Aphids are probably some of the most common and annoying plant pests, but they are far from a nuisance. These tiny insects are arch nemeses of many gardeners due to the damage they inflict by sucking the sap of plants, preferring young, developing tissues. A fully established colony can cause severe stress and literally suck the life out of the plants, and the worst part is – they are quick to establish. Aphids have a very high reproductive rate and can develop multiple generations in one season. Since some fertile females develop wings, they can colonize other plants and spread the infestation. Aside from sucking the sap from plant tissues and weakening them, aphids also spread plant viruses and attract other problematic organisms.
When aphids feed on the plant sap, they ingest a lot of sugars. However, since they don’t need as much for their own metabolic activities, they excrete the excess sugar in the form of a sweet, sticky substance called honeydew. The presence of aphids is often easily recognized by the trail of honeydew on leaves, stems, buds, and flowers. This sugary mess is not only an aesthetic problem – it also invites other harmful organisms, like sooty mold. The mold feeds on the honeydew sugars and spreads its dark mycelium over the plant tissues, reducing their photosynthetic activity. The sugary honeydew is also a food for ants, so they often exacerbate the infestation by securing their food source and protecting the aphids from predators.
Ignoring aphids can be very costly, so proper, timely prevention and treatment are crucial. In order to find the best approaches to suppress and get rid of aphids, we should first take a look at their life cycle.
The life cycle of aphids
Aphids receive a lot of attention from researchers and entomologists, and this interest isn’t rooted only in their significance in plant production but also in their variety of forms and the complexity of their life cycle. The small size and unobtrusive looks of aphids hide remarkable genetic adaptations. These tiny sap-sucking insects are some of the most incredible specimens of evolutionary gumption.
The members of the superfamily Aphidoidea have two types of life cycles – host-alternating and non-host-alternating. The life of aphids that prefer the latter type revolves around a single plant species or several closely related species, while the host-alternating aphids require at least two distinctive hosts (usually one woody and one herbaceous species). Although the host-alternating tactic is generally viewed as suboptimal in evolutionary terms, aphids have managed to utilize it successfully due to their peculiar adaptations and astonishing reproductive potential. These aphids make up about 10% of all aphid species but are more significant in plant production than non-host-alternating species.
Aphids survive low winter temperatures in the form of eggs, which are embryos of females that will hatch during spring. The highly fertile females that develop from these eggs are stem mothers, also called fundatrices – wingless females that establish the first generation of aphids in the new season. Shortly after hatching, stem mothers start to feed and produce offspring. They do this without mating by cloning themselves and giving birth to live individuals. The offspring of these viviparous females also reproduce asexually and are called fundatrigenia. This generation of aphids produces ‘migrants’ – forms with wings that migrate to secondary hosts during summer.
The migrants establish new populations on secondary host plants, which include both wingless and winged individuals. As the summer goes by and cold days start drawing nearer, fall migrants start to seek out partners to reproduce sexually. They lay eggs that will overwinter and produce stem mothers during next spring.
Reproduction of aphids
Aphids have the ability to reproduce sexually and asexually (without mating) and develop numerous generations in one year. In warmer climates, they can reproduce continuously, and some species can develop more than 40 generations. The number of offspring and generations per year depends on the species and environmental conditions (temperature, humidity, availability of resources, etc.), but can reach a hundred individuals per single female.
The quality that further accelerates their reproduction is the ‘telescoping of generations. Namely, the viviparous females give birth to daughters which are also pregnant with a daughter embryo. The telescoping of generations enables aphids to shorten the periods between generations and colonize plants very fast. These qualities give aphids significant evolutionary advantages, and it seems that their numbers are only limited by the availability of resources and the presence of predators.
If you are growing your plants in an outdoor setting, it is difficult to completely evade contact with aphids. However, what you can do is inspect new plants for their presence before introducing them into your garden. Look carefully at leaf undersides, buds, and flowers. Even a few individuals can create a mighty colony that can make a mess out of your beloved plants. This especially applies to greenhouses and indoor gardens – since they provide stable, favorable conditions for plant development, they also provide favorable conditions for plant pests. Always take a good look at the plants you plan to add to your indoor setup and investigate them thoroughly for signs of aphid presence, which include:
- live aphids
- aphid exuviae – the remnants of aphid exoskeletons after molting
- traces of honeydew
Aphids are more likely to attack and colonize weakened plants, so the key preventive strategy is to keep the plants as healthy as possible. Since soil plays a vital role in plants’ health, it is the best place to start. Ensure that the soil has a favorable structure, adequate water retention, nutrient content, and microbial activity.
