- Author: Lauren Fordyce
It may be after Labor Day, but some of your plants may still be wearing white, breaking that long-standing fashion “rule”! While many of us don't adhere to this old rule for our wardrobes these days, you may care about white stuff on your plants this time of year.
There are several white colored pest insects and diseases that you could be noticing on your plants.
Scales
Several types of scale insects are white. Each has their own host preferences, or plants they feed on.
- Cochineal scales feed only on cacti, usually prickly pears. On the outside they are white and waxy but have bright red bodies. If you scrape them off, you'll notice a red stain left behind.
- Cottony cushion scale feeds on a variety of woody ornamental plants. Common hosts in California are citrus, cocculus, nandina, and pittosporum. Adult scales are elongated and fluted.
- Euonymus scale feeds on many different plants but can be a serious pest to Japanese Euonymus (Euonymus japonica). Immature males are white and elongated with a brownish-yellow area at one end.
See the UC IPM Pest Notes: Scales to learn more about these and other scales and how to manage them.
Mealybugs
Mealybugs are whiteish-gray, oval, wax-covered insects. They can be found on many different plants, from fruit trees to houseplants, often hiding in protected places like branch crotches.
Powdery mildew
Powdery mildew often appears as white powdery spots (fungal spores) primarily on the tops of leaves. It can infect fruits and fruit trees, vegetables, and landscape plants in shady conditions when temperatures are between 60 to 80 degrees.
Whiteflies
Whiteflies are tiny insects with white, waxy wings and yellow bodies. They feed in large colonies on the undersides of leaves and have a very wide host range, including trees, vegetables, houseplants, and flowers.
Hackberry wooly aphids
This introduced aphid species infests Chinese hackberry and other hackberry (Celtis) plants. Hackberry wooly aphid adults appear in white, fuzzy masses on shoot terminals and leaves.
Spittlebugs
Adult spittlebugs are often brown, but they produce a white, foamy excrement on plants that looks like spit. They are known to feed on just about any plant.
Mushrooms
Many fungi produce white fruiting bodies that may pop up in your lawn, mulched landscape beds, or on trees and stumps. Puffballs and other fungi in lawns are mostly just a nuisance. However fungi on trees can sometimes cause wood decay and harm trees.
For more help diagnosing a plant problem or identifying a pest, see the UC IPM Plant Problem Diagnostic Tool or contact your local UC Master Gardeners.
- Author: Maria Murrietta
- Contributor: Dr. David Headrick
We're now seeing the aftermath of this long rainy season. The heavier than normal rain has given us velvety green hills carpeted with an abundance of wildflowers. But we cannot overlook the other less pretty things that benefit from a wet winter – lots of weeds and insects.
For now, we'll focus on insects. They're already showing up in a news-worthy fashion. KSBY did a feature story on whiteflies and spoke with UC Master Gardener Cathryn Howarth.
This story has been posted on various social media pages and people are wondering if this whitefly is a new pest, where it came from and what can be done about it. I wanted to follow up to fill in some of the gaps in information. So, I went to Cal Poly entomology professor, Dr. David Headrick who has done quite a bit of research on whiteflies over the course of his career.
Here's what he had to say after watching the news story.
"The whitefly pictured on hibiscus (in the news story) is giant whitefly which came to California from Mexico. It was first noticed in San Diego County in the early 1990s. Hibiscus is a favored host plant, but they also are seen on citrus, but more commonly on the landscape plant Xylosma. In the mid-1990s, a biological control program began with researchers at the University of California, Riverside. They successfully imported two beneficial species of tiny stingless wasps that feed exclusively on giant whitefly and achieved excellent control of giant whitefly."
"Giant whitefly and the two beneficial wasps (pictured above) all occur in San Luis Obispo County and normally the populations are all under good control. When the giant whitefly first invaded SLO county in the early 2000s, the populations were enormous and made the news then also. But the wasps soon brought whitefly numbers under control. However, in some years since then, environmental conditions have favored the giant whitefly and allowed them to outpace their natural enemies. Eventually, the wasps will catch up and population balance will be restored."
"There are many species of whitefly, most are native species that cause no serious harm to their host plants. The whiteflies that you see on broccoli, citrus, oaks, and poinsettias are all different species. Some of the invasive species of whiteflies, like giant whitefly, can indeed kill their host plants."
"As for management, spraying plants with the garden hose is a good approach when numbers are low."
Thank you, Dr. Headrick, for your insights.
UC Master Gardeners always recommend monitoring your plants regularly to catch pest populations before they become a problem. Whiteflies can be observed any time of day. Remember to flip over the leaves to look for evidence of nymphs and the waxy coating as pictured below. Insecticides, such as horticultural oils may provide some relief. However, direct contact with the insect is necessary to smother and kill them which is difficult to achieve, particularly for whiteflies that congregate under the waxy coating on the underside of leaves.
If you have questions about plant or pest issues, call the Master Gardener Helpline
Arroyo Grande: 805-473-7190
SLO Office: 805-781-5939
Templeton: 805-434-4105
Or email us at anrmgslo@ucanr.edu
Looking for more?
- Visit UC IPM for more photos and information: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html
- For more information about the early biological control program and findings -
UC Riverside research paper - http://www.nhm.ac.uk/resources/research-curation/projects/chalcidoids/pdf_X/BellowMe2000b.pdf
Cal AG article - http://calag.ucanr.edu/Archive/?article=ca.v051n06p5
- And if you really want o geek out, like I sometimes do, check out this catalog of introduced species - https://bugwoodcloud.org/resource/pdf/FHAAST-2018-09_Arthropod_Biological_Control.pdf
- Author: Jeannette E. Warnert
A common springtime nuisance, whitefly populations have escalated this year in California Central Coast areas, reported Megan Healy on KSBY Channel 6 in San Luis Obispo.
