You should have applied the most important bug spray of the year – the dormant oil sprays – during the winter.
Why then? Because that’s when insects sensitive to these sprays are most vulnerable.
Why didn’t you do it then? Because there was no problem.
Dormant oil sprays (sometimes called horticultural oils) are “insurance” sprays. Without insurance sprays, you will be spraying insecticides when you see problems. This forces you to exterminate the problem (and the bug causing the problem) when it is seen.
Examples of insect pests that you will encounter without dormant oil sprays include aphids, whiteflies, mealybugs, and mites – collectively called soft-bodied insects. Dormant oil sprays do not affect borers or ugly leaf-footed bugs.
Dormant oil sprays are applied on warm, windless days during the winter to trees and shrubs after pruning. The entire tree or shrub is sprayed from head to toe.
I usually give it a last shot of dormant oil just at the base of those trees and shrubs where insects like to find protection during the winter. Also, kill and remove any weeds that grow in the area. Wintering insects also like these places.
White grubs live underground in the soil by chewing on the roots of plants and end up transforming into many different types of flying beetles such as the so-called June bugs and hornworms which feed on the leaves of tomatoes, of grapes and other edible products. Because many white grubs live underground, you don’t see them until plants like lantana die.
The most effective and long-term controls for many types of white grubs are insect-eating nematodes which you can purchase from online sources. Remember that these should be applied to damp floors and kept in the refrigerator until applied.
Aphids first appear when new leaves and flowers appear. Preventing curled, sticky leaves is where you’ll likely see the greatest impact from dormant oils applied in winter. If not applied, the mother aphids begin to produce live young a few hours after they arrive on the leaves in early spring.
I generally recommend controlling nearby ants as they will help spread these aphids on the leaves. Ants are secondary to the problem but deserve to be controlled if they are in large numbers.
One evergreen tree forgotten during winter spraying of dormant oil is the Italian cypress. The spider mite is a common pest of Italian cypress that causes browning during the hot summer months. Spider mites are easily controlled by a late winter application of dormant oil rather than waiting for an infestation to appear.
Unfortunately, dormant oil sprays do nothing to control borers or the ugly leaf-legged plant bugs. These guys are good aviators and can enter your yard from your neighbors’ plants. Thus, unless there is a community effort to reduce these pests, methods other than insecticide spraying are available for their control.
Q: I see a big yellow bee flying around my newly planted oak tree. Do they sting?
A: Yes they do. Even the sweetest bees and wasps will sting if they are aggravated. I can’t tell you which bee has your tree, but if it looks like a bee and flies like a bee, then it’s probably a bee or a wasp. And most likely, it can sting.
This bee is probably attracted to the sticky, sweet honeydew produced by aphids. These aphids inhabit new, soft, tender shoots as soon as they appear. If your tree is evergreen, like most oaks, these aphids have likely overwintered in the shelter of this tender springtime food source.
Mother aphids keep their wings through the winter and fly to new growth when it seems to be starting a new colony of young. These newly born aphids are wingless.
Dormant oil applications during the winter would have significantly reduced or eliminated them in the spring. At present, several soap sprays directed at the leaves will dissolve the sticky honeydew and reduce its appeal to bees as well as the number of aphids.
Do this until the temperatures get warm. As with any organic spray, soap sprays need to be applied more often to achieve the same results and are indiscriminate in what they kill.
Q: You talk about wood chip mulch instead of rocks on the soil surface for plants, but I find wood chips provide refuge for cockroaches and other pests that cause other problems. For this reason, I prefer to use rock.
A: The main value of wood chips applied to the soil surface instead of rock is to provide organic material to the soil as it decomposes or decomposes. Rocks decompose, but they end up mineralizing the soil instead of adding organic matter to it.
This mineralization can take three to five years after planting, but some plants will turn yellow during this time, while others will not. Some plants grow best when organic matter is continuously added to the soil instead of minerals.
Insects like cockroaches and bedbugs are decomposers that help break down wood chips into small pieces. Earthworms are often next in this process, followed by microorganisms like bacteria and fungi which break them down into plant nutrients.
If you have a lot of woodchip applied to your landscape, expect to find a lot of decomposers as well. These pest critters need to be managed so that they do not become a problem.
They are drawn to water, food and heat. Use your favorite insect repellent inside the irrigation boxes and keep wood chips 2 to 3 feet from the foundation of the house.
The water applied shouldn’t be any closer to the foundation than this anyway. Remember that the roots of the plants follow where the water is applied. No water applied, no root growth.
Your home is welcoming when the outside temperatures are not. In late spring and early fall, apply a barrier spray to keep these insects away from your home.
Your favorite barrier spray applied to the foundation of the house helps repel pest creatures when they need a change in temperature. The same principle applies to strawberries and other red fruits like tomatoes touching the ground.
Q: I have roses with sticky leaves from aphids that feed on them. I need to get them under control before they become a problem. I have found insect repellent and disease control sprays at the nursery that have worked in the past. Have an opinion about them?
A: They work, and if you’re not averse to using insecticides, use them. If you look at the ingredient list, the pesticides that control insects and others used for disease control contain both an insecticide and a fungicide. You usually don’t need both ingredients.
In climates with high humidity, rose diseases like black spot, powdery mildew, and botrytis are common. In these climates, it’s probably a good idea to apply a multipurpose insecticide plus a fungicide spray.
In our climate, diseases are less of a problem due to lower humidity. I would focus on an insect repellent chemical rather than a combination of two chemicals. Why apply fungicide if you don’t need it?
Q: One of my roses is wilting even though it is watered regularly. He receives the morning sun. My other roses don’t do that. What is happening?
A: It looks like damage from western flower thrips to rose petals. Thrips are very small insects that tear and shred rose petals into crumbs. You cannot see them unless you use a magnifying glass.
They will spend the winter waiting for new growth to appear before attacking. As soon as the rosebuds form, they attack the soft petals.
Thrips tear and shred soft plant tissue and coat the fluids released by the tears like a dog licking water from a bowl. When these insects are in large numbers, they attack the pink flowers while they are still in their buds, causing them to remain closed but damaged. They also attack the fruits of the nectarine when they are very small, causing scarring.
The usual remedy is to remove all the flowers and rid the plant of this pest, then spray a dormant oil if it is early in the season. Spinosad sprays have been effective in warding off thrips. They do not kill them like other more toxic sprays but offer a safer alternative because spinosad is considered a natural product.
Bob Morris is a horticultural expert and professor emeritus at UNLV. Visit his blog at xtremehorticulture.blogspot.com. Send your questions to [email protected]