Ten Common Flower Garden Insect or Insect-Like Pests

It’s a given that insects will always be with us.  They fly, crawl, hop, chew, suck, bore, and mine. If their damage is minor, then it’s generally OK to adopt a “live and let live” attitude toward them in our flower gardens.  But when they consume our coneflower petals, deposit clusters of unappetizing “spit” in the yarrow, or chew huge holes in the hostas, that’s when we are forced to declare war!

The iconic book Good Bug, Bad Bug explains that 90% of all insects are either benign or beneficial.  Of the remaining 10% of insects, the damage they inflict depends on whether they are chewers or piercers/suckers.  It also depends in many cases on whether the insect is in the larval, nymph, or adult stage of their development.  This article provides examples of insect pests belonging to both categories, the type of damage they inflict, and recommendations for managing them.

The non-chemical management controls described are meant to be as non-toxic and environmentally friendly as possible.  But if chemical controls are indicated, then refer to the Virginia Cooperative Extension’s (VCE) 2022 Pest Management Guide and the plant fact sheets listed under Resources at the end of this article for information on pesticides and additional controls. To avoid inadvertently harming beneficial insects or doing damage to the environment, comply fully with directions on pesticide labels.

CHEWING INSECTS

Chewing insects have mouthparts that allow them to feed on all parts of a plant.  They chew and swallow bits and pieces of plant tissue, leaving an unsightly, ragged leaf or flower margin.  Here are some clues that indicate the culprit is a chewing insect:

1. Leaves or entire plants are defoliated.
2. Leaves are “skeletonized” (nibbled between the veins).
3. The outer edge of a leaf or flower is devoured.
4. The outer layer of plant cells is nibbled, causing the chewed areas to turn brown and unsightly.
5. Tiny holes in leaves resulting in a “shot hole” effect.
6. Plant dieback or loss of vigor due to consumed roots or stems.

Four examples of chewing insect pests include blister beetle, iris borer, earwig, and leaf miner.

BLISTER BEETLE 

Of the blister beetle family of insects, the black blister beetle (Epicauta pennsylvanica) is commonly found in gardens east of the Rockies.  This chewing insect has a slender, elongated body and soft wing covers that don’t cover the tip of the abdomen.  Blister beetles produce cantharidin, a defensive oil that can irritate skin and cause blisters – hence the common name of this insect.  The adult beetles feed on leaves and flowers of a wide range of plants, including aster, calendula, chrysanthemum, dahlia, dianthus, delphinium, goldenrod, Japanese anemone, lupine, phlox and zinnia.  In large numbers, these insects can quickly defoliate a plant.  The larvae are predaceous; that is, they feed on the eggs of grasshoppers as well as wild bee larvae.

Signs of Blister Beetle Damage:

• Defoliation of leaves and flowers, particularly at the top of the plant.
• Dark, irregular fecal spots on remaining leaves and stems.

Blister Beetle Management Control:

• Pick off and squish by hand (be sure to wear gloves).
• Knock them into a container of soapy water. If they fall to the ground, they will pretend to be dead.  So make sure they fall into the soapy water.
• Keep ragweed (Ambrosia spp.) and pigweed (Amaranthus spp.) under control. Both weeds attract the beetles.
• Manage grasshopper populations as a preventative measure. Blister beetle larvae consume grasshopper eggs. Blister beetle populations may be high in areas where grasshoppers were dense during the previous year.

  

IRIS BORER

Not all insect pests limit themselves to chewing leaves and stems. Some, like the larval form of the iris borer moth (Macronoctua onusta), do their dirty work underground and out of sight.  Using its chewing mouthparts, the caterpillar tunnels down into the underground iris rhizome and consumes it undetected. Unfortunately, iris borer damage can set the stage for bacterial soft rot, which typically spoils the rhizome and often spreads to other nearby iris rhizomes.  By the time borer damage becomes noticeable, it is generally too late to save the affected portion of the plant.

Signs of Iris borer damage:

• Tan or water-soaked streaks in young iris leaves in late spring or early summer.
• Brown tips on iris leaves.
• Small holes or notches in the iris leaf, which are the points of entry for the borer.
• Partially or completely eaten rhizomes.
• A foul-smelling odor from bacterial soft rot, which causes the leaves to yellow and collapse.

