Putting insecticidal soap to work

Insecticidal soap can be an effective against many of the small, soft-bodied insects and mites that attack landscape shrubs, trees, flowers, vegetables, and houseplants. They may be used on fruits and vegetables until the day of harvest and are one of the few alternatives available to homeowners for control of spider mites. However, these products must be used carefully because of the potential for plant tissue burn – phytotoxicity.

Knowing a few key points will allow you to get the most from these products.

Recognize target pests

Develop your identification skills. Some pests are very vulnerable to insecticidal soaps; others are not. Many sap feeders, such as aphids, scale crawlers, plant bugs, mealybugs, thrips, and spider mites usually can be controlled by insecticidal soap. However, caterpillars, sawfly larvae, and many beetles are not affected, for a variety of reasons.

How insecticidal soaps work

Insecticidal soaps are contact insecticides. The exact impact is unknown but it appears that soaps penetrate and disrupt the pest’s exoskeleton, its internal cell membranes, or block the spiracles- openings through which arthropods breathe. Consequently, pests must be thoroughly wetted to be killed. The spray needs to be applied to runoff to upper and lower leaf surfaces. There is no residual effect after the spray has dried. It is best to treat early or late in the day when the spray droplets will evaporate most slowly.

Pest control afforded by insecticidal soap should be apparent within two days of application. Something in the range of 50% to 60% control would be normal because not all individuals are hit by the spray, some may hatch from eggs present when the treatment was applied, or some may arrive after treatment.

Check treated foliage two or three days after an application to evaluate control. Several treatments may be needed to significantly reduce heavy infestations. Follow label directions on re-treatment intervals; overtreatment may injure the plants.

Product use tips

Read the product label carefully; look for warnings of plants that can be damaged by insecticidal soap. Some of the plants listed on labels as being sensitive to soap sprays include:  Azaleas, Begonias, Camellias, cherries, ferns, Fuschia, Impatiens, jade plants, hawthorn, palms, sweet pea, plums, Portulaca, and some tomato varieties.

Application of insecticidal soap to stressed plants, young, tender seedlings, or when temperatures are high may result in injury, even to plants that normally tolerate the spray. When in doubt, treat a small area and check in a day or two to for phytotoxicity (leaf burn or distortion). Don’t spray during the hottest part of the day or when temperatures are above 90F.

Substituting other soaps and detergents

Insecticidal soaps are potassium salts of fatty acids that have been designed and formulated for use on plants and have been evaluated for potential to injure plants. Some liquid hand soaps and dishwashing detergents are used successfully for insect control but there is a greater chance of phytotoxicity when using these products.



Backyard bug-zappers

Insect electrocuters provide some impressive crackles, flashes, and sizzles on summer nights but apparently little pest control around the home landscape. Homeowners expect to rid the area of mosquitos, and perhaps a variety of other pests, by using these devices. Research has generally shown little positive effect. A study from Delaware investigated the “catch” from electrocuters placed around some homes near potential mosquito breeding sites during June and July. Of the 13,000+ insects eliminated by the electric grids, only 0.2% (31 specimens) were biting flies. On the other hand, more than 1,800 beneficials- including predators and parasites- were dispatched (about 13% of the catch). Not much return on the investment -Especially when considering that more than 45% were aquatic insects that were at least, harmless and at best, important fish food.

Several factors contribute to this performance. For example, UV lamps that give off a lot of visible light are less attractive to mosquitoes and other night-biting flies than those that give off only UV light. That is, the more visible the light to us, the poorer it is in luring biting flies. Also, many species of mosquitoes are not attracted by black lights at all. For species that do respond to UV, only a portion will actually make it to the charged grids.

Phylloxera: Leaf bumps and leaf drop of hickory and pecan

Several species of Phylloxera, small aphid-like insects, live on hickory and pecan. Chemicals injected as the insects feed stimulate gall production on leaves, leaf petioles, twigs, or nuts. These hollow plant growths provide food, shelter, and protection for the insects. Their impact on the tree can vary depending on how many are present and where they settle to feed.


Phylloxera gall on petiole

Galls on petioles can cause premature leaf drop, which has been seen in several areas of the state this year. Usually, light to moderate defoliation does not affect healthy, established trees but several years of significant leaf loss can stunt them and make them susceptible to other stresses. New trees in the landscape are much more vulnerable to injury.


