Growing plants indoors has multiple advantages over outdoor cultivation – you can fine-tune environmental conditions optimally for each plant, and protect your herbs and veggies from many pests and diseases that lurk in the great outdoors. However, even though indoor cultivation reduces the risks of unpredictable weather and exposure to many plant enemies, it provides favorable conditions for pests that thrive in sheltered environments. Unfortunately, these pests are often quite persistent once they establish, causing damage to plants and significantly reducing yields. For this reason, it is best to inspect the plants regularly and try to catch the problem as soon as it emerges. To help you identify the pest that is most likely harming your plants, we have gathered a list of the most common indoor plant pests and the symptoms they cause.
Despite the popular belief, these tiny creatures are not insects – they are more related to spiders. Unlike their strict carnivore relatives, many mite species opted for a parasitic way of life. Their hosts include a wide range of animals (birds, mammals, insects, etc.) and plant species. In crop production, they are most problematic in dry and warm climates. Their reputation is notorious in greenhouses and other indoor growing environments due to their adaptability, the abundance of food, and favorable temperatures. In such environments, spider mites can feed and reproduce all year long.
Considering that spider mites are pests to over 180 plant species, it is highly likely that you will find them in your indoor garden at some point. They usually occupy the undersides of leaves, where they feed on plant’s sap and weave their tiny webs. Although spider mites are a nuisance if present in small numbers, once they colonize the plant they can severely deplete its health by depriving it of food and reducing photosynthetic surface. This can happen rather quickly, especially in greenhouses, as spider mites have a high reproductive potential which enables them to increase their numbers very fast in favorable conditions.
The first symptoms of a spider mite infestation are tiny yellow or brown spots on leaves and webbing on the undersides. The spots appear as a result of their feeding. These small arachnids feed on the plant by piercing its tissue and sucking the nutritious sap. As the feeding continues, and colonies grow larger, the leaves will turn yellow, become dry, and fall off.
Spider mites are very hard to see with the naked eye, so use a magnifying glass to inspect the plant. You can also take a piece of paper, put it under a symptomatic leaf, and shake the leaf gently. The mites will fall onto the paper, and you will see them as tiny specks that look like ash or pepper.
Most common species: Two-spotted mite (Tetranychus urticae), the tomato spider mite (Tetranychus evansi), the spruce spider mite (Oligonychus ununguis).
Aphids are considered as one of the most notorious pests in agriculture, and for a very good reason. These tiny insects form large colonies that feed on young plant tissues and spread various diseases. They have a complicated life cycle that includes both wingless individuals, and the ones with wings. Aphids can reproduce by laying eggs, and interestingly, also by giving birth to live young. The latter way of reproduction is the most common in indoor environments. Aphids spend most of their life cycle eating, sucking the plant’s sap on flowers, young fruits, and leaves. As they also reproduce very fast, they can be very destructive to buds and young fruits, decreasing both quantity and quality of yields.
Aphids are a very large insect group, including about 5 000 different species, 450 of which attack plants. These persistent insects can be found in colonies on many plant species. They are especially harmful in the production of fruits, vegetables, and ornamental plants. Aphids come in a wide range of colors, depending on the species and the host plant – from yellowish-green to intense green, pink to reddish-brown. Some species are even completely black, like the black cherry aphid (Myzus cerasi).
The presence of aphids is easily noticeable, as they are often found in large clusters on flowers, young leaves, and fruits. Another indicator of their presence is white cast skins on leaves and soil around the plant. Heavy infestations result in stunted growth, curled leaves, and poorly developed fruits. Aphids secrete honeydew and often cover leaves and stems with this sticky, sugary substance. Honeydew invites sooty mold and ants, further exacerbating the problem. The fungus reduces the photosynthetic surface of the plant, while ants form symbiotic relationships with aphids and help them to increase their numbers by protecting them from predators.
Once aphids establish and form large colonies, they can be very difficult to completely eradicate. It is very important to inspect your plants weekly, especially during the flowering phase.
