Controlling Garden Insects

Numerous insects will be observed visiting or foraging in the garden throughout the growing season. Many of them are simply passing by, while others such as honeybees are attracted to the flowers and utilize the nectar as a food source while causing no harm to the flowers or the plants. But some insects feed on garden plants and can cause a significant amount of damage. Most insects are harmless and are often considered beneficial- either pollinating flowers or consuming other insects. In most instances, the damage to garden plants from insect feeding is minimal and does not require control strategies. In the landscape, there is often a natural balance that keeps most insect populations from rising and crop injury to a minimum. However, if insect feeding is beginning to make certain garden plants look ragged and unsightly, then it may warrant taking steps to control the pests causing the injury. It is not possible to cover every type of insect that may visit your garden in this article, but we think it is important to review several of the pests that are likely to cause injury to the annuals, perennials, and shrubs in your landscape and to provide some practical management strategies you can implement to reduce the amount of injury these pests may cause.

Insect Injury

The type of feeding damage to garden plants varies widely by the type of insect feeding on the plants. Insects with chewing mouthparts most commonly cause the leaf tissue to be consumed or destroyed. Some insects have piercing-sucking mouthparts, which cause the leaves to appear stippled, spotted, or distorted after feeding.

 

Here are some brief descriptions of the injury commonly caused by various garden pests.

 

  • Caterpillars and cankerworms often consume the entire leaf.
  • Leaves with only the margins consumed, are likely to have been eaten by black vine weevils or leaf cutter bees.
  • Japanese beetles often cause the leaves to appear skeletonized.
  • Leaf rollers cause the leaves to become rolled.
  • Some insects, like leaf miners, mine between the upper and lower leaf surfaces, leaving a visible path referred to as a mine.
  • Many insects such as Japanese beetle larvae or chafers prefer to prune or girdle plant roots.
  • Borers and bark beetles make tunnels under the bark of woody plants.
  • Leafhoopers, plant bugs, and mites cause the leaves to appear spotted or mottled with small dots.
  • Aphids cause the leaves to become distorted.
  • Other insects such as wasps and midges cause small galls, or swellings on the stems or leaves.


Once you see signs of insect feeding injury, it is helpful to identify the pest causing the damage. Since most insects prefer to feed on tender new growth or recently expanded leaves, look there first. Look for both insects and eggs on both the upper and lower leaf surfaces. Next, look at the leaf petioles and the stems. Finally, look at the leaf buds (growing point) and flowers for the presence of possible pests.

Insect Management

The first line of defense is to grow healthy plants; by providing plants the care they need to remain healthy, they will be naturally more resistant to insect feeding. Mother Nature is the second line of defense as there are a great number of beneficial allies that consume of parasitize insect pests.

 

However, there are instances when pest populations escalate even with the first two lines of defense are in place. In these instances, a third line of defense, such as the application of environmentally safe pesticides, may be necessary. There are a number of biological and botanical pesticides, such as insecticidal soaps, that are effective at controlling various insect pests. And finally, as a last resort when biological control strategies are not effective, it may be necessary to apply insecticides to control the pest population.

 

Insect control in the garden involves a well-rounded approach that includes good garden practices, prevention, and biological or chemical controls when needed. When such a strategy is in place, garden plants will remain comparatively problem free and appear beautiful year after year.

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Aphids

Aphids have small soft, pear-shaped bodies, measuring 1 to 3 millimeters in length. Most aphids can be identified by the two tubes projecting from their abdomen called cornicles, which resemble small tailpipes. Aphids are the only insects that have these tubes on their abdomens. The color and size of the aphids vary with species, environmental conditions, and the host plant. Common colorations include green, brown, black, purplish, red, or pink.

 

Aphids are usually all females that give live birth to young nymphs. They are usually wingless, but winged aphids may be observed on occasion. These slow moving insects have piercing-sucking mouthparts they use to remove plant fluids from within the plant. Aphids have a very wide host range and are commonly observed on a wide variety of garden plants.

 

They are usually spotted on the young succulent stems of flowering or non-flowering plants, but are commonly present on nearly every above ground plant part. They are gregarious and quickly form large colonies on young succulent stems of flowering or non-flowering plants. Aphids produce a digestive by-product which contains sugars and other chemicals called honeydew.

One signal that aphids are present is the presence of small white skins left behind during the molting or growth process. These skins are sometimes easier to see than the live aphids themselves. The presence of aphids can also be signaled by the presence of ladybird beetles, parasitic wasps, and lacewing larvae, all of which utilize aphids as a food source. Ants are commonly observed feeding on the honeydew of aphids.