Another factor that plays a pivotal role in plant health is exposure to stress. Plants damaged by frosts, droughts, overwatering, high and low temperatures, etc. are more likely to host families of aphids. It is best to pay extra attention to such plants and mitigate the damage as much as possible.
Kaolin clay and China clay are two common names for the silicate mineral kaolinite, which is most famous for its use in manufacturing fine porcelain. Aside from its significance in various industries, kaolin clay is also a valuable part of organic farmers’ toolbox. When applied to plant tissues, it creates a film that defends the plant by deterring and irritating insect pests. Kaolin clay works best when applied preventively before the aphids establish.
Mix 1.5 cups of kaolin clay with half a gallon of water and add a few drops of pure liquid soap to ensure better distribution of clay particles. Add the water to the clay gradually to prevent clumping. Pour the mixture into a spray bottle and shake well before applying. Spray the plants generously, fully covering the foliage.
Frequency of application
Spray the plants with kaolin clay every 1-3 weeks for at least 40 days.
Since kaolin clay has a preventive effect, it should be applied during spring through early summer to cover the period when stem mothers activate until the migrants move to secondary hosts. Spray the plants on a dry, cloudy day. Kaolin clay is easily washed by rain, so if it rains soon after application it is necessary to reapply it.
Horticultural mineral oils (HMO)
Horticultural mineral oils are refined paraffinic oils commonly used in organic farming primarily against scale insects and mites, but also other soft-bodied insects like aphids, mealybugs, whiteflies, and thrips. These oils affect pests in a number of ways – by blocking their airways, disrupting feeding, and reacting with compounds essential for their metabolism. HMOs are also very effective at reducing and preventing the spread of plant viruses, making them a very valuable environmentally-friendly tool against these dangerous organisms. HMOs are not harmful to pollinators and most other beneficial organisms, since they are applied during dormancy. Another significant advantage of these compounds is that they don’t produce resistant populations of pests due to their mechanical, multi-site mode of action. However, they should be nevertheless used sparingly, as their accumulation in the environment could lead to negative effects.
Some plants may be sensitive to mineral oils and display symptoms of phytotoxicity, especially if drought-stressed or exposed to intense UV lighting, so it is important to pay attention to timing and frequency of application.
Follow the instructions on the HMO product label to mix the optimal concentration of the oil in the solution. Don’t add more oil than the label says since a more concentrated mix can harm the plants. Shake the mixture well before applying, and make sure to spray the plants thoroughly, paying special attention to leaf undersides, crooks, and crevices.
Frequency of application
Once every 3-4 weeks during the critical periods of plant dormancy. Avoid using HMOs during the active season since they can harm the plants, as well as beneficial organisms.
HMOs are applied during the dormant period – in late fall and early spring before the buds open. Since they don’t have a residual effect, HMOs display desired activity when applied directly onto the pests or their eggs, so applications in early spring and fall are the most effective. Spray the plants early during the day or in the evening. Avoid spraying if the rain is about to fall within a day after the application and if the relative air humidity is 90% or above, as well as if there is a risk of frost or high temperatures within 3-4 days after application. HMOs are best applied at temperatures between 40°F and 70°F.
You did your best to prevent the little buggers from invading, but nevertheless found them plaguing your plants? Worry not. There are effective solutions if the preventive measures fail. Don’t immediately grab a synthetic insecticide to solve the issue, as there are less toxic options that work just as well. Aside from toxicity, another great disadvantage of many synthetic pesticides is the danger of resistance development. Due to aphids’ adaptability and high reproductive potential, there are numerous populations that have developed resistance to a wide variety of insecticides – carbamates, organophosphates, neonicotinoids, and pyrethroids. Some of these pesticides are also highly toxic to humans and beneficial organisms, so you’d like to keep them out of your garden. There are alternatives to
If only a few aphids are present or they are in the initial phases of development, you can simply remove them by hand or clean them off with a wet cloth. In case of a greater population, you can use a water hose and a nozzle to spray the aphids off plants. Aphids are fragile and not very mobile, so you don’t have to worry about them getting back.
