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Rose Curculio

by Betty Mott, Master Rosarian

The Western Rose Curculio is one of the most destructive enemies of the rose. Just a few beetles are capable of destroying the entire spring bloom cycle on small bushes. For example on large clumps of roses the injury ranges from about 10 percent to almost 100 percent. The adults will also feed on raspberry buds, and other hosts such as boysenberries, blackberries, and hollyhocks, as reported by a San Jose rose gardener.

Rose Curculio has found its way into rose gardens in 11 western states; Washington, Oregon, Idaho, Montana, Wyoming, Colorado, Arizona, New Mexico, Utah, California, North Dakota, and North Carolina.

This insect has been observed to feed readily on all forms of wild and cultivated roses. Although in my garden they seem to favor Old Garden Roses first, then move on to just about anything left standing in the rose garden. After years of trial and error methods to keep deer away from my roses and eventually finding a solution, I am as determined to fight off this rose pest without the use of chemicals and insecticides. My garden has been free of chemicals, pesticides and herbicides for over 15 years. I rely on beneficial insects, birds, organics, handpicking to remove insects, compost, mulch, and tolerance to keep my roses happy.

My first lines of defense with the rose curculio began with frequent observations and documentation in my garden journal, reading articles and research on the Western Rose Curculio, and gathering information from fellow rose gardeners experiencing the invasion of this destructive insect. Finding a solution that works for your gardening practices begins with getting to know your enemy. The research is fascinating and the rose curculio is one clever animal with many unique adaptations making it a tough challenge.

This June I unfurled the petals of an infected rose bud to discover the pearly white egg, peeled open mummified buds to find larvae, and found what I hoped to be the last curculio, placed it in a jar with infected buds and watched it go to work. I observed the curculio using the strength of its entire body to push its long black beak deep into the bud making several holes. My desk lamp by the computer seemed to activate the curculio and I sacrificed a few fresh ‘Cecile Brunner’ buds adding them to the jar. It was like watching a seagull on french fries. Antennae flying it proceeded to seek out the new buds taking between 5.23 and 5.36 minutes from start to finish to infect the bud. When a bud was showing color the process took less than 50 seconds. I was observing what I had read about. The adult captured in the jar made feeding punctures feverishly for three days then died.

And So I begin with a simple question, “Why worry about a few curculio in your rose garden?”

Rose Curculio Damage The feeding punctures of the adult beetles are seen as small holes in the buds with a thick sticky black secretion on the buds that have been punctured. Most of the punctures are made in the petals, sometimes through the sepal into the petal or in the base of the bud. They are also made on the stem below the bud. When buds are not plentiful the young tips of shoots may be eaten off or the stem punctured so that the tip will curl and dry. Some of the buds that have been punctured from feeding will open and the petals will be riddled with holes, giving the flower a ragged, unsightly appearance. It is difficult to distinguish the feeding punctures from those where the eggs are deposited. In addition to the egg punctures the female will puncture the stem of the bud so that it will wilt and usually bend over and dry, leaving a mummified bud in which the larva develops. In a lab study a single pair of beetles in rearing cages made as many as 99 punctures in rosebuds in 24 hours. In that same study the maximum number of eggs obtained from one egged female was eight. This means as many as eight buds may be destroyed daily by one female in egg deposition, and in addition she may make a large number of feeding punctures, which disposes of rose buds rather rapidly. All sizes of buds are ruined from very small to those ready to open.

As a Master Rosarian, consulting with many distraught rose gardeners with curculio infestation, I decided to gather as much information as possible to help those of us dealing with this rose pest. Depending on which methods of control you choose to apply in your rose garden it is best to understand the life cycle of the pest to begin to get a handle on the problem.