 

Low populations of aphids can be controlled using a couple of simple strategies. To control them without using any biological or chemical strategies, simply spray the infested plants vigorously once a day in the early morning for 3 to 5 days to knock the aphids off of the plants they are feeding on. When washing them off, be sure to deliver this forceful spray to the undersides of the leaves. Aphids can also be controlled by spraying them with natural products such as insecticidal soaps (try our Garden Safe Insect Soap), neem oil, or pyrethrum. Spraying a diluted dish soap solution also provides good control of aphids. If these strategies do not work, they can be easily controlled using chemicals containing bifenthrin, cyfluthrin, deltamethrin, esfenvalerate, imidacloprid, and malathion.

Caterpillars

There are a number of insects, namely butterflies and moths, whose immature or larval forms, often referred to as caterpillars, feed on garden plants and often cause significant amounts of plant damage. These caterpillars of various sizes include a diverse group of larvae including armyworms, cutworms, leaf rollers, and loopers.

 

Caterpillars only cause injury to plants while they are in the larval stage. Once they become adults they either do not eat or only feed on nectar. Caterpillars have strong jaws, allowing them to consume large amounts of foliage, tender stems, and even flowers. Many times, their feeding causes serious plant injury, leaving behind ragged, unmarketable plants. Caterpillars are usually host specific, feeding on certain types of plants or plants within a particular family. The damage left behind varies with the species present and includes consumption of part or entire leaves, rolling leaves, or tunneling through stems. Besides the damage to plants, the occurrence of fecal deposits (frass) on plant leaves is a good confirmation that caterpillars are present.

 

Depending on the population, control methods can range from simply removing these pests by hand to controlling them using various biological or chemical strategies.

The most common biological method is using the bacterium Bacillus thuringiensis var. kurstaki (Bt). Bt, marketed as Dipel, is very effective at controlling caterpillars and is practically non-toxic to insects, animals, or humans. The active ingredient must be consumed by the foraging larvae and needs to be applied when they are first observed. Multiple applications of Bt may be necessary to increase the residual activity. Caterpillars can also be controlled naturally using one or two spray applications of pyrethrum. There are numerous chemical insecticides (bifenthrin, carbaryl, cyfluthrin, esfenvalerate, malathion, and permethrin) which effectively control caterpillars.

Japanese Beetles

During the summer months, many gardeners are often plagued with severe Japanese beetle (Popillia japonica) infestations. As the adults emerge from mid-June thru early August, there seems to be an endless supply of these voracious feeders which consume a wide array of herbaceous and woody ornamentals. They generally feed only during the day, and can consume plant foliage quickly, particularly when there are lots of them feeding simultaneously.

 

Adult beetles measure 0.375 inch (10 mm) long by 0.25 inch (7 mm) wide; the front of the beetle is metallic green and the wing covers are metallic brown. Other key identifiers are the two patches of short white hairs on its rear and the five white hair tufts along each side of the dorsal abdomen. The adults are skeletonizers, feeding on the leaf tissue between the veins leaving the veins behind; the injured leaves resembles a lace-like appearance and often withers and dies in the days ahead. They occasionally consume the entire leaf and flower petals of some plant species.

 

The eggs are laid beneath the soil surface in July and August and do not emerge as adults until the following summer. The larvae, known as the white grub, are easily identified measuring about 1.25 inches (32mm) long (the last 3 instars).

Controlling these insects is very difficult as the most effective control strategies involve controlling the larval stages. Unfortunately, even when the grubs are effectively controlled around the home and garden, the adults will likely fly in from outlying areas. Larvae can be controlled in the lawn and garden naturally using Bacillus popilliae (milky spore disease) or chemically using products containing imidacloprid in the fall.

 

The adults can be difficult to control since new adults may fly into the treated areas and re-infest the crop. Pheromone traps are highly effective at attracting beetles from over 0.25 mile (0.402 km) away. These traps should not be placed near the garden as they attract far more beetles than they can capture; place them far, far away from the plants you are protecting. The adults can also be hand-picked from the plants and crushed or dropped into a pail of soapy water; this method is time consuming and fairly ineffective as new beetles are likely to fly in and replace the beetles you’ve removed.

