Begonias attract a surprisingly consistent lineup of sap-sucking insects and other small creatures, and most of them can be handled without resorting to heavy-duty chemical sprays. The pests you’re most likely to encounter are mealybugs, aphids, spider mites, thrips, fungus gnats, whiteflies, and scale insects. Outdoor begonias also deal with slugs and snails, and greenhouse-grown varieties face an especially sneaky problem in foliar nematodes. Knowing what each pest looks like, where it hides, and what damage it causes makes the difference between catching an infestation early and losing a plant.
Mealybugs
Mealybugs are probably the single most complained-about begonia pest among indoor growers. They look like tiny tufts of white cotton clustered in leaf axils, along stems, and on the undersides of leaves. Each “tuft” is actually an insect covered in a waxy, powdery coating that protects it from water and many contact sprays. Mealybugs feed by inserting their mouthparts into plant tissue and drawing out sap, which weakens the plant over time and causes yellowing, leaf drop, and stunted growth.
For a small infestation, dip a cotton swab in rubbing alcohol (70% isopropyl) and dab each visible mealybug directly. The alcohol dissolves the waxy coating on contact and kills the insect underneath. For heavier infestations, spray the entire plant with insecticidal soap or horticultural oil, making sure to coat every crevice where mealybugs hide. You’ll need to repeat treatment every seven to ten days for at least three rounds, because eggs tucked into crevices will keep hatching after the adults are gone. Neem oil works as both a contact killer and a feeding deterrent, though it’s slower-acting than insecticidal soap.
One reason mealybugs are so persistent on begonias is that begonia stems tend to be thick and succulent with plenty of joint crevices, giving the insects sheltered spots that are easy to miss during treatment. After every application, check the base of each leaf petiole and the crown of the plant where new growth emerges. If you keep finding stragglers after three rounds, consider a systemic insecticide containing imidacloprid applied as a soil drench, which the plant absorbs and distributes through its tissue.
Aphids and the Sooty Mold Problem
Aphids are small, soft-bodied insects that cluster on new growth, flower buds, and leaf undersides. On begonias they’re usually green or pale yellow, though some species appear pinkish or nearly translucent. A few aphids won’t do much harm, but populations explode fast. A single female can produce dozens of live offspring without mating, so a handful of aphids can become hundreds within a couple of weeks in warm conditions.
The direct feeding damage from aphids shows up as curled or distorted new leaves and stunted flower buds. But the secondary problem is often worse. Aphids excrete a sticky, sugar-rich waste called honeydew that coats leaves and stems. That honeydew serves as a food source for various fungi, and research on the cotton aphid has shown that honeydew actively promotes fungal growth, supporting both beneficial fungi and plant pathogens alike.1Turkish Journal of Agriculture – Food Science and Technology. Effect of Honeydew Secreted by Aphis gossypii Glover (Hemiptera: Aphididae) on Fungal Growth On begonias, the result is usually sooty mold: a black, powdery fungal coating that doesn’t infect the plant tissue directly but blocks light from reaching the leaves. Photosynthesis drops, the plant weakens further, and the whole thing looks unsightly.
A strong blast of water from a hose or spray bottle knocks most aphids off begonia leaves and is the simplest first step. Follow up with insecticidal soap or neem oil if they return. Outdoors, encourage natural predators like ladybugs and lacewings by avoiding broad-spectrum pesticides. If sooty mold has already formed, wipe it off with a damp cloth or spray with a diluted soap solution once the aphids are controlled. The mold won’t keep growing without its honeydew food source.
Spider Mites
Spider mites are nearly invisible to the naked eye, and that’s exactly what makes them dangerous. By the time you notice the telltale fine webbing stretched between leaf veins or along stems, the colony is already well established. The first visible sign of damage is usually a stippled, speckled pattern on leaves, as if someone pricked the surface with a pin hundreds of times. Heavily infested leaves turn bronze or gray, dry out, and fall off.
Spider mites thrive in hot, dry conditions, which is why they tend to show up on indoor begonias during winter when heating systems drop indoor humidity. Raising humidity around your plants is the single most effective preventive measure. Grouping plants together, using a pebble tray filled with water, or running a small humidifier nearby all help. Misting the leaves directly doesn’t raise ambient humidity much, but it does physically disturb mites and their webs.
For active infestations, insecticidal soap and horticultural oil are effective, but coverage is critical. You need to coat the undersides of every leaf, where mites concentrate. Neem oil also works but may need repeated applications. One important caution: many common insecticides are ineffective against spider mites because mites are arachnids, not insects, and they’ve developed resistance to several chemical classes. Miticides exist for severe cases, but for most home growers, persistent spraying with soap or oil at five-day intervals will bring the population under control within a few weeks.