Physical removal methods can significantly curb aphid numbers and prevent any greater damage. However, this method has to be done repeatedly and can be quite time-consuming with a greater number of plants, so it works well on a smaller scale. Larger-scale grow op and persistent infestations require a chemical approach.
Diatomaceous earth (DE)
One of the main tools of organic farmers is diatomaceous earth – a naturally-occurring sedimentary rock composed of fossilized diatoms, which are silicate algae that once inhabited rivers, lakes, and streams. When ground into a powder, this rock has powerful insecticidal properties, acting as an abrasive and desiccant, damaging the waxy protective layer of insect exoskeletons and drying it. DE is effective against a variety of pests, including aphids, weevils, beetles, roaches, fleas, earwigs, snails, and slugs. Since its mode of action is mechanical, the pests cannot develop resistance toward it. DE is also completely harmless to humans, and rarely harmful to beneficial organisms, so it is not a surprise why it is one of the favorite insecticides amongst organic farmers.
DE is most commonly applied in its powdery form with a dust applicator. Dust the plant and the soil around it generously, creating an even layer of the material. You can also spray the plants with a liquid solution, mixing one cup of DE with half a gallon of water. It is important to note that DE is easily removed with water, so it needs to be reapplied if the rain strikes shortly after application.
Frequency of application
DE is best applied every four weeks during the season (spring-fall). In case of a heavy raining season, it can be added more often, but just like with other insecticidal agents, it is necessary to use it sparingly.
Apply DE during dry periods to ensure prolonged and effective action, since rain and irrigation can wash it away. If you are using DE in powdery form, avoid applying it on windy days.
Note: DE is not always as effective as necessary, so to ensure complete protection against aphids, it is best to combine it with biocontrol methods and the application of other protective compounds, like insecticidal soaps or neem oil.
Insecticidal soaps are based on potassium salts of fatty acids and do a fine job in controlling not just aphids but numerous soft-bodied insects like thrips, spider mites, mealybugs, lace bugs, whiteflies, scale insects, and leaf hoppers. The active compounds of insecticidal soaps work by disrupting the waxy protective layer of the soft-bodies insects, which causes dehydration. Insecticidal soaps can also efficiently remove honeydew stains and sooty molds, providing a complete aphid treatment.
There are many commercially available insecticidal soaps, but you can also create a cheaper and equally effective alternative at home, using store-bought ingredients. The basic recipe for a ready-to-use insecticidal soap solution includes:
- 5 tablespoons of plant essential oil (cottonseed, lemon, orange, peppermint, thyme, anise, etc.)
- 5 tablespoons of pure, refined liquid soap, preferably Castille soap
- 1 gallon of water
When making your own mixture, it is crucial to pay attention to the ingredients you are using. Detergents and soaps intended for washing applications, especially the ones that are in dry form and contain bleach, degreasers, and similar, often include compounds that are toxic to plants. It is also important to note that some plants are highly sensitive to the active ingredients of insecticidal soaps.
Sensitive plants: Plums, cherries, sweet peas, some tomato varieties, horse chestnut, portulaca, hawthorn, and a variety of ornamental species like Japanese maple, gardenias, maidenhair ferns, bleeding heart, some succulents, and plants with waxy leaves.
If you are not sure if the plants you want to treat are sensitive to the insecticidal soap, test the mixture by spraying it onto a small area (for example, on 2-3 leaves). Wait for at least 24 hours to see if the plant reacted to the active compounds. If there are no visible changes, it is fair to assume that the mixture is safe to apply.
Pour the mixture into a spray bottle and shake well before applying it to ensure an even distribution of active ingredients. Spray the whole plant, especially paying attention to crooks and leaf undersides.
Frequency of application
Depends on the severity of the infestation. The basic treatment includes spraying the plants every 5-7 days over the course of 2-3 weeks.
The ideal period to treat the plants is early in the morning or late in the evening. The sun wasn’t strong at that time, so the likelihood of phytotoxicity and low efficiency are minimized. High air humidity also significantly helps since it enables prolonged activity. Applying insecticidal soaps in dry and warm conditions can lead to poor results and low efficacy. The mixture loses its potency as the water evaporates, and high temperatures can exacerbate phytotoxic activity.