Iris Borer Management Controls:

• Remove weeds, as a preventative action.
• Cut affected iris foliage off at the base.
• Remove plant stalks at the base in fall to prevent overwintering eggs or pupae.
• Burn or place affected foliage in trash. Do not add to compost pile.
• For suspected borer damage to rhizomes, dig them up in July and inspect for borer damage. If damage is minor, cut out the affected parts, soak roots in a 10% bleach to water solution, and replant in a different location.  If damage is significant, destroy the rhizomes.

  

EUROPEAN EARWIG

The European earwig (Forficula auricularia) is a chewing insect that is both a pest AND a beneficial insect.  It is a pest because both adults and nymphs significantly damage the foliage and flowers of a wide variety of plants.  It is beneficial because it preys on other insects such as aphids, armyworms, maggots, mites, slugs, and snails. It also eats dead or decaying flower and vegetable plant matter.  It is easily identified by the fierce looking large pincers on the hind end.  Earwigs are nocturnal and feed on a variety of plants including dahlia, chrysanthemum, clematis, and zinnia plus a variety of fruits and vegetables.  During the day, these insects hide in dark, cool places.  Their populations can explode during periods of wet, warm weather.

Signs of earwig damage:

• Numerous large, ragged holes in leaves.
• Chewed flowers.
• Chewed leaf margins.

Earwig Management Controls:

• Encourage natural predators such as toads, birds and predator insects such as assassin bugs, praying mantis, and tachinid flies.
• Remove debris from the garden that might harbor earwigs.
• Trap them in shallow containers of vegetable oil.
• Roll several layers of newspaper into a loose tube. The earwigs will crawl into the tube and can then be easily shaken out into a container of soapy water.

  

AGROMYZID LEAFMINER

In its larval form, the Agromyzid Leafminer fly differs from other chewing insects by feeding within leaves.  In spring, the flies lay their eggs on the underside of leaves.  After the maggots hatch, they burrow between the upper and lower surfaces of the leaves to feed on ornamental plant species such as columbine, chrysanthemum, butterfly weed, daylily, mist flower, and verbena.  The tunnels increase in size as the maggot grows, significantly affecting the appearance of the plant. This is not a serious pest.  Their damage is more cosmetic in nature than harmful to the plant.

Signs of leafminer damage:

• Unsightly long-winding light-colored tunnels in leaves.
• Blotches in leaves where tunnels have coalesced.

Management controls for leafminer insects:

• Encourage predator insects such as parasitic wasps.
• Either ignore minor damage or simply snip off and destroy any infested leaves.
• Clean up and destroy debris from infested plants in fall.

  

PIERCING/SUCKING INSECTS

Insects belonging to this group are known as true bugs. They are members of the Hemiptera order of insects, which comprises roughly 80,000 species including aphids, assassin bugs, cicadas, leaf hoppers, planthoppers, and shield bugs.   A true bug has a rigid beak (also called a proboscis) that is designed to pierce and suck fluids from plant cells.   Some insects such as bees or butterflies, which belong to the Hymenoptera family of insects, also have a beak-like proboscis.  However, their proboscis is retractable whereas a true bug’s proboscis is non-retractable.

Here are some clues that indicate a pest may be a piercing/sucking insect:

1. Affected leaves or stems are yellowed (chlorotic), twisted, or curled.
2. Leaves have tiny white or yellow speckles (stippling effect).
3. Leaves have large, darkened spots.
4. Leaves are covered with black sooty mold growing on honeydew deposits.
5. Stunted growth on plants.
6. Abnormally developed new leaves.

Four examples of piercing/sucking insect pests include four-lined plant bug, milkweed bug, phlox bug and spittle bug.

FOUR-LINED PLANT BUG

The piercing/sucking four-lined plant bug (Poecilocapsus lineatus) pierces plant cells in a destructive manner called “lacerate-flush.”  After it sucks out the juices from the cells, it flushes the damaged area with digestive enzymes, which result in localized areas of dead cells.  The bright red and black nymphs hatch out around midspring and mature into their adult form around July. The adult is greenish-yellow with four black lines that extend the length of the wing covers.  They are shy and difficult to spot but their damage gives away their presence. This insect feeds on over 250 herbaceous plants including chrysanthemum, lavender, Russian sage, and shasta daisy.

