Fern Pests

Ferns in the outdoor landscape rarely suffer from pests and are some of the easiest perennials to grow. Houseplant ferns can suffer from common pests due to the artificial atmosphere indoors, primarily low humidity caused by heating or cooling systems. Sap-sucking insects like aphids, scale, and spider mites are most common indoors, although in dry climates spider mites may be an outdoor pest during periods of high temperatures.

In some parts of the United States, seasonal caterpillars may be an issue, although any damage to the fronds will be replaced by new growth the following growing season. To correctly identify a possible pest on a fern, pay attention to the regular spore formation on the undersides of the fronds. Often a regular and symmetrical pattern of dots is just the normal spore organs full of sporangia. Insects tend to converge on a plant in more random and clumping formations. 

Aphids

Aphids are sap-sucking insects that can reproduce multiple generations in one growing season. Damage is often seen as dieback of individual stems and browning of fronds. A sticky honeydew may be present if the aphids have been allowed to multiply. Indoors the honeydew can stain and damage furniture or attract sugar ants that can cause further problems with infestation. Aphids rarely kill mature plants if they are treated quickly. 

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Treating Aphids on Ferns

The quickest treatment is to remove any affected fronds to prevent the spreading of the aphids. Spray the plant with a strong stream of water from a hose outdoors or in a shower indoors to physically wash the insects off. Ferns are more delicate than they appear and are not tolerant of most pesticide sprays, including many soft soap sprays. Ensure that any treatment spray is specifically listed for your type of fern on the label before applying. 

Preventing Aphids on Ferns

Often under or overwatered plants are the most common reason that aphids attack. These plants may be significantly weakened and not able to fend off a pest effectively. When plants grow close together in an artificial environment like an indoor garden, pests can spread quickly. Quarantining of infested plants is a must to stop the cycle of infection. Do not return the quarantined fern to its normal growing location until all signs of the pest are gone and healthy new growth has emerged.  

Fern Scale

Scale insects are another type of sap-sucker common on houseplants, especially during the winter months. Fern scale has a protective armor which makes it more difficult to treat, but not impossible. The females are brown while the males are bright white with a brown stripe down the middle of their bodies. The fern scale lives on the underside of the fronds and can sometimes be confused for the spore organs.

Scale causes slow dieback as they suck any moisture in the fronds preventing nutrients from reaching other parts of the plant. Outdoors, scale insects are more likely to be seen in gardens that have very mild winters and hot summers like the south and west of the United States.

Treating Fern Scale on Ferns

Treating armored scale insects with a horticultural soap spray is often not effective, while stronger chemical treatments can cause serious damage to your fern. Using rubbing alcohol and a cotton swab or pad to remove the scale is highly effective and tolerated by most ferns. Cut out affected stems if the infestation is large. Quarantine your fern from the rest of the houseplants until there is absolutely no sign of another generation hatching. 

Preventing Fern Scale on Ferns

Ensure that any new plant is completely free of pests before adding it to your indoor collection or landscape. Outdoors, plant your true fern away from other plants that are known to be hosts for fern scale. Citrus trees and other woody plants grown in warm climate areas above USDA zone 9 are hosts for many scale species. Tropical house and landscape plants like Bird of Paradise, Staghorn fern, Monstera, and Dracaena also host scale without showing any damage themselves. 

Spider Mites

Spider mites are sap-sucking arachnids that typically appear as reddish dots that are just barely visible to the naked eye. The damage to the leaves appears as yellow stippling that is much more obvious. When populations increase they begin to spin fine webs to move between fronds. Multiple generations are produced in areas with mild climates where the adults overwinter in the stem joins or top growth of ferns that remain evergreen year-round.

Hot, dry weather supports rapidly exploding populations in garden landscapes. Spider mites are also common on houseplants during the winter months when indoor humidity levels may be at their lowest due to heating systems.

Treating Spider Mites on Ferns

Pruning out affected fronds of ferns is the best way to control a small breakout of spider mites. Once webbing begins to appear, controls may only be possible by spraying a horticultural soap spray. Ferns are delicate and do not tolerate many of the common pesticides. Ensure that any soft soap spray you use is listed specifically for use on your particular fern variety.

Soap sprays and horticultural oils should not be used when temperatures are over 90F. Increasing the humidity around infected plants may help control the spread and protect unaffected parts of the plant. 

Preventing Spider Mites on Ferns

Spider mites thrive in hot and dry conditions. Regular watering of outdoor gardens and surrounding hardscaping is necessary during periods of extended heat and drought. Plants that are growing in their preferred location with well-draining soil and plenty of soil moisture are much less likely to contract spider mites.

Washing down outdoor plants once or twice a season can remove dust and pollen from the leaves so that the plants are able to photosynthesize properly and remain healthy throughout the growing season while temporarily increasing the relative humidity.  

Caterpillars

Across the United States, there are a range of moth caterpillars that like to feed on native and introduced ferns. The damage is seen as holes chewed from the middle of the plant or ragged edges of fronds. Typically the caterpillars feed through the summer in preparation for pupating later in the year. Wooly bear caterpillars are most common and do the most damage late in summer to prepare for overwintering in a chrysalis. Eggs will be found on the undersides of the fronds and are placed in a pattern that is regular but different from the typical spore pattern. 

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Photo by Odd Wellies, unedited, Flickr, Copyright CC BY 2.0 DEED

Treating Caterpillars on Ferns

The only way to control caterpillars is to hand-pick and dispose of them on a regular basis. Scouting missions are generally successful at any time of the day, although the moths will be seen during the evening hours hatching from cocoons or laying eggs. Tiger moths, leopard moths, and hawk moths all start out as a type of wooly bear caterpillar. Any damage to fronds is quickly replaced with new growth of herbaceous ferns and can be pruned out of evergreen types. 

Preventing Caterpillars on Ferns

Check any new plants brought into the garden for signs of caterpillars or damage from caterpillars that could indicate the possibility of eggs being brought into your landscape. Keep the caterpillars to a minimum by encouraging upper-level predators such as birds and other small mammals that find them to be a great snack.

Limit the use of broad-spectrum pesticides that harm all insects including the beneficial ones and provide plants that provide shelter and food. Use bacterial spore preparations only if populations are unmanageable and apply early in the season when caterpillars may be small. Ensure that the usage label specifically states for use on your fern variety for the most effective application. 

Common Fern Pests Chart

PestIdentifyingTreating
Aphids Browning of frond tips and possibly sticky honeydew residue on leaves or other nearby surfaces Remove affected fronds, spray down the plant with a stream of water with a house or under a shower 
Fern Scale Brown shield-type growth on the underside of the frond, browning of the frond  Prune out affected parts of the plant, rub away the scales with cotton swab dipped in rubbing alcohol
Spider mites Yellow stippled leaves, small red dots, webbing between fronds Remove affected foliage, soap sprays designated for your plant type
Caterpillar Chewed foliage and rolled fronds where the caterpillars reside Pick off visible caterpillars and dispose of them in a bucket of soapy water, Bt only early in the season

Sources: “Florida Fern Caterpillar”, Mississippi State University Extension. www.extension.msstate.com