 

There are several insecticides containing bifenthrin, carbaryl, deltamethrin, malathion, or permethrin that effectively control the adults with contact sprays. However, repeated applications may be necessary because these products have relatively short residual effects and new adults may enter the production site following the each application.

Leafminers

Leafminers are small flies belonging to the family Agromyzidae that commonly feed and lay eggs in numerous garden plants. The most common species of leafminer (Liriomyza trifolii) are tiny yellow and black flies measuring only 2 millimeters in length. Although the adults feed on the plants, they do not cause significant injury to them. It is the larva that causes unsightly injury to ornamentals.

 

Female leafminers make small punctures on the upper leaf surfaces with their ovipositor (egg-laying organ). These punctures have two purposes, the males and females feed on plant sap that exudes from them, and the females will lay a single egg in some of them. The punctures turn white and take on a speckled appearance over time. After they hatch, the small larvae slash open surrounding plant cells, using their sickle-like mouth parts. As the plant cells rupture, the larvae move through them, destroying more cells as they move forward. As they travel between the upper and lower leaf surfaces, they weave winding trails through the leaf. These trails are commonly referred to as ‘mines’ and greatly reduce the aesthetic appearance of the plant.

In most instances, the adults will not be observed in the garden since they are not usually present in large numbers. When ‘mines’ become visible, pick or prune off and destroy all infested leaves. In instances where leafminers are problematic year after year, they can be repelled by weekly applications of insecticidal soap beginning in mid June. Insecticides containing acephate, bifenthrin, and permethrin are also effective at suppressing leafminers when they are applied weekly.

Slugs and Snails

Slugs and snails are some of the most frustrating pests many gardens face. Although they are often considered an insect pest, they are not insects at all. They belong to the phylum Molluska, which contains clams, crustaceans, oysters, and octopus. Even though they are not related to insects in any respect, they are very similar in their biology, and behavior. The brown garden snail (Helix aspera), the gray garden slug (Agriolimax reticulates), and the greenhouse slug (Milax gagates) are some of the most common species threatening ornamental crops.

 

Slugs and snails have bodies that are soft, unsegmented, and slimy. The head has one pair of short tentacles located near the front, used for touching and smelling, and another longer pair with an eye at the distal end of each tentacle located on the top of the head. They move by gliding along on a muscular ‘foot’. This muscle constantly secretes mucous, leaving behind the distinguishing ‘slime trail’ used for detecting their presence. The biggest distinguishing characteristic between these pests is snails have an external shell large enough to house their entire body and slugs do not have a visible shell.

Both slugs and snails are active mostly at night, but can frequently be seen moving about on cloudy overcast days. During sunny days, they seek refuge from the heat and bright light in plant debris, under boards, or any other cool dark place that is protected from the sun.

 

They chew irregular holes with smooth edges; feeding injury often resembles the injury from some caterpillars, often causing some confusion for gardeners. To confirm the injury was caused by slugs or snails look for the distinguishing silvery mucous slime trails they leave behind.

 

Controlling these pests often requires a combination of methods. Reducing the number of ‘hiding places’ they can hide during the day can greatly reduce their survival. Anything setting on the ground including boards, stones, boxes, debris, and weeds can provide adequate shelter for them during the day. Reducing these hiding places will decrease their ability to survive, causing them to look elsewhere for a better habitat.

 

Many gardeners make snail traps and homemade baits to attract and trap these pests so they can be destroyed. One method used to trap slugs and snails involves laying boards with ½ inch runners on the bottom side to allow slugs to enter under them, or placing old pieces of wet carpet on the ground. Check under the boards or carpet regularly, removing and destroying any slugs found. Another technique commonly used, involves burying traps at ground level with deep vertical sides to keep the snails and slugs from crawling out, and a top to reduce evaporation of an attractant such as beer, sugar-water yeast mixture, or grape juice. Jars or plastic containers coated with soap or grease can also be sunk into the ground. Once these pests enter the slippery-sided container, they cannot climb back out. All of these trapping methods are only good at trapping slugs or snails that are in the vicinity of the traps and do not work well for trapping pests over large areas.

 

Baits containing metaldehyde or iron phosphate (try our Garden Safe Snail & Slug Bait) are available for controlling slugs and snails. Baits work by several modes of action: metaldehyde baits cause over production of mucous- leading to desiccation and iron phosphate baits cause them to stop feeding- leading to starvation. Broadcast baits throughout the garden, particularly where the slugs seek refuge during the day. They are best applied under moist conditions, conducive to slug and snail activity, and less effective during very hot, dry weather conditions or cold times of the year, as these pests are less active during these periods. Baits are valuable tools used to reduce slug and snail populations, but will not eliminate them altogether.

