There is no getting around it, if you grow plants, you will have to deal with insect pests at some point. The types of insects, severity of the outbreak or infestation, and control methods will depend on your growing environment, crops, growing methods (organic or conventional), options available to you, and of course your budget. There are hundreds of possible pests that can affect your crops, but for the most part it is the same few that tend to do the most damage, at least in regard to indoor crops.
The same conditions that are ideal for plant growth are typically also ideal to bring on the appearance of insect pests. Many greenhouses and interior structures also offer protection for insect pests by keeping out most beneficial insects and organisms that would help control the population of pest species if grown outdoors or in the field. Fortunately, there is a wide range of control options for dealing with insect pests. Properly identifying what is present and doing damage is the first step for choosing the correct control measures.
Greenhouse Insect Pests:
While there are thousands of insect species that could potentially become a nuisance in your greenhouse or growing area, there are only about 10 that cause most growers consternation. Whiteflies, aphids, mealybugs, scale, thrips, gnats, leafminers, caterpillars, spider mites, and nematodes are generally the pests that a grower may encounter. It is important to know what pest is present before attempting a control strategy aimed specifically at eradicating them. Identification is the most important, and the first step to remedial action.
It behooves any grower to become familiar with the appearance and damage symptoms that these pests cause. It is easy to mistake some insect damage for environmental, fungal, or bacterial pathogens and attempt the wrong corrective actions. Fortunately, there are many resources for growers who might need some help in this area. Online sources of insect identification are numerous and very accurate. Growers can also send samples of pest species found to university or government labs to be identified, usually for a nominal fee. Sending clear pictures of pests or their damage to entomologists working for your state or province should suffice. Sending those same pictures to researchers found online if they are expert in greenhouse pests can also work sometimes. Nonetheless, it is still useful to gain this knowledge as there are indeed times where a quick solution is needed and the time waiting for a response would result in further damage.
Most insects cause damage by either chewing or sucking. This is an important distinction as the remedy for one type of insect may not be as effective on another. Let’s consider some of the damage of the more common pests and their identification.
Sucking pests:
Aphids - these are small but very visible pests that can be winged or wingless. They are often green, but there are species that are black, and shades of grey or red. Cannabis aphids tend to be more clear. They suck the fluid from the phloem, often causing distorted leaves in their wake. Having some aphids present does not amount immediately to plant damage. As they reproduce and their populations increase, action is needed. They excrete a sticky substance known as “honeydew”. This substance can attract ants or other insects who feed on the sugar-rich substance. A type of mold (sooty mold) can grow on the honeydew causing the leaves to look mottled.
Whiteflies - these insects are aptly named as they are highly visible, bright white, and fly. Their damage can be similar to that of aphids though there is not usually as much distortion of the leaves. They too suck out fluid and excrete honeydew which attracts ants and molds.
Thrips - these are a very small insect, difficult but not impossible to see without magnification. They suck out fluid from leaves, but their damage appears as small, silvery scars, often dotted with darker spots which is their excrement (frass). They can attack leaves, buds, or fruit. They are also responsible for transmitting plant pathogens as they feed form one infected plant they can transfer the pathogens to another.
Fungus gnats - these are small flying insects often found on the surface of wet soils. The adults feed on decaying matter and can spread spores, but the real damage is done by the larvae which are in the soil. They can feed off a plants’ roots, just like many types of harmful nematodes. The symptoms often appear as areas of host plants start to collapse. If there are adult fungus gnats, it is likely that there are larvae in the soil. They are attracted to moist areas. Letting plants dry out in between waterings can control them.
Mealy bugs and scale insects - these are two different insects but are often mistaken for each other as they are both found on stems and on the underside of leaves often. They range in size from that of a pencil tip to a pencil eraser. They are easy to spot, but not in areas of dense foliage. They can vary in appearance but often both “park” themselves in an area and stay put, sucking out fluids form the phloem. These sometimes have to be picked off.
Spider mites - there are several species of spider mites. Control of them can be very difficult because they are not truly insects (they are arachnids), so insecticides do not often have any effect on them; miticides must be used to control them. Symptoms of spider mite damage can start as small, speckled areas from where they are sucking out fluid, but that usually leads to larger cellular damage becoming larger dead areas. They also leave webbing in the areas where they are feeding, which is very visible, even though the mites themselves are difficult to find.
