Whether indoor or outdoor, all plants are potential prey for the right insect pest, and some plants are much more susceptible than others. Knowing how to identify and eradicate these pests is integral to ensuring our plants live long and happy lives while keeping their natural, beautiful aesthetic beauty.
Due to the ideal conditions in indoor and ornamental gardens, insect pest populations can explode very quickly so we have to be vigilant in looking for signs and symptoms of insect attack.
If a plant looks sick all of a sudden, take a closer look because there’s a good chance that an insect pest is to blame.
Infestations can be severe, and plants that have been ignored may have the majority of their leaves damaged rendering them irrecoverable.
Munching insects making my mint their lunch. It could be a caterpillar, a munching bug or something different. Disregard the orange speckling, which is just dust from the recent dust storms we’ve had in Melbourne. Photo via Plants Grow Here.
Common Insects That Attack Plants
While it’s not an exhaustive list, here are some of the most common garden insects that I see all the time and that are common around the world:
Aphids: are usually found in a group form just below the leaves. They are found in plants of both indoor and outdoor gardens. Like many other members of the Sternorrhyncha suborder, they are a sucking bug that eats sap and secrets a sticky substance called honeydew.
Notice the brown and green aphids on the fresh buds. They usually don’t have wings, but a few may grow some if the population gets large. Photo via Plants Grow Here.
Scale: are small barnacle-like Sternorrhyncha sucking bugs that can be seen attached to the leaves and stems of plants. They leave a mark where they have been attached to a leaf for long enough that will never heal, so early detection is important for ornamentals.
Serious scale infestation on a cycad with loads of honeydew. Image source
Mealy Bugs: are unarmoured scale Sternorrhyncha sucking bugs that leave cotton wool-like honeydew behind. They’re visible in the leaves, fruits and stems of many fruits and ornamental plants.
Mealy bugs look like soft scale bugs that have just been dusted with icing sugar. Image source
Whiteflies: are Sternorrhyncha flying sucking insects that are present in the leaves, fruits and flowers of many indoor and outdoor plants.
Whitefly pupae on a leaf. Adults look similar to small aphids but you’ll notice they all have wings. Image source
Fungus Gnats: are flies present in the surrounding soil and foliage of indoor gardens, including greenhouse and kitchen gardens. Tiny larvae with black heads and translucent/white bodies may be visible in the soil.
Dark winged fungus gnat. Image source
Thrips: flying sucking insects present on the leaves, stem and flowers of many indoor and outdoor plants. They literally suck the life out of plant tissue, leaving ugly necrotic (dead tissue) sections.
Thrips are tiny flying insects that may or may not be immediately visible upon first inspection. Image source
Spider Mites: fast-reproducing teeny-tiny pests that are not actually insects but instead arachnids. Like thrips, these guys are sucking insects that feed on the sugars in a plant, and they often leave a silvery appearance on leaves and spiderweb.
Spider mites feeding on a leaf and making a mess. Image source
Caterpillars: munching larvae that cause holes in leaves. Not all caterpillars are so bad; in fact, many native caterpillars are to be encouraged and will grow into beautiful and beneficial moths and butterflies. Caterpillars are just one example of a larval form of insect that can cause munching damage to plants.
This Helicoverpa armigera caterpillar is considered to be one of the hardest agricultural pests to control in the world due to its tolerance to pesticides. Image source
Each of these examples has many individual species, so your local variants may look slightly different. This means you might need to do a Google search to see what your local pests look like before you coordinate your attack. You don’t want to kill anything beneficial!
How to Identify Insect Pests In Plants
By looking at the physical appearance of a plant it’s usually easy to identify an insect attack. Below are the most common signs and symptoms I use to identify insect pests.
Aphids and other sap-sucking bugs in the Sternorrhyncha suborder can usually be seen by the naked eye. In extreme cases, they can cause stunted growth, curling, as well as yellowing leaves (or “chlorosis”). The presence of a sticky substance on the leaves and stems (called “honeydew”) which can encourage a fungal growth known as sooty mold, causing the leaves and stems to appear black. Read more about sooty mold here.
Wilt is another prominent symptom of insect pests. Plants depend on the water in leaves and stems to stay perky and in position; without this water the plant will wilt which means that the tender parts of the plant hang down limply. If the lost water is not restored in the plant then it will eventually die.
Wilt is commonly (though not always) caused by insect pests, which may be present in the soil feeding on the roots or in the stems and leaves. Wilting caused by insects often means that leaves develop a yellow color (chlorosis) with V-shaped sectors between the major veins as they draw back chlorophyll. The leaves eventually die and fall from the plant.
Leaf spots are also common symptoms of insect attack. These start quite small but can enlarge with the passage of time. Insects that have sucking mouth parts can leave leaf spots.
Insect pests attack plants in groups, so by closely looking at the plants you may be able to physically see pests with the naked eye.
Dry edges and discolouration of the leaves are another indicator of insect pests, though may be caused by other issues such as nutrients, water, sunlight, or pathogens.
Bite marks out of leaves are usually due to munching insects such as larval forms like caterpillars, as well as a wide variety of other offenders like some species of bug.
Galls can be created by insect pests, but can also be created by bacteria or fungi. When insects create galls, they use them as a home and a food source.
