Garden Doctor: How to identify, prevent black widow spiders

Answer: The western black widow spider, Latrodectus hesperus, is quite common in California garden environments. Well adapted to our hot, dry climate; it likes cracks, crevices and clutter. The “black widow” emerges as a sexy femme fatale from folklore, where females supposedly cannibalize their mates. Males do look different and are less toxic, but not all the girls eat the boys, and males are toxic, too.

Proper identification is important as this spider is quite venomous. Black widow bites contain a neurotoxin and can be very serious, especially for children and older people. Damage may be hard to see, like a painful pinprick, and can present with flu-like symptoms. If you have been working in the garden and suspect a black widow bite, seek medical attention.

Mature females have big, fat, shiny black bodies about a half-inch long, excluding legs, with a characteristic red hourglass on the abdomen that can vary in shape and size. Their babies are much harder to identify, being tan with white bellies.

Proper identification should exclude the false black widow spider, or Steatoda grossa, which is a European import commonly found here. It has the familiar fat belly, but the spider is dark brown, has no characteristic red mark, and has a bite of fairly low toxicity. (For good pictures of western black widow spiders and their eggs, see http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74149.html, UC IPM Pest Note 74149.)

Look for irregular, surprisingly strong webs. Black widows are predators and hang, undersides up, from their webs at dusk. If you go looking as the light fades, you can see all kinds of spiders setting up shop. Often shy and retiring, black widows will be quite aggressive if they have eggs or babies with them.

The main attraction in the garden for black widows is a cluttered habitat and places to hide. Get rid of dropped leaves and old wood and rock piles and seal up open bags of compost, dirt and manure. A garden relatively clean of debris will prevent many future problems — overwintering pests, soil-borne diseases and bad spiders. When cleaning up your garden, good garden gloves and sturdy footwear prevent bites and stings.

Q: What are some good rules of thumb for watering and fertilizing citrus in Davis?  I’ve seen some yellow leaves on my trees, and wonder if they are due to overwatering, or not enough water or nutrients.

A: Citrus are never fully dormant because they are an evergreen subtropical shrub/tree that have adapted to the many of California’s climate zones. Because they are never fully dormant, they have year-round fertilizer and water needs. A mature tree probably will need fertilization with nitrogen on a fairly regular basis. Fall application of nitrogen should be avoided because it can affect fruit quality by delaying fruit coloring and creating a tough rind. Apply in January or February just prior to bloom with a second application in May. A third application can be made in June.

One of the most common causes for yellow leaves is nitrogen deficiency, shown as light yellow-green leaves and veins that are slightly lighter in color than the foliage. New leaves will be smaller and tend to be thinner. The mature leaves will become mottled, irregular yellow blotches that can cover the entire leaf. When this happens, the leaf falls from the branch.

If your tree is lacking magnesium, you’ll see those irregular shaped blotches at the base of the leaf, growing close to the fruit. Sunburn is another cause of yellowing and will show on the west and/or south sides of the canopy.

The roots of citrus are shallow and can extend twice as far as the canopy. Are you watering out far enough? A layer of compost mixed with a mulch under the tree but not touching the trunk will help to protect the roots, keeping them cooler in summer and insulating and adding nutrients to the soil in winter. Mulch will also reduce the incidence of weeds and prevent damage from soil borne diseases such as phytophthora. Try to water in the morning to give bark and foliage plenty of time to dry.

UC IPM Online has some very good, quick to read pages on citrus watering and fertilizing. Check out http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/CULTURAL/citfertilization.html and

http://www.ipm.ucdavis.edu/PMG/GARDEN/FRUIT/CULTURAL/citruswatering.html

— Send questions, addressed to the “Garden Doctor,” by email to mgyolo@ucdavis.edu, voice mail to 530-666-8737, or regular mail to UCCE Master Gardeners, 70 Cottonwood St., Woodland, CA 95695. Be sure to include your contact information because any questions not answered in the Garden Doctor column will be answered with a phone call or email to you.

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