Chewing insects:
Leafminers - these are very small insects that chew trails between layers of leaves. They leave what look like “squiggly” lines where they have been. They are difficult to control as they are protected between the layers of the leaves they are in.
Caterpillars - This is a very broad category as these are the larvae of hundreds of different species of mostly flying insects. They often chew leaves from the margins inward but can also chew holes. They can be nearly invisible to several inches long.
This is by no means an exhaustive list. These are just the more common insect pests that growers are likely to encounter in a greenhouse or indoor growing area. Growers need to be extra vigilant at all times as they will not often know when insect pests have arrived until there is visible damage.
Control options
Once you are sure that what you have is an insect problem in your greenhouse or indoor growing space, it is time to figure out what type of control methods you will employ. Will you try one or numerous? Will you start with the least invasive method first, or will you go right for the most toxic? It is important to weigh a number of factors before selecting your strategy. First, you should understand all of the control options at your disposal.
Greenhouse insects can be controlled with a range of options. The very design of greenhouse space should include some allowances to try to exclude insects from entering. These types of controls are mechanical. The way that you grow your crops can have an impact on whether or not insect pests come to visit. These methods are cultural. Spraying synthetic pesticides to control the problem is a form of chemical control. Using beneficial insects or plant-derived solutions is organic or biological control of insect pests.
Cultural Insect Control Options
In some regards, cultural insect controls are the easiest to implement because they don’t have to cost anything. They certainly can become pricey if new environmental controls are added, but there are definitely many cultural practices that require no additional costs. They are those methods that relate to the practice of growing plants and doing it in a way that minimizes the opportunity for insect pests to appear. Though this is largely a preventative approach, it is also one of the most critical to stay on top of.
Sanitation is one of the most important cultural methods to control insect pests. While keeping the physical growing space clean is critical, sanitation does not end there. It is very important to make sure that no weeds are allowed to grow inside of a greenhouse or growing environment. This is not only true for the pots themselves, but for floors and even the area immediately outside of the greenhouse or growing area. Weeds themselves can play host to dozens of different insect pest species, all which can then migrate onto your crops in a blink of an eye. Sanitation goes beyond just weeds though.
Continual pruning and removal of any diseased or damaged plant material will help to keep plants healthy and less of a target for insect pests. Proactive removal of bottom branches or bracts, if they are heavy, or rest on the ground or table will not only reduce the chance for harboring insects, but also improve air circulation around the plants. Also, don’t hang on to sick plants. It can be very tempting to try to nurse a sick plant back to health, but this can be the invitation for insect pests to set up shop in your growing area and spread to the other healthy plants. Cull all sick plants once they have moved beyond the stage of showing light stress.
Maintaining the ideal environment in the greenhouse or growing area will help to prevent some insect pests form appearing. Healthy plants are the best defense against the ravages of insect pests, who are more likely to favor plants under stress already. Managing the light, temperature, humidity, and air circulation is equal in importance of pest control as is making sure your plants are in the proper media, with the appropriate pH, moisture, and nutrient values.
Another cultural insect control method involves inactivity. This can be a difficult step, especially for commercial growers. It is good practice to have a completely fallow growing area between crops or at least at certain times of the year. This “emptiness” will remove all potential food sources for insect pests that may be hiding and give the grower a chance to sanitize all tables, benches, floors, walls, and growing equipment so it is at optimal hygienic levels for the next crop. If total shutdown is not possible, then separate growing facilities should be used where one can be completely empty while the other(s) still can be a production space.
Mechanical Insect Control
Mechanical greenhouse pest control utilizes a variety of physical barriers and methods to prevent insect pests from entering the greenhouse space. The most common are screens and traps. Screening is incredibly important in a greenhouse since they require frequent ventilation which provides opportunities for insect pests to enter the structure. Screens with a small mesh pattern can be installed inside of roof vents, side vents, door vents, or any other location. At no time should the greenhouse be open to the outside world without having a screen in place to prevent unwanted pests from entering the space.
Increased screening does decrease air flow and to compensate, additional ventilation areas should be installed. Proper screening does an excellent job of keeping out larger insect pests such as whiteflies, leafminers, aphids, and various types of moths (Lepidoptera). Even the tightest screens, however, won’t exclude all insect pests. Thrips are small enough to enter even the tiniest of holes. For thrips and other pests that sneak in, traps are the next line of mechanical defense.