There are a wide range of insect galls, but the most common type I come across are found on citrus trees, created by “gall wasps”. The best thing to do is usually prune affected parts of new growth, which can be a bit sad. Last year’s galls have probably already been vacated by the pest so if they’re down near the base of the plant you may as well just leave them to save all the growth above.
8 Natural Ways to Treat Insect Pests
Sure, it’s possible to use chemical pesticides to treat insect pests, but why not use something a bit nicer on your plants and the environment instead.
After you’ve chosen the right plant for the right place (that’s a loaded statement), here are 8 natural methods that help treat insect pests on plants:
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Start with good soil: Good soil filled with composted organic matter can actually discourage the attack and growth of insect pests. Decaying organic matter, like compost, is integral to healthy soil and can be tilled in when the growing season begins. If you have serious insect pest problems, once the tilling is done cover the soil with black plastic for at least 6 months. The heat that builds up beneath this plastic will help to kill the insect pests and their eggs. After the removal of the plastic, new plants or fresh mulch can be applied.
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Sweep nets: Sweep nets are used manually to eradicate insect pests by physically catching them in a net, and are effective for all insect stages except eggs, where the insects are easy to catch such as those in grass and wildflowers. The standard sweep net is 38cm in diameter, 70cm deep with a 120cm long handle.
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Traps: Another method to remove insect pests without pesticides are traps which insects become physically stuck to. Sometimes these traps have pheromones, sometimes they don’t. Sometimes they are sticky, and sometimes they use liquids or other methods to trap the insects.
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Shaking insects off the plants: In order to do this, place a length of plastic bags (the type you buy in a roll for bin liners at the supermarket) between rows and then forcefully shake the plants over the bag to avoid them dropping onto lower foliage. Discard the plastic bag once you’re done.
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Beneficial insects: It’s important to note that not all insects are pests, and that many play an important role in a plant’s life, from pollinating to preventing pests and diseases. Predator insects keep pest populations at bay by feeding on all life cycles of pests and are an awesome long-term solution, however they may take time to respond when pests first arrive. They may be naturally present in your garden or you can purchase native Aussie predators through Bugs For Bugs.
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Crop rotation: By growing the same crop in the same place each year, the bugs in that specific area will remain there. Crop rotation of annuals not only helps in keeping soil nutrition from depleting but it’s also a helpful strategy in combating the attack of insects on your plants.
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Cut off infected areas: Once you first notice any infected plant areas like leaves or shoots, cut or pinch them off in order to prevent contamination to other parts of the plant body.
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Use natural and homemade insecticides: There are many natural insecticides which can kill insects without being harmful to the environment and human health. These include soap spray and horticultural oils which are very effective, but cause less damage to the ecosystem than harsh chemical insecticides. These natural insecticides usually work by smothering insect pests and their eggs. The downside is that they affect beneficial insects just as much as pests, but the upside is that they don’t stay in the plant’s internal system to poison beneficials for days or weeks to come.
Chemical Pesticides For Insect Pests
Using chemical-based insecticides should be the last option in a healthy garden as they’re harmful to the environment and kill insects indiscriminately, including beneficial ones that our plants rely on for optimal health.
In case of chemical treatment, always start with the least toxic and most specific option available and try to apply these chemicals in the evening time.
Below is a list of both moderately and highly toxic chemicals which can help in the defence against insect pests.
5 Moderately Toxic Chemicals:
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Boric Acid
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Neem Oil
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Copper
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Horticulture Vinegar
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Lime Sulphur and Sulphur
5 Highly Toxic Chemicals:
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Copper Sulphate
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Sabadilla
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Rotenone
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Pyrethrins
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Spinosad
My Favourite Brands & Products:
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Eco-Organic: a company focusing on more natural options, including fly bait, eco (white) oil and neem oil.
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Mavrik: a non-systemic chemical made by Yates that focuses on chewing and sucking insects and doesn’t kill bees but does kill caterpillars (butterflies and moths). I usually use white oil, however the benefit of Mavrik is it targets specific sucking and chewing pests.
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Confidor: created by Bayer, this is a heavy duty pesticide that I have used a couple of times a last resort if safer methods won’t control the problem. I don’t generally recommend use of this product because it’s a systemic chemical that kills everything including bees, which may have consequences on your garden and plant’s health that you don’t intend.
Always check the label, mix the correct measurements, and wear the appropriate personal protective equipment (PPE).
Conclusion
The continuous use of chemical pesticides encourages insects develop resistance towards these chemicals rendering them less effective over the long term, not to mention the destruction of beneficial insects such as pollinators and predatory insects that eat pests.
Believe it or not, in the “olden days” gardeners would use chemical pesticides as a preventative method against pests that were not even present yet. Although this pest strategy is declining, it’s still a method that is practiced to this day by poorly educated gardeners.
Modern thinking is taking us away from barbaric strategies such as these, and toward Integrated Pest Management strategies.
Where To From Here?
Now that you know how to identify insect pests, I recommend learning about how to control them through an Integrated Pest Management (IPM) response.
Maybe it turns out you don’t have an insect pest after all. You can begin the process of finding out what the real plant health problem is with these 17 questions.
Good soil health and flowering plants (including “weeds”) help create an environment that encourages beneficial predators that eat pests.