflies.
Broad spectrum pesticides: Use of these often results in a spider mite infestation.
Pesticides must be alternated with those with a different mode of action to avoid pests developing pesticide resistance.
Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension. For more information, visit rose-harmony.com.
San Diego Rose Society Rose Show and Auction
The San Diego Rose Society will host its 97th annual Rose Show from 1 to 5 p.m. May 3 and 10 a.m. to 3:30 p.m. May 4 at the Ronald Reagan Center, 195 E. Douglas Ave., El Cajon. A Rose Auction will be held at 2:30 p.m. on May 4. For more information on these free events, visit sandiegorosesociety.com.