Sticky traps are a tried and true methods of non-chemical pest control. Some insects are attracted to the color, in other cases, pheromones are added to attract the insect pests to the trap. In any event, traps should be placed both near entrances and near the plants that the pests are drawn to. They should be at or just above the foliage level so it is more likely that they will be caught by the trap. At a minimum, one trap should be placed for every 1,000 square feet of growing space. More traps will be more effective at catching insect pests. Traps should be inspected often, at least weekly if not daily. If exclusionary tactics and traps aren’t enough, then it is time to consider other methods, such as chemical or organic insect control methods and products.
Organic Insect Control
There are many times when an organic insect control solution is not only desired but required. Any crop that is intended to be sold as “organic” or “natural” or any other such claim whether certified or not, can only use applications that are certified organic by an agency such as the Organic Materials Review Institute (OMRI) or similar, third-party registry. Even if there is no requirement to use organic pesticides, there are many reasons to do so. In some (not all!) cases, organic insecticides are safer to use than chemical ones. To be clear, this is not always the case. There are caustic organic materials and benign chemicals to be sure. It is a general statement and should not be assumed as a given.
Other viable reasons to select organic pesticides are that they are generally “easier” on the environment and on beneficial insects, including pollinators. Some may be characterized as “soft” on beneficial insects or may be labeled as a “bio-rational” pesticide. These are formulations that are derived from natural substances such as plant oils, bacteria, or minerals, and are not manufactured with artificial ingredients. Commonly used organic pesticides like these include Neem oil, derived from the Neem tree, Pyrethrin, derived from a species of chrysanthemum, and insecticidal soaps which are almost always potassium salts of fatty acids.
Other organic insect control methods include the release of beneficial insects that will parasitize or prey on pest insects. This is much easier in a greenhouse or enclosed setting as compared to field-grown plants. There are numerous options of beneficial insects from microscopic organisms like beneficial nematodes, to large insects like praying mantids. Parasitoids will lay their eggs inside or on the targeted pest species. The egg will hatch within the pest species and feed its way out, killing the host, or on it and feed on it until it dies. Beneficial parasitic insects are usually host-specific, and several kinds may need to be released if there is pest pressure form more than one type of insect pest.
Predatory insects are often less “picky”, but do have their favorite prey. The larvae of lady bugs have voracious appetites for aphids. Adult ladybugs eat them too, but in nowhere near the volume of the young ones. Beneficial nematodes will feed on the eggs of fungus gnats that are found in the substrate or media below the surface. Beneficial insects are not always easy to keep around though. Once they have taken care of the problem, they move on in search of more food or die due to lack of it. Managing the population of beneficial insects requires planning. Growers can “inoculate” their growing area by releasing small amounts of beneficial insects early in the growing season, in the hopes that they reproduce and build up their populations, or they can inundate a pest outbreak by releasing, large numbers of them onto growing areas that are being affected by pest species. Other planned releases should occur throughout the growing season to keep a baseline population of beneficial insect.
Pest insects are also subject to a host of diseases that can be introduced intentionally so long as they are harmless to humans, the crop, and other organisms. Bacillus thuringiensis is a great example. These bacteria often sold as Bt, are deadly to numerous species of insects in the larval stages and prevent their maturation into adulthood.
Other benefits to using organic sprays include a shorter re-entry interval (REI) between the time the pesticide is applied to the time people may re-enter the space safely. This is especially beneficial in a retail environment. What may be the most important reason to use organic pesticides, is that the consumption of chemical pesticides can have harmful effects on the human body. Traces of chemical pesticides are often detected in all types of crops exposing the consumer or end user of the product to potentially harmful or even deadly contaminants. To get “cleaner” crops, organic pesticides should be used if any pesticides do in fact need to be used.
Chemical Insect Control
Chemical control of insect pests in the greenhouse or growing space should be considered a final alternative, only after all other methods have been exhausted. Chemical insecticides are very effective at killing pests or otherwise impeding some aspect of their necessary behaviors, which then causes them to die without reproducing or due to starvation. Among the reasons to use chemical pesticides sparingly or as a last resort are due to the potential harm caused by exposure to the applicator and those working in areas that have been treated with chemical pesticides, as well as potential exposure to the end user.