Most common species: Green peach aphid (Myzus persicae), foxglove aphid (Aphis gossypii), cotton aphid (Aulacorthum solani), potato aphid (Macrosiphum euphorbiae).
The primary habitat of these small insects are tropical and subtropical areas, but they have been introduced elsewhere via seedlings and other plant material. Now they are present in most temperate regions, where they’re considered one of the worst greenhouse pests. Aside from deteriorating the plant’s health by sucking its sap, whiteflies are also vectors of various plant viruses. Just like aphids, they produce honeydew and attract sooty molds.
Whiteflies lay their eggs on leaf undersides. The eggs look like small white sesame seeds and are often laid in a circular shape. As eggs mature, they turn from white to yellow, then finally brown. Larvae are very small, oval-shaped, and almost transparent. They go through four stages, and only the first stage (called the crawler) is mobile. Other larval stages spend their development attached to the plant, sucking the sweet juices from its tissues. Once the final larval stage is over, the adult whitefly is ready to spread its wings, and continue to eat. And potentially spread some viruses.
Whiteflies cause the greatest damage with their vector activity – they are known to spread a number of highly destructive plant viruses that have a wide range of hosts. This applies especially to the Silverleaf whitefly.
When looking for whiteflies, it is best to first take a look at the leaf undersides. If you see their eggs, larvae, or adults flying about, or notice a white waxy material and sticky honeydew stains on leaves, it is a clear indicator of a whitefly problem. As these pests feed by sucking the plant’s sap, you may also notice disfigured leaves and stunted growth. The honeydew invites sooty molds, thus contributing to the further decline of plants.
Most common species: greenhouse whitefly (Trialeurodes vaporariorum), Silverleaf whitefly (Bemisia tabaci)
Mealy Bugs (Pseudococcidae)
These peculiar looking creatures are scale insects, but unlike many species that belong to that group, mealybugs are unarmed. Instead of having a shell, they secrete waxy materials over their dorsal area. This thick waxy layer protects mealybugs from potential enemies and gives their body a characteristic shape. Many species have long, waxy filaments at the hind end of their body, resembling a tail, and shorter ones at the edges. Just like aphids, mealybugs produce honeydew and form symbiotic relationships with ants. They provide the honeydew to ants, and in turn, they protect mealybugs from predators.
Mealybugs are sexually dimorphic, which means that females and males have different looks. Female mealybugs are nymph-like and wingless, while the males look like flies or gnats. Mealybugs reproduce by laying eggs in pouches, or by giving birth to live young. The pouches look like small rolls of cotton wool and are often laid on leaf undersides and leaf axils. A single egg pouch can contain between 50 and 600 eggs, depending on the species and the reproductive potential of the female.
There are thousands of mealybug species, but only a few are considered pests in plant cultivation. When present in small numbers, mealybugs are just a nuisance. However, if their colonies expand, they can become a serious threat to plant health. Just like previously mentioned pests, mealybugs harm plants by sucking on their sap and producing honeydew that attracts sooty molds. They are most often found in clusters on leaf axils, stem tips, and roots.
Plants infested with mealybugs initially display stunted growth and leaf yellowing. If the plant is not under physiological stress and the number of mealybugs is very low, the symptoms can be very subtle and cause little damage to the plant. However, in case of a severe infestation, the plants often suffer from defoliation and wilting, which can significantly affect yields. If mealybugs are present in your indoor garden, you will probably also notice their waxy secretions on leaves and stems, as well as their cotton-like egg pouches. Considering that mealybugs produce honeydew, the infested plant might also suffer from sooty molds.
Most common species: Citrus mealybug (Planococcus citri), longtailed mealybug (Pseudococcus longispinus), root-feeding mealybugs (Rhizoecus spp.), Madeira mealybug (Phenacoccus madeirensis), obscure mealybug (Pseudococcus viburni).