Thrips
Thrips are slender, fringed-wing insects barely a millimeter or two long. They scrape open plant cells and feed on the contents, leaving behind silvery streaks and tiny black dots of excrement on leaves and petals. On begonias, thrips damage often shows up first on flowers, which develop brown edges or streaky discoloration. Leaves may become distorted or develop papery patches.
Beyond cosmetic damage, thrips pose a more serious risk as virus carriers. The western flower thrips is the only species currently known to transmit Impatiens necrotic spot virus (INSV), which has been documented on greenhouse begonias. In one case involving Hiemalis begonias in a Canadian greenhouse, the virus caused stunted growth and necrotic spots on leaves, progressing to mosaic patterns, concentric rings, stem and vein necrosis, and eventually severe foliage damage resembling sunburn or chemical injury.2PubMed. First Report of Impatiens necrotic spot virus on Hiemalis Begonia (Begonia × hiemalis) in Canada Once a plant is infected with INSV, there’s no cure. The plant should be removed and destroyed to prevent the virus from spreading to other susceptible plants nearby.
Controlling thrips starts with monitoring. Blue or yellow sticky traps placed near your begonias catch flying adults and give you an early warning. Insecticidal soap and spinosad-based sprays are effective against thrips, though spinosad tends to work better because it’s ingested by the insect rather than requiring direct contact. Repeat applications every five to seven days for at least two to three cycles, since thrips pupate in the soil and emerge as new adults after treatment. For greenhouse growers, biological control agents like the predatory mite Amblyseius cucumeris can keep thrips populations low when released preventively.
Fungus Gnats
Fungus gnats are the tiny dark flies you see hovering around the soil surface of potted begonias. The adults are mostly harmless and just annoying, but their larvae live in the top layer of soil and feed on organic matter, fungi, and fine root hairs. In large numbers, larval feeding can damage young roots enough to cause wilting, poor growth, and increased susceptibility to root rot pathogens.
Fungus gnats thrive in consistently moist soil, and begonias, especially rex and rhizomatous types, are often grown in rich, moisture-retentive mixes that the gnats love. The most effective non-chemical control is simply letting the top inch or so of soil dry out between waterings. This kills a large proportion of eggs and young larvae, which need moisture to survive. Yellow sticky traps catch adults and reduce the breeding population. For persistent problems, a soil drench with Bacillus thuringiensis subspecies israelensis (Bti), sold under various trade names as mosquito dunks or gnat-specific products, kills larvae without harming the plant or beneficial soil organisms. Watering with a diluted hydrogen peroxide solution (about one part 3% hydrogen peroxide to four parts water) also kills larvae on contact.
Whiteflies
Whiteflies look like tiny white moths and flutter up in a cloud when you disturb an infested plant. Like aphids, they’re sap-feeders that produce honeydew and can lead to sooty mold problems. They tend to colonize the undersides of leaves and are especially common on begonias grown in warm, still environments like greenhouses or enclosed patios.
Whiteflies can be tricky to eliminate because of their life cycle. Adults lay eggs on leaf undersides, which hatch into immobile scale-like nymphs that are resistant to many sprays. The nymphs go through several stages before pupating and emerging as new flying adults. This means you need repeated treatments timed to catch each generation as it hatches. Insecticidal soap and horticultural oil are effective against nymphs if sprayed directly, and yellow sticky traps help reduce adult numbers. Reflective mulch or aluminum foil placed around outdoor begonias can disorient whiteflies and reduce landing rates. For heavy infestations, a systemic insecticide applied as a soil drench reaches all life stages through the plant’s tissue.
Scale Insects
Scale insects are easy to overlook because they don’t look like insects at all. They attach themselves to stems and leaf veins, then secrete a hard or waxy shell and stay put, feeding on sap. On begonias you might see them as small brown bumps along stems or on the undersides of thicker leaves. Like mealybugs and aphids, they produce honeydew and can lead to sooty mold.
The protective shell makes scale insects resistant to many contact sprays. The rubbing alcohol approach used for mealybugs works here too: dab individual scales with an alcohol-soaked swab to kill them, then scrape off the dead shells. For broader coverage, horticultural oil smothers both adults and the crawler stage (the brief mobile phase when young scale insects disperse to new feeding sites). Timing is important, because crawlers are far more vulnerable than settled adults under their armor. If you can spot the tiny, pale crawlers moving along stems, that’s the ideal window for a spray application.
Foliar Nematodes
Foliar nematodes are microscopic roundworms that live inside leaf tissue and are a particularly insidious problem on begonias, especially in commercial greenhouse production. They enter leaves through stomata or wounds and feed on cells from the inside, causing brown or black wedge-shaped lesions between leaf veins. The pattern is distinctive: because the nematodes can’t cross major veins easily, the dead patches tend to follow a geometric pattern bounded by veins, giving the leaf a stained-glass appearance.