Neem or nimtree (Azadirachta indica) is a plant native to the Indian subcontinent which produces a variety of phytochemicals with insecticidal properties. Its oil is extracted from fruits and seeds and has diverse medical, cosmetic, and chemical purposes. In plant cultivation, neem oil is one of the primary natural insecticides effective against soft-bodied insects like aphids, mealybugs, whiteflies, and thrips, as well as against spider mites. The main active ingredient of neem oil is azadirachtin – a chemical that disrupts the feeding of insects, their development, and reproduction and repels adult females from laying eggs on the treated plant. Although neem oil doesn’t negatively affect most beneficial organisms, it repels them, so it is best to use it sparingly and apply it only to affected plants.
Neem oil has a delayed effect on insect pests. Once the pests ingest the active compounds by feeding on the treated plants, they will gradually stop eating and eventually die of hunger. So don’t panic if you see live and moving insects shortly after application – it takes a couple of days to notice the insecticidal effects.
Mix five teaspoons of cold-pressed neem oil concentrate with half a gallon of water and one teaspoon of pure liquid soap to help the oil mix with water. Pour the mixture into a spray bottle and shake well before application. Spray the plants thoroughly, making sure that leaf undersides and crooks are covered.
Frequency of application
Apply the neem oil spray every few days for 2-3 weeks. You can cease the application earlier if there are no live individuals on the plants.
Just like with all oil-based foliar sprays, it is best to apply neem oil in the evening or any other time of day when the sun’s radiation isn’t strong. Otherwise, there is a risk of burning plant tissues.
Note: Aside from controlling soft-bodied insect pests and mites, neem oil is also effective against powdery mildews.
Biological control with natural enemies
Besides the traditional mechanical and chemical methods of pest removal, organic farmers also employ natural enemies to reduce the abundance of harmful organisms. Considering that aphids sit pretty low in the food chain, there is a variety of predators and parasitoids that can significantly curb their populations. These organisms include predators like spiders, ladybugs, hoverflies, ground beetles, lacewings, earwigs, bugs from the family Anthocoridae and Nabidae, social wasps, and parasitic organisms like braconid wasps, some species of gall midges, and parasitic wasps of the family Aphelinidae. The predators are generalists, so they are not very picky with their food. They would gladly eat any aphid species, as well as many other invertebrates. On the other hand, parasitic organisms are usually specialists, and their hosts come from a single genus or family. The most effective protection against aphids is achieved when these two types of natural enemies work together.
How to get natural enemies of aphids?
There are commercially available products that include natural enemies of aphids that you can introduce to your garden. However, since these are live organisms, most of which are able to fly, their effectiveness can significantly vary. It can often fail to reach the desired level of control in open fields. Adding natural enemies is most effective in greenhouses and other indoor gardens, where their populations can be controlled and maintained.
If you want to keep aphids at bay with the help of natural enemies in your outdoor garden, instead of buying commercial products, a far better approach is to attract native beneficial organisms already present nearby. This method is significantly more advantageous, as it can employ multiple species instead of one. It also fortifies the natural biocontrol processes that help your plants deal with harmful organisms. Maintaining biodiversity in the garden is very important since it provides a wide variety of ecological services. These services are valuable because they highly benefit plants and require little to no human input. However, to get the best out of natural enemies, it is necessary to know which plants they prefer and when they are active. Here is a quick list as a reference.
Ladybugs are notorious aphid killers. Both adults and larvae are carnivorous. Just like all insect juveniles, ladybug larvae cannot fly, so they usually stay on the plant they hatched on. Since they have an insatiable hunger, they systematically eat everything in their vicinity, especially preferring soft-bodied, juicy aphids. Ladybug larvae can eat their own weight in aphids every day, and continue to ravage aphid populations in adulthood. It is estimated that a single ladybug can eat about 5000 aphids over the course of its lifespan.