Two-Spotted Spider Mites

The two-spotted spider mite (Tetranychus urticae) is the most common mite encountered by homeowners. Spider mites are not actually insects; they are related to spiders and ticks as they have eight legs, two body regions, and lack antennae. Although mites do have their favorite host plants, they can be observed feeding on a wide range of garden plants. Spider mites thrive during hot, dry conditions, but are a threat to plants in the landscape throughout the year.

 

Two-spotted spider mites can be easily identified by the two dark green spots on each side of their oval shaped, light yellow to green colored bodies. The adult female is less than a millimeter (1/50 inch or less) in size. They can be hard to see without a magnifying glass and most commonly feed on the undersides of plant leaves.

 

The most common injury symptom is small pinpoint sized spots, often described as a mottled or speckled appearance on the upper leaf surface. Usually there will be a patch of these small spots on one area of the leaf, but severe infestations will cause the entire leaf surface to appear mottled with these spots or the entire leaf may turn yellow and drop off. 

When high populations of spider mites are present, they produce fine silk webbing that becomes apparent and can eventually completely cover the leaves, flowers, or the entire plant.

 

When examining plants for spider mites, it is useful to turn over the older leaves first, looking for tiny white round eggs and immature or adult mites. Then work your way up the stem paying particular attention to buds and bloom for the presence of mites. The eggs and empty egg cases are usually located along the leaf veins. Under low mite populations, they are most commonly found under the lower surfaces of the leaves. With severe infestations, mites are often observed on all above ground plant parts and they plants may be covered with the characteristic webbing of the mites, which is where their common name comes from.

 

Similar to controlling aphids, spraying the infested plants vigorously once a day in the early morning for 3 to 5 days to knock the mites off of the plants they are feeding on and discourages them from becoming established. When washing them off, be sure to deliver this forceful spray to the undersides of the leaves. Spider mites can also be controlled naturally with spray applications of insecticidal soap, neem oil, or pyrethrum. Effective chemical controls include acephate, bifenthrin, or malathion. With spraying natural or chemical control products, be sure to provide good spray coverage of the upper and lower leaf surfaces. It often takes multiple spray applications to control two-spotted spider mite populations.

Whiteflies

The greenhouse whitefly (Trialeurodes vaporariorum) and silverleaf whitefly (Bemisia argentifolii) are the most prevalent whitefly species to infest plants in the landscape. Whitefly adults appear, like their common name suggests, as small white flies measuring 1/16 to 1/12 inch long.

 

It is difficult to distinguish between the various species of whiteflies. The greenhouse whitefly is larger than the silverleaf whitefly and holds its wings fairly flat over its abdomen in such a way they appear almost parallel to the leaf surface. The silverleaf whitefly holds its wings roof-like over its abdomen, at approximately a 45 degree angle with the leaf surface and has a slight yellow coloration.

 

Female whiteflies lay numerous spindle-shaped white to gray eggs on the undersides of the younger, upper leaves, often in a crescent-shaped pattern. The immatures are scale-like, cream to clear colored; they and are immobile also located on the underside of leaf surfaces.

Whiteflies have piercing, sucking mouth parts and in many instances do not cause detectable injury symptoms to garden plants. Severe infestations could cause some plants to turn chlorotic and mottled from feeding, but this type of symptom is fairly uncommon. Occasionally, with high populations of whiteflies, honeydew may become visible, causing the leaves to become shiny and sticky.

 

Whiteflies can easily be detected by brushing up against the leaves, or shaking the plant, if whiteflies are present, the adults will fly from one plant to another. All developmental life stages are normally located on the undersides of the leaf surfaces. For the earliest detection, it is useful to check the lower leaf surfaces as opposed to the newer ones.

 

Whiteflies can be controlled by spraying them with natural products such as insecticidal soap, neem oil, or pyrethrum. There are numerous chemical insecticides (bifenthrin, cyfluthrin, esfenvalerate, gamma cyhalothrin, imidacloprid, and malathion) which effectively control whiteflies. With all life stages being located on the undersides of leaf surfaces, it is difficult to reach them with sprays and often takes multiple applications to control them.

 

Written by Paul Pilon: Perennial Solutions Consulting

Photos Courtesy of: Paul Pilon: Perennial Solutions Consulting