Description of Stages

The eggs are about one-twentieth of an inch long and can be easily seen with the naked eye. They are elliptical in shape and pearly white in color. Under a microscope the surface is pitted. The eggs are deposited in punctures of the rosebud at depths just beneath the surface of the bud to about 2mm below the surface. These punctures are plugged and are hard to see except as a slight scar on the bud, or a small protuberance on the side of the wilted rosebud. Eggs are most commonly found in the folds of the petals rather than the base of the buds. One female observed in the act of oviposition spent 8 minutes enlarging the puncture before turning around and in less than 50 seconds deposited the egg. Then the opening was plugged with the beak by chewing off bits of petal along the edge of the puncture and placing them in the opening to close off the hole. This part took another 2 and 1/2 minutes to complete. In one study the average number of eggs deposited by females were 37. The maximum for one female was 143 and the minimum was 16. The largest number deposited by one female in 24 hours was eight. Most of the eggs are deposited from the latter part of May to the middle of June and as late as August, but this is highly unusual.

Rose Curculio Larva When first hatched the larvae measure about one-twelfth by one twenty-fifth inch coiled, and are white. The full grown larva is about one-fourth inch long and one-eight inch wide. The larva is a legless grub and pale yellow or straw colored. The small larvae, in the dry buds are found among the stamens in most cases. The pollen is their main food when first hatched. Later, when the larvae become two-thirds grown or almost full grown, they are found in the base of the dried bud. The buds remain attached to the plant or fall to the ground. With some varieties of roses the injured buds drop readily. When full grown the larvae leave the buds and enter the ground, where they pass the winter in small earthen cells about 6 mm in diameter and about 1/2 to 2 inches below the surface. Larvae have been dug up from the soil beneath rose bushes in October, November, February, March, April, and early May. Some Larvae may not transform to pupae the following spring but remain over for two winters as a safety factor against unfavorable conditions. In the study larvae emerged from the buds July 16 to October 6, periods of 93 and 142 days. This would indicate that the larvae period may be as short as 45 to 50 days.

The pupae are uniformly white and the length being about one-fifth of an inch. The head is bent downward with the beak along the sternum. This takes place in the earthen cells.

It takes 4-5 days to become hardened and a day or two to work its way to the surface. In dry soil the adults were unable to work their way to the surface, but when the soil was moist it reached the surface and began feeding. Weather conditions have a lot to do with the timing of each cycle. The beetles begin feeding on the young growing tips of the rose shoots, eating the young leaves, stems, or rose buds. The tip of the shoot may be killed. “Could this also play a part of blind shoots we see on some rose bushes?” As the adults emerge they feed largely on the early varieties of roses and as buds become scarce move on to hybrid teas and other later bloomers. Feeding punctures may be made at the rate of four to five per minute and as many as 40-50 a day. When disturbed the beetles draw in the feet and depress the beak, usually dropping off the plant and remaining motionless for some time. When falling at the base of the rose plant among the prickly stems they are well protected and difficult to find. Rarely do they fly when disturbed. Mating takes place at all times of the day. Each male and female may mate several times with different individuals. Males were seen fighting, using their beaks raised above their opponents and bringing them down with considerable force.

Length of life
The average length of life of the adults after emerging is about 7 weeks. The average length of life of 109 beetles was 41 days. Males averaged 43 days and females 40 days.

The adults are robust bright red the under surface beak and antennae are black. The beak is almost as long as the head and thorax. The beak of the female is typically longer than that of the male. The adults are strong flyers and this is probably the most important means of spreading.

Summary of the Life Cycle

The winter is passed as full-grown larvae in the soil. About April or May the larvae pupate in the soil and in about 9 days transform into adults, emerging from the ground about 2 weeks later. The adults feed on the young rose shoots and buds; they mate and deposit eggs in the rosebuds, puncturing the stem beneath so that it withers and dries, sometimes falling to the ground.

The eggs hatch in about 10 days, and the larvae feed inside the dry rosebud, leaving it in the fall to enter the ground, where winter is passed. Thankfully, there is one generation a year.

Photos by the author, Betty Mott









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