Control whiteflies as soon as you see them

A: Whiteflies are a bad insect problem for any plant. I would rank them with the hard-to-control list of insects. In small numbers, they can be tolerated, but their populations grow so quickly that small numbers lead to large numbers very fast. For that reason, it’s important to get them under control early, as soon as you see them, in late spring and early summer. Don’t wait.

Damage is associated with their feeding. Their feeding supports a quick growth in their population as summer temperatures get hotter. They love the heat, and they’re not seen around much until June or July. Once you see them and it’s hot, watch out. You are behind the curve and they are way ahead of you.

Control is two-pronged. First, spray pomegranate bushes on a warm day in December and then again in January with a dormant oil. You can use the commercial brands of dormant oils and horticultural oils, or you can use soybean oil, canola oil, rosemary oil or cinnamon oil.

Spray all of the branches after pruning top to bottom, and spray a little extra shot at the base of each tree. This spray suffocates insects that might be hiding out during the winter months. It is the most important spray for controlling insects that will be problems in 2020.

In late May, start weekly inspections of the leaf undersides for whitefly adults. The adults look like living dandruff. You probably won’t see the eggs or immature forms because they are so small, but you will see the adult females preparing to lay eggs or protecting their young so that they can build their populations as quickly as possible.

Pull these leaves off when you see them infested and dispose of them or vacuum them. This practice slows their populations way down.

At the beginning of June, hang bright yellow or bright blue thick construction paper in the tree smeared with Vaseline. Whiteflies fly toward bright yellow and bright blue objects, and the Vaseline causes them to get stuck. As these traps start to fill, replace them with new ones. It might look a little funny, but light reflected from aluminum foil repels them.

If populations are totally out of control, spray with an insecticide to get their populations back under control. As a homeowner, use pesticides when all other attempts fail. Spraying with insecticides, however, have other consequences and should be a last resort.

Q: Our gardener thought a worm of some kind was causing damage to some of our 8-year-old bushes in our backyard. A few of them are OK, and a few have yellowed with dead branches at the tops. I bought some Bioadvanced Tree and Shrub liquid to protect them but want your advice before I apply it.

A: Judging from the pictures you sent and small red fruit still hanging on the bushes, they look like pyracantha, aka firethorn. I am 99 percent sure the dieback you mention is from borers earlier in the year. Pyracantha damaged by intense sunlight attracts borers, particularly if they are pruned incorrectly and planted in very hot locations. I will get to the chemical you bought in just a second.

The yellow leaf color can happen for many reasons, but in this case, it is an early sign of dieback from borer damage. Yellow limbs turn brown nearly overnight during the heat of the summer.

Pyracantha shrubs generally do well in our desert climate, but avoid planting them in extremely hot locations in the yard. They don’t like to be surrounded by rock. Planting them in desert landscapes surrounded by rock is not a good thing. If your shrubs are treated this way, they will continue to die over the next few years, faster if they are pruned incorrectly.

Pyracantha grows best on the north and west sides of buildings. These locations provide relief from the hot sun in late afternoons. They grow well when planted in soils amended with rich compost or a planting mix with added fertilizer.

After planting, either cover the soil surrounding them in wood chips or use rock large enough so that compost can be sprinkled and washed through them with a hose every one to two years. Covering the soil with rock and not adding compost to it exhausts any organics in the soil. Plants surrounded by rock start to suffer after three to five years.

I rail against the use of hedge shears in landscapes, but in this case, pruning with hedge shears is a faster option. Pruning with hedge shears will encourage thick, shady growth and protect larger branches from sun damage. Otherwise, hedge shears are a no-no unless pruning a hedge.

You got the right chemical to apply because it contains imidacloprid. Don’t apply it in December but wait until after the pyracantha flowers in late spring. After the pyracantha finishes flowering the spring, apply this chemical as a soil drench soon after flowers are gone.

Q: Can you recommend a good drip irrigation valve to use? I currently have Irritrol 205 valves and replacement parts are readily available, but I’m wondering if it would make more sense to just replace them with a newer product?

A: Years ago, irrigation valves might or might not work with drip irrigation because of the low operating pressure and the sometimes small amount of water that flowed through it. Not anymore. Most irrigation valves can be used for drip irrigation now. The irrigation industry doesn’t want to lose out.

The Irritrol 205 series of valves are workhorses, and unless the body is cracked, I would keep it. Replacement parts are easy to get, and it operates at low pressure and can handle very low amounts of flowing water. It is a reliable and inexpensive valve for drip irrigation.

Make sure you match the drip irrigation equipment you’re using with an appropriate pressure regulator and use a 150-mesh filter. Always flush your drip irrigation system on a regular basis, usually monthly if it’s clean water, and every time it is repaired.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.