Developing green gall


Maturing red gall will turn black

Galls may stunt the growth of infested twigs; ones that are heavily infested may die. Galled nuts may be deformed and drop early.

Galled leaves are dramatic, especially when the galls turn from green to red but are less problematic.

Phylloxera spend the winter in the egg stage inside old galls or in bark crevices of hickory and pecan. The eggs in the spring and the tiny insects crawl to newly expanding foliage. After finding a suitable spot, they settle and begin to feed. This stimulates growth of surrounding tissue, forming the gall.

open gall

Opened gall with large female Phylloxera, Eggs are visible in left corner of picture

Some species have one generation each year, others have several. In addition to providing food and a favorable environment, the gall protects the insects from natural enemies and insecticide sprays.

Phylloxera usually don’t cause significant injury to established, thriving landscape trees but high infestations can stress or stunt newly established trees. Application of a dormant oil in the winter may help to reduce potentially damaging infestations.

Calico scale – a serious pest of landscape trees

Adult female calico scales are easy to see on the trunk of this infested tree. They are large, about 1/4 inch in diameter, and distinctive: dark brown and white. It’s as if they are asking for attention, and they should get it!

Prying up a few of these large females should reveal hundreds of small eggs that will hatch into active crawlers soon after the females die. The crawlers will move to feed on sap from expanded leaves during the summer. This scale is most vulnerable now and the opportunity should not be missed. Once dispersed into foliage, control is much less successful. The crawlers will not return to bark until autumn, just before leaf fall. There, they will spend the winter and mature. There is one generation each year.


Insecticide applications, timed to coincide with emergence of young crawlers, will break the cycle of development and help to reduce further plant stress.  The ideal time to treat is when about 75% of the globular adult females have turned brown, which should be soon.

Several tree and shrub insecticides can provide good control. Read the product label to be sure scales and your tree species are listed. Thorough coverage of infested twigs, branches and adjoining leaves is important. Eggs hatch over a period of several weeks so a second application 2 to 3 weeks after the first may provide more complete control. Horticultural oil, insecticidal soaps, and tree injections of systemic insecticides have not been very effective against this scale insect.

Calico scales infest honeylocust, hawthorn, hackberry, sweet gum, yellowwood, dogwood, flowering crabapple, and sugar and Norway maples. During part of their life, they cover the trunk and branches, feeding on the phloem tissue. Like other soft scales, they excrete large quantities of nutrient rich liquid waste (“honeydew”) that drops onto leaves and branches.  Soon, the deposits are covered with sooty mold fungus.  The sweet liquid also will attract bees, wasps, and flies.

Infestations usually don’t kill trees but severely weaken them, making them highly vulnerable to wood borers, drought and other stresses.

Understanding insect repellents



Insect repellents are chemicals that protect us from blood-feeding arthropods:  primarily mosquitoes, ticks, black flies, and biting gnats. Using a repellent doesn’t make us invisible to pests. They will still be attracted to our warmth, movement, and exhaled CO2. Repellents work at the skin level to disrupt the feeding process just before they bite.

You can search for repellents on this EPA web page: http://cfpub.epa.gov/oppref/insect/#searchform. You pick the desired protection time and types of pests that will be encountered and get a list of registered repellents. Always read the label carefully before buying and using repellents. It will state precautions and any age restrictions. If no age limit is listed, then the EPA has not required a restriction.

Knowing something about the major types of repellents and how to get the most out of them can make time outdoors a lot more enjoyable and may reduce health risks. Here are some points to consider:

1) Likely pests. Some repellents are more effective against certain blood feeders than are others. Knowing what you will be facing will help in the selection process. For example, some repellents may be more effective against mosquitoes than ticks.

2) Potential disease threat. No one wants to be bitten but bites become much more important if you can be exposed to carriers of Rocky Mtn. Spotted fever; Lyme disease; or encephalitis, such as West Nile virus. Repellents are particularly important if there is a disease threat but there are other things that you can do, such as frequent self-examination for ticks.

3) Exposure period and pest intensity. Activity of biting pests varies with location and time of day. More frequent applications or use of products with greater concentrations of the repellent chemical are needed when you are outdoors for long periods of time or where biting arthropods are very troublesome. Look for pests and protection times on product labels.