These nematodes spread through water splash. When an infested leaf gets wet, nematodes migrate to the leaf surface and are carried to neighboring leaves or plants by irrigation, rain, or even handling wet foliage. This makes overhead watering a major risk factor. Switching to bottom watering or drip irrigation dramatically reduces spread. There are no effective chemical treatments readily available to home growers for foliar nematodes. Infected leaves should be removed and discarded (not composted), and severely infested plants should be thrown away. Keeping foliage dry, spacing plants for air circulation, and sanitizing tools and pots between uses are the main defenses.
Slugs and Snails
Outdoor begonias, particularly tuberous varieties with their lush foliage, are magnets for slugs and snails. These mollusks feed at night, leaving irregular holes in leaves and shiny slime trails as evidence. They’re especially active in cool, damp conditions and tend to hide under pots, mulch, and debris during the day.
Hand-picking at dusk or after dark with a flashlight is surprisingly effective if you’re consistent about it. Beer traps (shallow containers sunk into the soil and filled with beer) attract and drown slugs. Copper tape around pot rims creates a mild electrical charge on contact with slug slime that deters crossing. Iron phosphate bait pellets are the safest chemical option: they’re effective against slugs and snails but nontoxic to pets, wildlife, and beneficial insects, unlike older metaldehyde-based baits that pose poisoning risks. Reducing hiding spots by clearing debris and elevating pots on feet or stands also helps by making the area less hospitable.
When Damage Looks Like Pests but Isn’t
Not every mark on a begonia leaf means you have a pest problem. Several common cultural issues mimic pest damage closely enough to cause confusion. Leaf scorch from direct sunlight produces brown, papery patches that can look like thrips or nematode damage. Overwatering causes mushy stems and collapsing leaves that resemble severe mealybug damage or root-feeding larvae. Fertilizer burn creates crispy brown leaf edges easily mistaken for spider mite damage. And the bacterial leaf spot that begonias are prone to produces dark, water-soaked spots that can look like the early stages of foliar nematode infestation.
Before treating for pests, take a close look with a magnifying glass or your phone’s macro lens. If you can’t find actual insects, webs, slime trails, or frass (insect droppings), the problem is more likely environmental. Adjusting light levels, watering habits, or fertilizer concentration will fix cultural issues far more effectively than any pesticide.
A General Approach to Prevention
Most pest problems on begonias share the same underlying invitation: stressed plants in stagnant conditions. A begonia that’s overwatered, overcrowded, or sitting in still air with poor drainage is far more susceptible to infestations than one growing in good conditions. There are a few habits that reduce your odds of dealing with any of the pests described above.
- Quarantine new plants: Keep any new begonia isolated from your existing collection for two to three weeks. Inspect it regularly before introducing it to the group. Many infestations start with a hitchhiker on a plant from a garden center.
- Water at the base: Keeping foliage dry reduces the spread of foliar nematodes, discourages fungal diseases, and makes leaf surfaces less hospitable to soft-bodied insects.
- Improve air circulation: Space plants so air moves freely around them. A small fan on low in an indoor growing area makes a real difference. Stagnant, humid air is the ideal environment for nearly every pest on this list.
- Inspect regularly: A quick weekly check of leaf undersides, stem joints, and the soil surface catches problems early, when a cotton swab and rubbing alcohol can handle what would later require weeks of spraying.
- Use clean potting mix: Reusing old soil or incorporating garden soil into pots can introduce fungus gnat larvae, nematodes, and other soil-dwelling organisms. Fresh, sterile potting mix eliminates that risk.
The best pest management on begonias is honestly just paying attention. These plants give visual clues early: a sticky leaf here, a speckled patch there, a single cottony dot in a leaf axil. Catching those signals before a population builds means most problems can be solved with soap, water, and a bit of persistence rather than escalating to systemic chemicals.
Biological Controls Worth Knowing About
If you grow begonias in a greenhouse or in large numbers, biological pest control is worth considering as a complement to or replacement for chemical sprays. Predatory insects and mites are commercially available and can maintain themselves in enclosed growing environments once established. Lacewing larvae are voracious aphid predators and will also eat mealybugs, thrips, and whitefly nymphs. The predatory mite Phytoseiulus persimilis is highly effective against spider mites in warm, humid conditions. Parasitic wasps in the genus Encarsia target whitefly nymphs specifically, laying eggs inside them.
Biological controls work best as preventive measures, released before pest populations explode rather than after. They also require you to stop using broad-spectrum insecticides, which kill the predators along with the pests. This trade-off is worth it for many growers, since predatory insects provide ongoing suppression without the repeated application cycles that sprays demand. The catch is that biological agents are living organisms with temperature and humidity requirements, so they’re most practical in controlled environments rather than on a windowsill with three begonias.