Experts tackle readers’ garden questions.
Q: I have two gardenias that both seem to have something eating at them. It’s chomping on both new and old foliage but I can’t find any evidence (of pests such as snails) on or under the leaves, on the stems, or under the plant. One plant isn’t super healthy (not enough sun, I suspect) and is nearly defoliated. The other is pretty healthy but appears to be under similar attack.
Sacramento’s “Bug Man” Baldo Villegas: I doubt they are snails or slugs. I hope that David checked for the slime trail typical of snails and slugs.
The damage that I see is probably weevil damage such as Fuller rose weevil or black vine weevil or maybe another weevil. These weevils, cause this damage especially on the edges of the leaves. They are most active at night but one can usually find them if one turns enough leaves as they feed from the underside of the leaves.
I would put a piece of cloth under the plants and shake them for a quick look to see if indeed there are weevils or maybe even a moth larvae.
If they are indeed weevils, the control is tricky as the larvae feed on the roots of the plants and there may be an established population of the weevils (in the soil). However, I would try to remove them by vigorously shaking the plants over a piece of cloth and dispose of any of the insects that might be dislodged by this procedure. If weevils are found, then an insecticide that lists soil insects on the label might be used to control the larvae.
More than 1,000 species of weevils or “snout beetles” are found in California. According to University of California integrated pest management research, black vine weevil (Otiorhynchus sulcatus) is the most common. These pests, which measure about half an inch long, attack a wide range of ornamental plants including azalea, rhododendron, euonymus, grapes and liquidambar. Its larvae, which attack plant roots, are white.
The adult weevils, which are black and do not fly, feed on foliage, notching the sides of leaves and creating jagged edges. Handpicking or trapping these weevils, then disposing of them, is the best method of control.
The Fuller rose weevil (Asynonychus godmani) does similar damage and attacks far more than roses. It eats foliage on many species of ornamental shrubs and fruit trees including acacia, box elder, citrus, oak, photinia, hawthorn, rose, plums and pears. These snout beetles are brown with bulging eyes. Its larvae are yellow.
Long ago, I saw similar damage later in the season – early to mid May – at (a Sacramento) house that was being done by an interesting species of leafcutter bee. When I first saw the damage, I guessed that it would be katydids or moth larvae, but as I was looking around, I saw the bees coming and going with pieces of leaf. I took pictures of it to show that not all leafcutter bees cause the same damage.
Baldo Villegas is a retired state entomologist and master consulting rosarian. The Bee’s Debbie Arrington contributed to this report.
Questions are answered by master gardeners at the UC Cooperative Extension services in Sacramento and Placer counties. Send questions to Garden Detective, P.O. Box 15779, Sacramento, CA 95852. Send email to email@example.com. Please put “Garden Detective” in the subject field and include your postal address. To contact UC Extension directly, call:
- Sacramento: 916-875-6913; 9 a.m.-noon and 1-4 p.m. Monday-Thursday
- Amador: 209-223-6838; 10 a.m.-noon Monday-Thursday; website: ceamador.ucdavis.edu
- Butte: 530-538-7201; 8 a.m.-noon and 1-5 p.m. weekdays
- Colusa: 530-458-0570; 9 a.m.-noon and 1-4 p.m. Tuesdays; website: cecolusa.ucanr.edu
- El Dorado: 530-621-5512; 9 a.m.-noon Tuesday-Friday
- Placer: 530-889-7388; 9 a.m.-noon Tuesday-Thursday or leave a message and calls will be returned; website: pcmg.ucanr.org/got_questions
- Nevada: 530-273-0919; 9 a.m.-noon Tuesday-Thursday or leave a message
- Shasta, Tehama, Trinity: 530-242-2219; email firstname.lastname@example.org
- Solano: 707-784-1322; leave a message and calls will be returned
- Sutter, Yuba: 530-822-7515; 9 a.m.-noon Monday-Tuesday and 1-4 p.m. Thursdays
- Yolo: 530-666-8737; 9-11 a.m. Tuesdays and Fridays, or leave a message and calls will be returned