Plants that attract them: dandelion (Taraxacum officinale), calendula (Calendula officinalis), dill (Anethum graveolens), parsley (Petroselinum crispum), garlic (Allium sativum), crane’s bill (Geranium sp.), wild carrot (Daucus carota), chives (Allium schoenoprasum), hairy vetch (Vicia villosa), marigolds (Tagetes sp.), nasturtiums (Tropaeolum sp.), butterfly weed (Asclepias tuberosa), basket of gold (Alyssum saxatilis), four-wing saltbush (Atriplex canescens), coriander (Coriandrum sativum), tickseed (Coreopsis sp.), fennel (Foeniculum vulgare), yarrows (Achillea sp.), caraway (Carum carvi), tansy (Tanacetum vulgare), bugleweed (Ajuga reptans), California buckwheat (Eriogonum fasciculatum), alpine cinquefoil (Potentilla crantzii).
Period of activity: Ladybugs are active from spring to fall, so they can keep aphids at bay during the whole season. The fall is the time when they thrive the most.
Lacewings are a prime example of how gentle looks can hide a ferocious beast. Adult forms are small, brown, or more commonly green, with large, transparent wings innervated like a fine net. Their eyes have a golden shine and are placed laterally to enable a broad visual field. Lacewing larvae look quite different from adults – they have long, flat bodies and conspicuous, strong jaws. Similar to ladybugs, both lacewing larvae and adults are carnivorous and are actively participating in curbing aphid populations. Just like most predators, they are generalists but prefer aphid eggs, larvae, and adults. Lacewing larvae can eat up to 100 aphids daily, so only a dozen of them can make a visible contribution to the health of your plants.
Plants that attract them: dill, dandelion, coriander, caraway, tansy, prairie sunflower (Helianthus pauciflorus), angelica (Angelica gigas), golden marguerite (Anthemis tinctoria), four-wing saltbush, fennel, wild carrot, cosmos (Cosmos bipinnatus), purple poppy-mallow (Callirhoe involucrata).
Period of activity: Lacewings are active during spring and summer. The adults are active at night, while the larvae can be seen roaming the plants during the day.
Some of the most feared predators in the insect kingdom are spiders, the ancient arachnids that can eat anything they can catch in their webs and incapacitate with their venom. Although spiders are highly effective in reducing the number of insects in your garden, they are not very picky with their food and will eat pests and beneficials alike. Although this is generally viewed as a disadvantage, we shouldn’t overlook the importance of spiders as pest control agents in our gardens.
Plants that attract them: Spiders are not attracted to any specific plant species, but rather to the abundance of available prey. However, they do like tall and branchy plants, on which they can easily weave their webs (for example, sunflowers (Helianthus sp.), hollyhocks (Alcea rosea), larkspur (Delphinium sp.)). They can be also found on a variety of plants that attract their prey. Most insects they eat are attracted to light, so they prefer to place their webbing in a well-lighted spot.
Period of activity: Spiders operate from early spring to late fall, so their activity covers the entire growing season.
Hoverflies include about 6 000 members of the family Syrphidae, some of which are very important biocontrol agents, as well as plant pollinators. Their name refers to their style of flight – they flutter their wings so quickly that they seem to hover over plants and other objects. Hoverfly larvae are among the most ferocious aphid eaters, only being topped by ladybugs and lacewings. The larvae are significant as pest biocontrol agents since the adults are pollinators and eat only plant food. Aside from aphids, hoverfly larvae also feed on other small, soft-bodied insects like thrips, scale insects, and some caterpillars.
Plants that attract them: dill, coriander, cosmos, yarrow, caraway, golden marguerite, wild carrot, fennel, spearmint (Mentha spicata), pennyroyal (Mentha pulegium), lemon balm (Melissa officinalis), parsley, marigold ‘lemon gem’ (Tagetes tenuifolia), basket of gold, alpine cinquefoil, four-wing saltbush, stonecrops (Sedum), masterwort (Astrantia major), poached egg plant (Limnanthes douglasii), crimson thyme (Thymus serpyllum), wild bergamot (Monarda fistulosa), English lavender (Lavandula angustifolia), California buckwheat, purple poppy-mallow, hedgenettle (Betonica officinalis), and many other herbs.
Period of activity: Hoverflies operate from spring to late fall, but their activity peaks during the summer months.
Minute pirate bugs (Anthocoridae) and damsel bugs (Nabidae)
These inconspicuous tiny hemipterans are among the most effective biocontrol agents against a wide variety of small, soft-bodied pests, including aphids, thrips, leaf miners, spider mites, and scale insects. There are commercially available species that can be applied in greenhouses and open gardens, but you can also attract the natives with the right plants.
