FIMBY: Battling early-season tomato pests

This is another installment in our Food in My Back Yard series, focused on edible gardening.

Here it is early June in the vegetable garden. The tomato plants are green and thriving, with their cute little green tomatoes tucked  under the big leaves. Harvest is several weeks off, which means you can just let the plants be for now, right?

Nope. The tomato gardener needs to be on top of threats at all times, and this part of the season brings a trio of insect pests that are most easily battled right now.

The big three:

— Tomato hornworms

— Leaf-footed bugs

— Spider mites

Any of these three can sneak into the garden and do damage before the gardener is aware of what’s going on. And then the impulse is to run for some chemical to obliterate them. Which probably won’t work.

So be on watch now, and fight these invaders before they get a stronghold in the garden — using integrated pest management techniques as recommended by the UC IPM program. Each one has a specific, non-toxic approach in the home garden:

Tomato hornworms

These are the green caterpillars of a variety of sphinx moth (Manduca quinquemaculata), a large moth with a 5-inch wingspan. The hornworms feed primarily on tomato plants, but on occasion can be found on other members of the Solanaceae family, including eggplants, peppers and potatoes.

They’re named, of course, for the small dark spike or horn on the end of the abdomen. (Tobacco hornworms, which are similar, have reddish spikes.) They are voracious eaters, demolishing leaves, stems and even fruit if allowed to grow big enough. And they are well camouflaged, with their stripes resembling the underside of tomato leaves, which is where they often hide.

How to find them? The clue is sprinklings of their black frass (poop) on leaves — look above that for the culprit. At dusk or later, another way to find them is with a blacklight (ultraviolet) flashlight — they glow in the dark.

In either case, the hornworms can be plucked off the plant (I tend to clip off whatever they’re clinging to). Chickens apparently love them; so do my neighborhood birds. Alternately, drop them into a bucket of soapy water or squish ’em with your garden boots.  (A co-worker once told me she stopped growing tomatoes simply because she hated dealing with hornworms — a sad decision.)

If you have a lot of birds in your neighborhood, you may never see hornworms — the moths as well as the caterpillars are their prey. This is true of my community garden, though one fellow gardener who tends to shade all his tomato plants recently found hornworm damage.

A severe hornworm infestation can be fought with Bt spray or dust on the plant. Bacillus thuringiensis is “a selective, biological insecticide that is derived from a naturally occurring soil bacterium,” the UC IPM site notes. “After eating the spores of this active ingredient, insects die of infection and starvation.”

However: Bt needs to be used specifically and sparingly. It also will kill other caterpillars, including ones that produce butterflies.

Leaf-footed bugs

Leaf-footed bugs will not kill tomato plants, but they will suck juices out of the fruit, discoloring them or causing distortions. Note: They like pomegranates as well as tomatoes.

These nasties are why it’s important to keep weeds away from the vegetable garden: The adults overwinter in weeds, then move into the nice fresh growth of spring and summer veggies. They lay eggs end to end in straight rows, and the shiny orange-bodied nymphs tend to hang out in packs. That’s when it’s best and easiest to stop them, since the whole crew can be knocked into a bucket of soapy water.

Adults are brown with a white zigzag across their wings, and their hind legs have the distinctive leaf-shaped enlargements. They are harder to knock into buckets, since they can fly. However, these insects have several predators, including birds, spiders and assassin bugs.

UC IPM recommends against insecticides for adult leaf-footed bugs. “The most effective insecticides against leaf-footed bug are broad-spectrum, pyrethroid-based insecticides, such as permethrin. However, these products are quite toxic to bees and beneficial insects. Insecticidal soap or botanicals, such as neem oil or pyrethrin, may provide some control of young nymphs only.”

Spider mites

These tiny pests are my personal nemeses. Like leaf-footed bugs, they do damage by piercing/sucking, and can obliterate half the red on the skin of a ripe tomato if given the chance. But their first damage can be very hard to notice — and the mites themselves are almost impossible to see with the naked eye.

Spider mites are members of the arachnid family, and they prove it with the fine webbing that shows up on plants with a severe infestation. Early on, however, light speckling of leaves may be the only clue they have moved into the garden.

Here’s the biggest clue about spider mites: They like hot, dusty conditions — see: Central Valley in June through September. They also migrate to plants under water stress, so adequate irrigation of those tomato plants is important. The mites won’t kill a tomato plant, but they can make one a real mess, and extensively discolor the fruit.

So fight them with water! As the weather heats up, in addition to keeping plants well-irrigated, wet them down in the morning. Aim a strong spray from the bottom of the plant up, so the undersides of the leaves (where the mites hang out) get the force of the spray.

Spider mites also have predators, including predatory mites, but they may not show up until late in the season. One year I had such a bad early infestation that I ordered predatory mites from a commercial supplier.  You need to have a lot of mites for the predatory ones to stick around, and I did. They managed to slow down the damage at least, but it was a hard lesson.

Meanwhile, applying insecticides actually might worsen the problem, UC IPM says. “Carbaryl, some organophosphates, and some pyrethroids apparently also favor spider mites by increasing the level of nitrogen in leaves. Insecticides applied during hot weather usually appear to have the greatest effect, causing dramatic spider mite outbreaks within a few days.”

Oils and soaps should not be used on hot days (over 90 degrees), and in any case only are effective on contact with the mites.