Signs of four-lined plant bug damage:

• Round brown or gray pockmarks on leaves, which become transparent as the plant cells die.
• Small round holes on leaves where the plant cells have died.
• Distortion of developing leaves, buds, and flowers, which may be mistaken for a plant disease.
• Bud drop.
• Feeding damage generally concentrated on the upper parts of the plant.

Four-Lined Plant Bug Management Control:

• Encourage natural predators such as assassin bugs, damsel bugs, praying mantis, or spiders.
• Handpick nymphs and adults and either squish them or drop them into soapy water.
• In midsummer, after the insects disappear, cut back the damaged plant to a point below the damage to remove any eggs laid inside stems.
• In fall, clean up and destroy all stems and leaf litter from the damaged plant.

  

MILKWEED BUG

In addition to aphids, milkweed species (Asclepias) are also affected by milkweed bugs (Oncopeltus fasciatus). The nymphs and adults of this piercing/sucking insect species can be found feeding on milkweed leaves, stems, and seeds in mid to late summer.  They usually appear in summer after milkweed seed pods form. Their bodies contain toxic compounds from the sap they suck, which means birds and predators don’t typically bother them.  Also, their colorful markings warn predators to stay away.

Signs of Milkweed bug damage:

• Deformed seed pods but little damage to plants otherwise.
• If the infestation is particularly heavy, they may crowd out Monarch butterfly populations.

Milkweed Bug Management Control:

• If infestation is minor, no management controls are necessary.
• Hand pick and drop them into a container of soapy water.
• Dislodge with a strong spray of water from a hose.
• Remove leaf litter and spent stalks in fall to eliminate overwintering sites.

  

PHLOX BUG

The colorful black and reddish-orange phlox bug (Lopidea davisi) is a piercing/sucking insect that can inflict significant damage to both cultivated and wild phlox species.   It feeds on leaves, stems, flowers and seeds but may be hard to detect because it hides out on the underside of leaves.

Signs of Phlox bug damage:

• Bleached looking white or light green stippled spots on leaves.
• Leaves turn brown, curl up, die and fall off.
• Plant may appear stunted and die.

Phlox Bug Management Control:

• Cut back infested phlox stems and leaves and destroy.
• Remove stem and leaf litter from flower beds in fall to eliminate overwintering phlox bug eggs.

  

SPITTLEBUG

Both adults and nymphs of this aptly named piercing/sucking species (Philaenus spumarius) feed on a wide variety of herbaceous and woody plants as well as edible crops.  The adults are small, hopping insects that resemble leafhoppers.  The nymphs create white, sticky, frothy masses, which hide them from predators such as birds, protect them from extremes in temperature, and prevent dehydration in a low humidity environment.  The masses appear on the stems of plants where the nymph pierces the stem and sucks the plant juices.  They are often seen on ornamental plants such as roses, shasta daisies, goldenrod, lavender, and grasses.  Fortunately, spittlebugs don’t typically do much damage to annuals and perennials, but they can be a problem on woody plants and their feeding can affect yields on food crops.  As an aside, the spittle comes not from the bug’s mouth but from the other end.

Signs of Spittlebug Damage:

• Unsightly “spittle” masses on plants.
• Leaf spotting and sometimes death of grass blades caused by spittlebug saliva.
• In an extreme infestation, spittlebug feeding can stunt or weaken plants.

Spittlebug Management Control:

• Direct a strong spray of water to dislodge the nymphs and their frothy masses.
• Control weeds in and around gardens to eliminate them as a food source for spittlebugs.
• If evidence of spittlebugs is minor, then no action is needed.

  

INSECT-LIKE PESTS THAT DESERVE MENTION

A few insect-like species are included here because the damage they do is consistent with that of many insect pests.  Equipped with either chewing or piercing/sucking mouthparts, the damage they inflict can be significant.

SLUGS AND SNAILS

These nocturnal creatures are not insects.  They are members of the mollusk family and have more in common with clams and mussels.  They can inflict significant damage on ornamental plants with their chewing mouthparts. They need moist conditions to stay alive, so they stay hidden in dark, damp sites during the heat of the day and emerge after dark to feed.  Because they are nocturnal, their slimy trails are often the only indication of their presence besides the obvious damage they do.  Best known for their damage to hostas, they are attracted to many other mature perennials and annuals but avoid plants with highly fragrant or fuzzy leaves, such as salvias, lavender, lamb’s ear, and most herbs.

