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Hot weather brings out spider mites, mosquitoes

spider mite webbing on spirea
Telltale webbing of spider mites covers this spirea. The cure? Water. (Photo: Debbie Arrington)

Watch out for sunburned plants, too

Recent blazing hot weather has brought out heat-related pest and garden issues – while making other problems disappear.

First the good news: Gone is powdery mildew. It can’t survive in this heat. We shouldn’t see another outbreak of this fungal disease until October or later.

Also unable to cope with triple-digit temperatures are immature brown marmorated stink bugs and leaf-footed bugs. This heat wave likely toasted a whole generation of these destructive pests. That also means fewer bad bugs in August.

But some pests like it hot.

Leaf with stippling
These tomato leaves show stippling by spider mites.
(Photo: Kathy Morrison)

With no rain for weeks, it’s also dry and dusty – perfect conditions for spider mites. If you see their telltale webs or stippling on leaves, get out the hose. Wash the plant down, removing the dust and likely many mites.

Spider mites attack a wide range of plants including roses, fruit trees, berries, grapevines, beans, squash, melons, tomatoes and more. Miticides are not recommended; they kill many beneficial insects in addition to the mites, which often manage to evade pesticide sprays.

For more advice, see the UC IPM pest notes on spider mites:

Also out in force now are mosquitoes, which are a threat to people and pets as well as wildlife. According to the state’s West Nile Virus website, five dead birds found in Sacramento County last week tested positive for WNV. That brings the 2020 county total to 12. Four more Sacramento County mosquito samples last week tested positive, too, bringing that total to six.

County officials urge residents to be alert for dead birds and report them to the WNV bird hotline, 1-877-WNV-BIRD (1-877-968-2473).

Protect yourself, too. Wear mosquito repellent when working outdoors. At dawn and dusk when mosquitoes are most active, wear long sleeves and long pants.

For more info and the latest WNV statistics, visit .

While most plants need full sun, too much sun (and heat) can cause problems, too.

Shaded plant
A plastic plant flat serves as shade for a Patio Yellow potted tomato plant.
(Photo: Kathy Morrison)
Just like people, plants can get sunburned. If you notice crispy edges to leaves or pale patches on ripening tomatoes and peppers, that plant may need some shade. Construct a temporary shade structure to give plants some relief. It can be as simple as cardboard pieces on top of a tomato cage or burlap loosely draped over stakes.

Sunburn also can damage bark, especially on young trees or shrubs, opening up the plant to wood-boring pests and premature death. It tends to be worse on the west- and south-facing sides of plants. Painting the trunk and exposed lower limbs with diluted interior white latex paint (1:1 dilution with water) can protect a young tree's bark.

Besides a little shade, consistent irrigation can help plants survive too much sun.

For more tips on helping your garden deal with sunburn, see the advice of UC Cooperative Extension experts at .


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For week of Dec. 10:

Take advantage of these dry but crisp conditions. It’s time to get out the rake!

* Rake leaves away from storm drains and keep gutters clear.

* Fallen leaves can be used for mulch and compost. Chop up large leaves with a couple of passes with a lawn mower.

* Prune non-flowering trees and shrubs while they’re dormant. Without their foliage, trees are easier to prune.

* Rake and remove dead leaves and stems from dormant perennials.

* Make sure to take frost precautions with new transplants and sensitive plants. Mulch, water and cover tender plants in the late afternoon to retain warmth.

* Succulent plants are at particular risk if temperatures drop below freezing. Don’t water succulents before frost; cover instead. Use cloth sheets, not plastic. Make sure to remove coverings during the day.

* Clean and sharpen garden tools before storing for the winter.

* Brighten the holidays with winter bloomers such as poinsettias, amaryllis, calendulas, Iceland poppies, pansies and primroses.

* Keep poinsettias in a sunny, warm location. Water thoroughly. After the holidays, feed your plants monthly so they'll bloom again next December.

* Just because it rained doesn't mean every plant got watered. Give a drink to plants that the rain didn't reach, such as under eaves or under evergreen trees. Also, well-watered plants hold up better to frost than thirsty plants.

* Plant garlic (December's the last chance -- the ground is getting cold!) and onions for harvest in summer.

* Bare-root season begins. Plant bare-root berries, kiwifruit, grapes, artichokes, horseradish and rhubarb. Beware of soggy soil. It can rot bare-root plants.

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