People are mistaking them for clouds of pollen or ash; some leaves look like they're coated with a thin layer of new-fallen snow.
The reporter spoke to Cathryn Howarth, a UC Cooperative Extension Master Gardener in San Luis Obispo County. She said the office has received dozens of calls from residents wondering why the whiteflies are so abundant this year.
"It's probably a reflection of the increasing temperatures coming on top of all that nice water we have had, so we have had a flush of vegetation and the flies that came from eggs originally have just all hopped out," Howarth said.
The story referenced the UC Agriculture and Natural Resources Integrated Pest Management Program, which has produced a Pest Note on whiteflies that says signs of an infestation include:
- Tiny nymphs on the underside of leaves
- Sticky honeydew on leaves or a covering of black sooty mold
- Yellowing, silvering or drying of leaves that have whitefly nymphs on them
Howarth said home gardeners can spray the whiteflies with a stream of water from a hose or spray them with insecticidal soap to reduce the population. The pest has abundant natural enemies in the yard, including ladybugs, spiders, lacewings and hummingbirds. However, whitefly outbreaks can occur when the natural biological control is disrupted.
For more information on whitefly management, read the UC IPM Pest Note on whiteflies.
- Author: Ben Faber
Wow, woolly whitefly covered with waxy, curly filaments , Aleurothrixus floccosus.
One of the consequences of fire and the resulting ash is that the biocontrol agents that keep whiteflies, scale, mealybug and other pests in balance is that they will spend so much of their time preening that they don't have time to go after their prey. Lacewing larvae, minute pirate bug, ladybird larvae, parasitic wasps and others rely on moving around to get at their food sources. When they cant move fast, they stop and clean their joints to stay limber. Whitefly and scale insects just hunker down and don't need to do a lot of moving. They just breed, and without actively moving biocontrol agents, their populations can explode. Or that's my human analogy. In dusty areas or areas affected by ash, the particles get in their joints and they need to spend time cleaning in order to move fast.
Whiteflies suck phloem sap, which in some cases can cause leaves to wilt and drop when there are high numbers of whiteflies. However, the primary concern with whiteflies is the honeydew they produce. Honeydew excreted by nymphs and adults collects dust and supports the growth of sooty mold; large infestations blacken entire trees, including fruit, as well as attract ants, which interfere with the biological control of whiteflies and other pests. The sooty mold can also affect tree yields by reducing photosynthesis and requiring extra handling time for cleaning.
So pests under good control prior to a fire can get out of hand. This is a good example of a tree in the town of Ventura where ash was a problem. A seemingly clean tree, free of whitefly, started to defoliate with blotchy leaf spots. On the undersides of the leaves corresponding to the blotches are colonies of whitefly. And looking closely you can see that some of the nymphs have exit holes, indicating that they have been parasitized by a wasp. So nature is kicking in and taking it's course. The whitefly should get cleaned up soon too by some forager, such as lacewing larvae or pirate bug. No need to spray because it would just be a further disruption.
See more about whiteflies at:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html
Photos: Defoliating 'Meyer' lemon tree, blotches on upper side of leaf, whitefly colonies with exit holes in some of the nymphs
![lemon leaf drop lemon leaf drop](/blogs/blogcore/blogfiles/49738.jpg)
![lemon leaf blotch lemon leaf blotch](/blogs/blogcore/blogfiles/49739.jpg)
![lemon scale lemon scale](/blogs/blogcore/blogfiles/49740.jpg)
Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County
found on underside of leaf
Response from the Help Desk: Thank you for sending the photo of your citrus leaf with what clearly looks like a whitefly infestation. Whiteflies are tiny, flying insects that derive their name from the mealy white wax covering their wings and body. They suck the leaf's sap, which in some cases can cause leaves to curl and/or wilt and drop when populations are large. Whiteflies can be abundant in vegetable, fruit, and ornamental plantings, especially during warm weather.
However, the primary concern with whiteflies is the honeydew they produce. Honeydew excreted by nymphs collects dust and supports the growth of sooty mold, which is the black stuff you see on the leaf in your photo. (Sooty mold does not cause damage to the plant unless it's so heavy it blocks all sunlight from reaching the leaves.) The honeydew however attracts ants, which can "protect" the whiteflies from natural predators which would usually keep the whitefly population under control. These natural predators can also be harmed by the home gardener spraying with broad-range products such as insecticidal soaps.
Management of heavy whitefly infestations is difficult so prevention is best. Removing heavily infested leaves or hosing them with “sharp” water sprays is a good way to get a handle on an infestation. You will need to be vigilant in watching for whiteflies in the future. Since dusty leaves are more attractive to these insects, regularly washing off the plants with water should help a lot. Also, controlling ants will go a long way to allowing the natural predators to do their job. Placing sticky yellow traps (available at garden stores) on the trees (or plants or nearby) can also be used to monitor for whiteflies and will give you a warning that you need to increase your inspections and controls.
Here is more information from UC about whiteflies and their control: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7401.html and also information about ants and their control: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7411.html.
Please do not hesitate to contact us again if we can be of help in your gardening.
Happy gardening!
Help Desk of the UC Master Gardener Program of Contra Costa County (SEH)
Note: The UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog (http://ucanr.edu/blogs/CCMGBlog/).