Plants that attract them: caraway, fennel, spearmint, alfalfa (Medicago sativa), European goldenrod (Solidago virgaurea), cosmos, marigold ‘lemon gem’.
Period of activity: Minute pirate bugs and damsel bugs operate from spring to fall, so they can provide pest control for most of the growing season.
Parasitoid wasps are some of the most common biocontrol agents. The key to their popularity lies in the fact that they are specialists, targeting only the pest organism and nothing else. Once the target pests are eradicated, they will move to nearby areas in search of hosts or die off, so the likelihood of them increasing their population severely and destabilizing the biological balance in the garden is minimal. Parasitoid wasps can decimate aphid populations without negatively affecting other beneficials or plants. One of the most significant types of parasitoid wasps that attack aphids includes the subfamily Aphidiinae, which belongs to the family of braconid wasps. Parasitoid wasps act by laying their eggs into live aphids. The wasp larvae hatch within 48 hours after being laid and start feeding on the host tissues. They leave dried, dead aphids after they finish their development.
There are commercially available species of this wasp subfamily, successfully used in commercial biocontrol against aphids. However, there are also wild populations of parasitic wasps that can significantly contribute to the health of your garden.
Plants that attract them: There are no specific plant species that attract parasitic wasps. The wasps are rather attracted to the compounds plants release in specific situations. When plants are attacked by herbivores, they produce a cocktail of compounds that warn their cousins of the threat. However, this chemical signal is quite sophisticated – aside from warning other plants, it also attracts enemies of the given herbivore. Basically, this means that aphid-infested plants release compounds that attract aphid parasitoids, and this reaction is observed in many plant species.
Period of activity: Parasitoid wasps are active from late spring to summer. The egg-to-adult period can last from 9 to 23 days depending on the temperature, with the development being faster at higher temperatures. Wasps of the family Aphidiinae can develop multiple generations in a year. The exact number of the generations depends on the temperature and other environmental conditions. In greenhouses and other closed gardens, where the conditions are pretty much constant, parasitoid wasps can reproduce almost during the whole season and provide effective control against aphids.
Ground beetles (Carabidae)
Similarly to spiders, ground beetles are generalists that don’t discriminate. Lunch is lunch, and ground beetles will readily eat pests as well as beneficials if there is an opportunity. As their name indicates, ground beetles roam on the surface of the soil, hunting available prey on the ground. Because of this, they are not the primary pick when it comes to aphid control, but their contribution is nevertheless significant.
Plants that attract them: Various species of tall grasses, fescue (Festuca sp.) white (Trifolium repens) and red clover (Trifolium pratense).
Period of activity: Ground beetles operate from spring to fall, reaching peak activity during early summer. Since they can be active well into November, they can control pests when fewer natural enemies are available. Ground beetles mostly hunt at night, and you can recognize them by their swift running between the plants in search of prey.
Note: Many beneficial insects and spiders prefer to inhabit humid areas abundant in water, so it is best to add shallow water dishes filled with stones to your garden during dry bouts to maintain their populations.
- Ensure that the plants are growing in healthy soil and limit their exposure to stress
- Apply preventive substances like kaolin clay, horticultural mineral oil, and diatomaceous earth to the susceptible plants
- Check plants for the presence of aphids on a regular basis
- If you detect any aphids, remove them immediately manually or spray them off the plants with a nozzle
- Opt for physical methods of control when possible; in case of a greater or reoccurring infestation, use natural insecticidal solutions like insecticidal soap and neem oil
- Lure natural enemies of aphids into your garden by adding plants that attract them; avoid or minimize the use of insecticides and repellents that harm them
- Van Emden H. F. (Ed.) & Harrington, R. (Ed.) (2007). Aphids as crop pests. CABI.
- Zeni, V. (2021). Diatomaceous Earth for Arthropod Pest Control: Back to the Future. Molecules. 2021 Dec; 26(24): 7487.
- Pissinati, A, Ventura, MU. (2015). Control of cabbage aphid, Brevicoryne brassicae (L.) using kaolin and neem oil. Journal of Entomology, 2015, Vol.12, No.1, pp.48-54, ref.26.
- Ron Patterson (2016). Aphid Natural Enemies and Biological Control. Utah State University Extension and Utah Plant Pest Diagnostic Laboratory.