4) Exposure conditions and your “meal appeal”.  Use of a repellent with a higher concentration, or more frequent applications will be necessary if you are active and perspiring freely or periodically getting wet.  People vary in attractiveness to biting arthropods, some are attacked ferociously while companions are not bothered. “Meal appeal” is related to the amount of attractants, like lactic acid. and other chemicals produced by individuals. More frequent repellent applications may be needed to protect people that attract a lot of biters.

5) Active ingredients in the product and personal preference – biopesticide or conventional. DEET, Picaridin, IR3535, and oil of lemon eucalyptus are recommended for use by the Centers for Disease Control and Prevention (CDC) based upon their endurance and effectiveness against biting arthropods.

Here is a quick review of some of the active ingredients in common repellents; they are divided into biopesticide repellents and conventional repellents.

Biopesticide repellents

These essential oils are concentrated volatile/aromatic compounds extracted from plants.  The most common materials are:

Oil of lemon eucalyptus (PMD) is a plant-derived repellent that is effective against mosquitoes, black flies, biting midges, gnats, and ticks.  It is as effective as DEET but provides shorter term protection. It is an eye irritant and should not be used on children less than 3 years old.

Citronella comes from lemongrass and related plants. It is available in candles, torches, and some topical repellents and provides limited protection against mosquitoes.  Citronella is not effective against ticks and other biting pests.

Other essential oils are from catnip, geranium, peppermint, and soybean. They provide limited protection against mosquitoes and are not effective against other biting flies, gnats, and ticks.

IR3535, based on a naturally occurring amino acid, is moderately effective against mosquitoes, other biting flies, and ticks.

Conventional repellents

DEET is the standard by which repellents are judged. It provides very effective protection against mosquitoes and works well against other biting flies, fleas, and ticks. DEET is available in a wide range of concentrations. In general, the higher concentrations provide longer protection. However, use of concentrations greater than 30% may cause skin irritation and does not provide increased protection. According to the American Academy of Pediatrics, DEET may be applied by an adult to children over the age of 2 months. DEET can dissolve some synthetic fabrics and plastics.

Picaridin is derived from pepper. It can be as effective as DEET against mosquitoes, biting gnats, fleas, and ticks. In contrast to DEET, it is relatively odorless, not oily, and does not damage synthetic fabrics and plastics.

Non-skin application repellents

Permethrin is an insecticide with repellent activity against mosquitoes, other biting flies, fleas, chiggers, and ticks. It not to be applied to the skin; products are available for application to boots/shoes, clothing, tents, mosquito netting, etc.

Metofluthrin, similar to permethrin, is available in impregnated repellent strips that can be hung around activity spots or in battery powered devices worn by people.  Metofluthrin vapors from the strips or devices repel mosquitoes.

Other ways to reduce bites – Cover exposed skin when outside and avoid being outside when mosquitoes are most active.

Repellent is a general term but individual products may work in very different ways. We don’t understand a lot about how repellents work but we can tell if they are effective or not.


Insect repellents PM Fact Sheet 5105 http://umaine.edu/ipm/ipddl/publications/5108e/

Insect repellents: use and effectiveness http://cfpub.epa.gov/oppref/insect/#searchform

Updated information regarding insect repellents  http://www.cdc.gov/ncidod/dvbid/westnile/repellentupdates.htm

May beetle- defoliators in the night


May beetle

May beetles have the general shape of Japanese beetles but come in various shades of brown. They emerge from pastures and turfgrass, feed ravenously at night, and hide during the day. These beetles prefer, and may completely strip, the foliage from oaks, birch, elm, hickory, and walnut. Entire leaves may be eaten or petioles may be cut so leaves drop to the ground. May beetles also will feed on ash, fruit trees, hackberry, locust, Lombardy poplar, maple, plum, and willow. Damage often is heaviest on trees next to pastures or large expanses of turf – the source of these insects. May beetles have 2- to 3-year life cycles. Most of that time is spent as a white grub, feeding on the roots of grasses.


May beetle damage

Insecticides labeled for trees and shrubs can provide some protection from feeding by these insects. Small trees, especially those that are relatively new in a landscape, can benefit from treatment but it is impractical to treat large, established trees, and they generally can tolerate moderate defoliation without harm.