Fungus Gnats (Nematocera)
Fungus gnats are a type of very small flies (often less than 1/8-inch long), that include species from various families in the suborder Nematocera. You can find these tiny insects in and on the soil, as they are attracted to moisture and decaying organic matter. They thrive in very humid conditions, so they are often present in large numbers on overwatered plants. Considering that many indoor plants are cultivated in rich and moist crop substrates, and are prone to overwatering, it is not a surprise that fungus gnats are very common indoor pests.
Fungus gnats lay their eggs beneath the top layer of the soil, into the organic matter. Each female is able to lay up to 300 eggs. Fungus gnats have a simple and fast life cycle. The laid eggs hatch after three days, and new larvae need about two weeks to reach maturity. Adults live for only a week on average. For this reason, you can often find multiple generations and all stages of growth around the plant at the same time. Even though adults have wings, they are not very good at flying. They are often erratic and fly into people’s faces, food, and drinks. However, they can power walk and move rather quickly on the soil surface.
Fungus gnat larvae feed on soil fungi and decaying organic matter, but they can also take a bite of the plant’s root if other food is scarce, or in case of overpopulation. Both larvae and adults can spread pathogenic fungi like Pythium, Fusarium, and Verticillium, which are especially destructive to young seedlings, causing damping-off and often killing young plants.
Visually inspect the substrate around the plant. If you notice fruit fly sized insects roaming about, or whitish worm-like larvae beneath the top layer of the soil, it is very likely that you have a fungus gnat problem. If your plant has suddenly turned yellow and started to wilt, dig into the substrate to inspect the roots. The larvae feed on root hairs and create small brownish scars on the root. Finding these symptoms is a clear indicator of a fungus gnat infestation. If the infestation is severe, these small insects can cause serious root damage and deteriorate the plant’s health very quickly. Sudden wilting is often a result of the combined effect of larvae and fungal diseases like root rot (Pythium) and wilt (Fusarium, Verticillium).
Most common species: Bradysia impatiens, Bradysia coprophilia.
Thrips are very tiny insects, often less than 0.04-inch long. Their slender bodies and small size makes them very difficult to see with the naked eye. Order Thysanoptera includes more than 6000 species, and most of them are plant pathogens. Thrips feed by sucking the cell contents of plant leaves and like many sucking insects, they spread various viruses. Greenhouse conditions are very favorable for their growth and reproduction. Considering that thrips have a very high reproductive potential, are difficult to manage, and serve as vectors for more than 20 different plant viruses, they are widely considered as one of the most harmful pests in indoor plant cultivation.
It takes only two weeks on average for thrips to reach maturity. As a result, you can often find multiple generations and all stages of development present on the plant simultaneously. Adults live up to 7 weeks, and considering that females can lay 2-10 eggs daily, a single female can produce hundreds of eggs. For this reason, thrips can increase their numbers very quickly and cause very serious infestations. They can eat and reproduce all year long in favorable conditions, which makes them very persistent pests in indoor environments. Both larvae and adults are harmful to plants and cause leaf spots, yellowing, and wilting. Thrips can severely deteriorate plant health, especially in dry conditions. They are especially harmful to vegetables and leafy spice plants like basil. By reducing the plant’s vigor, thrips can increase the incidence of fungal diseases and decrease the quality of yields.
As these minute insects are very hard to see, you will most likely first spot the signs of their presence on plants. The first symptom you will probably notice are tiny yellow spots on leaves. Young leaves, buds, and fruits often grow distorted as a result of their feeding. As the infestation progresses, you can also see leaf silvering and curling on older growth. Previously yellow areas on leaves turn rusty red or brown and have black specks. Spotting on flowers and scarring on fruits are also very common symptoms. In case of a severe infestation, leaves can become completely dry and fall off.
Most common species: The onion thrips (Thrips tabaci), western flower thrips (Frankliniella occidentalis), black tea thrips (Heliothrips haemorrhoidalis).