Signs of slug and snail damage:

• Small, tender leaves are consumed whole.
• Edges of leaves chewed.
• Irregular damaged areas on foliage.
• Slime trails, which indicate where they have been active.

Slug and Snail Management Controls:

• Encourage natural predators such as birds, frogs, toads, turtles, garter snakes, salamanders, and predacious beetles and their larvae.
• Modify or eliminate the dark, moist conditions by removing decaying plant matter, eliminating weeds, and trimming grass around landscaped areas.
• Handpick (using gloves) and drop into a container of soapy water. The best time to look for slugs and snails is by flashlight about two hours after sunset.
• Trap by placing an overturned flowerpot, board, brick, piece of cardboard, or inverted grapefruit half in the garden.  It will provide the dark, moist retreat that slugs and snails prefer for daytime shelter. Then lift the shelter, hand pick the slugs or snails and drop them into soapy water.

  

SPIDER MITES

Related to spiders and ticks, adult spider mites have eight legs rather than six. The twospotted spider mite (Tetranychus urticae) is perhaps the best known of the mite species and is the most common mite pest of houseplants. Because they are so small, they are often difficult to spot, but they make their presence known through the damage they do to plants with their piercing/sucking mouth parts. The damage is most noticeable during hot, dry weather when plants are stressed.  In addition to houseplants, spider mites feed on a wide range of other ornamental plant species including salvia, daylily, marigold, phlox, pansies, and verbena.

Signs of Spider Mite Damage:

• Mottled foliage or fine stippling at feeding sites caused by the loss of fluids.
• Lighter green or yellowed foliage.
• Fine silk webbing, particularly at the site where leaves attach to plant stems.
• A generalized bronzing or reddish discoloration of foliage caused by heavy infestations.
• Premature leaf drop on heavily infested plants.
• Reduced plant vigor.

Spider Mite Management Controls:

• Encourage predator insects such as big-eyed bugs, damsel bugs, lacewings, ladybugs, and minute pirate bugs.
• Dislodge mites from plants with a sharp spray of water.
• Keep plants watered during hot, dry weather.
• Mulch around susceptible plants to reduce loss of moisture in the soil.
• Raise humidity levels around susceptible houseplants.

  

IN CONCLUSION

While most insects in the flower garden are beneficial, a few species can cause significant damage to plants.  Recognizing the signs of chewing versus piercing/sucking insects is important to determining the best course of action to take.  If damage is minor or merely cosmetic, a “live and let live” approach may be the wisest choice.  A diverse garden filled with pollinator-friendly plants will attract natural predator species that can help keep pest infestations under control for you.  Otherwise, use non-toxic control methods to the extent possible to manage pests while protecting beneficial insect species.  For further reading on additional insect pests such as Japanese beetles and aphids and their controls, see Eleven Common Garden Pests — Identification and Management, which appeared in the June 2022 issue of The Garden Shed.

 

Feature photo of chewing insect damage to Japanese Anemone foliage.  Photo:  Pat Chadwick

SOURCES

Garden Allies (Lavoipierre Frederique, 2021).

Garden Insects of North America (Cranshaw, Whitney, 2004).

Good Bug, Bad Bug (Walliser, Jessica, 2008).

2022 Pest Management Guide — Home Grounds and Animals, Virginia Cooperative Extension (VCE) Publication 456-018.

Blister Beetles, Bug of the Week website, Michael J. Raupp, PhD., University of Maryland Extension Specialist

Earwigs in Virginia, VCE Publication

Flower Garden Pests, Purdue University Extension Publication E-70-W

Flower Insects, University of Maryland Extension

Four-Lined Plant Bug, North Carolina State Extension Fact Sheet

Iris Borer, VCE Publication

Leafminers, VCE Publication

Milkweed Bug, North Carolina State Extension Fact Sheet

Phlox Bugs, Missouri Botanical Garden

Problems Common to Many Perennials and Annuals,  Missouri Botanical Garden

Slugs and Snails on Flowers,  University of Maryland Extension

Spider Mites, VCE Publication

Spittlebugs, University of Wisconsin-Madison Publication