Gardening: July's “least wanted” list

We have heard of the Interpol “Most Wanted” list, featuring international gun runners, armed robbers and other similar offensive culprits. As the dog days of summer approach, July gardeners face very similar offensive culprits, the “Least Wanted” list of garden pests.

By Charleen Barr
Colorado State University Extension Master Gardener in Larimer County

Here are some common pests facing our gardens in July:
Aphids are seen on most plants, from herbaceous to hardwoods, at various times of the year. They come in colors of green, black and red, and they suck the juices right out of a plant. The good news is they are relatively easy to control. Spraying a hard stream of water will knock them down. Most oils and soaps will do the job too. Keep an eye out for the beneficial insects that feed on them. Lady beetles feast on aphids, at both the larval and adult stages. (For more information, refer to “Aphids on Shade Trees and Ornamentals” Fact Sheet 5.511 at the CSU Extension website

Spider mite activity peaks during the warmer months of summer. One important spider mite is the two-spotted spider mite (Tetranychus urticae). This mite attacks a wide range of garden plants, including many vegetables (e.g., beans, eggplant), fruits (e.g., raspberries, currants, pears) and flowers. Dry conditions greatly favor all spider mites. At the same time, most of their natural enemies require more humid conditions and are stressed by arid conditions. One reason spider mites become problems in yards and gardens is the use of insecticides that destroy their natural enemies–minute pirate bugs, predatory thrips, lady beetles and predatory mites. Adequate watering of plants during dry conditions can limit drought stress on spider mite outbreaks. Periodic hosing of plants with a forceful jet of water can physically remove and kill many mites, as well as remove the dust that collects on foliage and interferes with mite predators. (Refer to “Spider Mites” Fact Sheet 5.508.)

Flea beetles are common pests of many vegetable crops. Adult beetles, which produce most plant injuries, are typically small, often shiny, and have large rear legs that allow them to jump like a flea when disturbed. Flea beetles produce an injury known as “shotholing.” The adults chew many small holes or pits in the leaves, which make them look as if they have been damaged by fine buckshot. Dozens of species of flea beetles are found in Colorado. Each type of flea beetle has a preference for certain plants. Some flea beetles feed only on potatoes, tomatoes and other members of the nightshade family. Others have a taste for broccoli, cabbage and other cole crops. Floating row covers or other screening can exclude the beetles. In isolated plantings, thick mulches may also help reduce the number of flea beetles by interfering with the activity of the root and soil stages. (Refer to “Flea Beetles” Fact Sheet 5.592.)

Grasshoppers can be the most noticeable and damaging insects to yards and fields. Over 100 species of grasshoppers occur in Colorado and their food habits vary. Among vegetable crops, certain plants are favored, such as lettuce, carrots, beans, sweet corn, and onions. Squash, peas, and tomatoes (leaves, not fruit) are among the plants that tend to be avoided. During periods when a local outbreak develops, control usually involves using sprays or baits. To be successful these need to be applied to developing stages of grasshoppers and concentrated at sites where egg laying occurs in late spring. By July, the ability to control grasshoppers declines as grasshoppers develop and migrate. (Refer to “Grasshopper Control in Gardens & Small Acreages” Fact Sheet 5.536).

Pear slugs are strange insects that can damage the foliage of cherry, plum, apricot, pear, hawthorn, mountain ash, cotoneaster. The larvae feed on the upper surface of the leaf, so they are well exposed. As they feed they avoid the main veins, producing a characteristic “skeletonizing” injury. There are two generations per year of pear slugs. Larvae will complete development in July and move to the base of the plant where they will pupate. Adults of these usually emerge again in late August and September to produce a second cycle of feeding injury. Pear slug is very susceptible to soapy water. (Refer to “Pear Slug” Fact Sheet 5.560.)

The codling moth is the “worm in the apple” — the larvae of the codling moth, Cydia (Laspeyresia) pomonella. These “gorging gourmets” hatch soon after the developing fruit is about three-fourths inch in diameter (early-to-mid-June), burrow through the fruit skin into the flesh and gorge themselves for three to four weeks until they’re fully grown. Then they eat out an exit hole and leave the fruit, ready to begin the cycle all over again (late July). The most effective control is to thoroughly spray your tree(s) shortly after flower petal fall and then every two weeks through the summer with permethrin or carbaryl. Never apply insecticides when the tree(s) are blooming, so as to avoid killing beneficial pollinators. Control for coddling moth should extend over the entire summer following the first egg hatch. (Refer to “Codling Moth: Control in Home Plantings” Fact Sheet 5.613)

The peach tree borer is the most destructive insect pest of peach, cherry, plum and other stone fruits in Colorado. External evidence of peach tree borer tunneling is a wet spot on the bark or the presence of oozing, gummy sap. Most injuries occur along the lower trunk beneath the soil line. Peach tree borer can be difficult to control because insecticides cannot reach the damaging larvae after they move under the bark. The most effective controls are preventive insecticide applications at the vulnerable egg and early larval stages, while the insect is on the tree bark. In Colorado, egg laying occurs during the middle of the growing season. In warmer areas, it may begin July 1 and continue into September. In general, peak egg laying occurs from mid-July to mid-August. (Refer to “Peach Tree Borer” Fact Sheet 5.566)

Oystershell Scale so named because it resembles a minute oyster attached to the bark of trees. Aspen, ash, lilac, cotoneaster, willow and poplars are among the many plants in the region highly susceptible to this insect. Oystershell scale can kill trees, often because of the increased susceptibility to disease. Encourage natural predators (parasitic wasps and lacewings) by planting flower plants near the infected trees. Horticulture oils, such as Neem, will also be affective. (Refer to “Oystershell Scale” Fact Sheet 5.513)

Mosquito management in the backyard indicates that the best control is still to minimize any standing water in the area and use an effective mosquito repellant that contains DEET. Chemical or biological control DEET methods (mosquito dunks, bricks, or sprays) will kill larval and/or adult mosquito populations in small areas such as a backyard. For more information visit, or

In conclusion, keeping insects out of a garden is not a realistic goal. A complicated food chain exists from the soil to the treetops, and insects are an essential part of a healthy ecosystem. Some are bad guys that damage plants or spread disease, while many more are allies in pollination and overall pest control. Understanding pests, their life cycles and what they feed on, will help in selecting the control and when that control should be applied. Gardening is full of challenges and rewards. Take the time and learn more about all aspects of the garden and enjoy.

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