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Some pests love hot, dry weather

Spider mites, tomato hornworms spiking now

Spider mites love hot, dry conditions. Ash and fallen leaves cling to spider mite webs covering a camellia. (Photos: Debbie Arrington)

Some of the biggest and smallest pests to attack our gardens are enjoying these last days of summer – much to our dismay. While many critters retreat in high heat, these invaders actually spike in numbers and activity as the mercury rises.

Recent dusty, hot and smoky weather has been terrible for people. Spider mites think it’s heaven. They don’t mind the ash clinging to their webs.

These itty-bitty arachnids thrive in hot, dry, dusty conditions, attacking water-stressed plants.

According to the UC Cooperative Extension master gardeners, spider mites can multiply especially rapidly during these conditions.

“If the temperature and food supplies are favorable, a generation can be completed in less than a week,” say the master gardeners’ pest notes.

No bigger than a pinpoint, a spider mite is a minute arachnid, the same eight-legged class that includes spiders and ticks.

Spider mites spin telltale webs, which are much more noticeable than the tiny creatures. They attack just about any plant, from strawberries to full-size trees, sucking out moisture.

Beneficial insects usually can outnumber the spider mites and keep them under control. But when the weather turns unusually hot, dry and smoky as it has been, the predators tend to retreat, allowing the spider mites to take hold.

The solution? Water. Take the hose and spray dust and ash off leaves, making sure to get the undersides of foliage, too. A strong shower disrupts spider mite paradise and holds them at bay until the real rains come (hopefully) in fall.

To keep them away, keep the garden well watered. Irrigation is key to spider mite control. They don’t like it wet.

Tomato hornworm on leaf
Tomato hornworms will munch on leaves, stems and fruit (both
unripe and ripe) of tomato plants.

Meanwhile, the heat also brought out a late-summer surge of one of the biggest bugs in the vegetable garden: The tomato hornworm.

Natural enemies usually keep their populations under control. But hornworm numbers can spike in late summer with high heat.

Fat as a finger and just as long, hornworms rank as Sacramento’s largest caterpillars. They eat big bites out of their favorite food: Ripe tomatoes. They’ll also eat green tomatoes, leaves and stems.

Their stripes let them hide in plain sight. They blend in so well with their surroundings, they can seem impossible to spot.

If you suspect hornworms, look for their poop. They leave large black or green droppings on or around the plant. If you see those droppings, carefully inspect the plant’s leaves and stems. When you find it, pick off the hornworm and dispose of it.

If the hornworm escapes capture, it burrows into the soil and pupates into a moth of equally gigantic proportion: The hawk moth. Emerging in spring, this brown and gray moth has a 5-inch wingspan.

Rototilling the tomato bed after harvest prevents those moths from ever developing – and laying eggs next spring.

For more on spider mites:

For more on tomato hornworms: ml


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Dig In: Garden Checklist

For week of June 4:

Because of the comfortable weather, it’s not too late to set out tomato and pepper seedlings as well as squash and melon plants. They’ll appreciate this not-too-hot weather. Just remember to water.

* From seed, plant corn, pumpkins, radishes, melons, squash and sunflowers.

* Plant basil to go with your tomatoes.

* Transplant summer annuals such as petunias, marigolds and zinnias.

* It’s also a good time to transplant perennial flowers including astilbe, columbine, coneflowers, coreopsis, dahlias, rudbeckia, salvia and verbena.

* Let the grass grow longer. Set the mower blades high to reduce stress on your lawn during summer heat. To cut down on evaporation, water your lawn deeply during the wee hours of the morning, between 2 and 8 a.m.

* Tie up vines and stake tall plants such as gladiolus and lilies. That gives their heavy flowers some support.

* Dig and divide crowded bulbs after the tops have died down.

* Feed summer flowers with a slow-release fertilizer.

* Mulch, mulch, mulch! This “blanket” keeps moisture in the soil longer and helps your plants cope during hot weather.

* Thin grapes on the vine for bigger, better clusters later this summer.

* Cut back fruit-bearing canes on berries.

* Feed camellias, azaleas and other acid-loving plants.

* Trim off dead flowers from rose bushes to keep them blooming through the summer. Roses also benefit from deep watering and feeding now. A top dressing of aged compost will keep them happy. It feeds as well as keeps roots moist.

* Pinch back chrysanthemums for bushier plants with many more flowers in September.

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