Another important reason to limit use of chemical pesticides is that populations of insect species, either locally or globally can and will develop tolerances to certain pesticides, meaning that their repeated use only makes them less effective. To avoid this, it is a good idea to use a range of control methods or even just a range of chemical control options that contain different active ingredients. Chemical pesticides can be formulated from natural ingredients, synthetic ingredients, or some combination of the two. It should be noted that insecticides derived from natural sources do not make them automatically “organic”.
One benefit to using chemical pest control is that it is usually non-selective, and the same pesticide will kill a range of greenhouse insect pests. Chemical insecticides allow for fewer individuals to control larger areas or number of plants due to the ability to perform broad applications of pesticides at a single time. There are additional drawbacks to using chemical insecticides beyond worker exposure and insect resistance. Chemical insecticides often harm non-targeted life. Because they are non-selective, they will kill beneficial insects and pollinators right along with the pest insects.
Chemical insecticides do not have to be avoided entirely and should be looked as one “tool” among many, that are available to the grower. Efforts can be made to lessen their potential harmful effects. Portions of crops can be selectively sprayed based on insect pressure instead of spraying the entire crops. Chemicals can be selected based on their level of potential harm to beneficial insects. Chemicals can also be mixed at less than the recommended rates to make them slightly less toxic. In an event, the use of chemical insect control in the greenhouse should be done cautiously as well as judiciously. The instructions found on the label of any commercially available pesticide describe in detail when to apply, how to apply, and any other precautions needed, such as the wearing of personal protective equipment (ppe). Remember that these instructions are not just suggestions; they MUST be followed. The label IS the law.
Field Insect Pests
The species list of insect pests that bother field crops is different than those that bother greenhouse crops. Many of those species require the heat of an indoor growing environment that cannot be replicated in most parts of the outdoors in North America, except for some of the southern states of the US. This does not mean that fewer pests bother field-grown crops, only that the range and types of pest species are different, though there is some overlap.
The same diagnostic tools for greenhouse pests exist for field pests. The types of damage caused by field insects is similar to that of greenhouse pests. There are those pests that cause damage by sucking fluids from leaves, stems, roots, buds, and fruit. There are also insects that chew leaves, as well as burrow between leaf layers. Soil-dwelling insect pests can be an issue in field crops just as in greenhouse crops.
Common field pests include caterpillars of various species, flea beetles, boring insects of a wide range of species, aphids, whiteflies, snails, and slugs to name a few. The options for controlling these pests are similar to those available to the greenhouse grower, though again, with some exceptions and modifications.
Control options:
Cultural Controls
In many cases, cultural controls can be the most effective methods to control pest insect in field crops. Just like in the greenhouse, these methods rely on best practices and good judgment instead of relying solely on chemicals. The most common one is hand-picking of insects. This cannot be done easily in a greenhouse since the pests tend to be smaller but in the field, many common pests are large enough to see and to be picked off and dropped into a bucket of soapy water.
Selecting pest-resistant field varieties is a great place to start. These cultivars are bred and selected for their ability to either withstand more pest pressure, or not to attract them to begin with. Practicing crop rotation is another cultural method in the field that can help to eliminate or control the incidences of pest pressure. When a crop is planted in a different field, then any pests that may have overwintered in the soil will be disappointed when they emerge to find no food source. This practice only works on a large scale. In a garden setting, this is a beneficial practice but is not likely to impact the amount or severity of insect pests. Crop rotation also helps to ensure that nutrients and minerals are not depleted out of the soil, causing for weaker, subsequent crops. Weaker plants attract far more pests than healthy ones.
Planting a trap crop is another strategy to control insect pests. This practice includes planting a “sacrificial” crop of a variety that pests prefer, in the hopes that they leave your more valued crop alone. These trap crops can then be removed and destroyed, with the insect pests along with them.
Tilling the area to be planted and letting it sit for a short time before planting can help too. This exposes many insect pests to predators like birds and frogs. The timing of planting can also be an aid to the reduction of insect pests on a crop. This is especially true to avoid some types of maggots and other pests that feed in the larval stage but are not so harmful as adults.
The best cultural method to control pests has got to be practicing good sanitation. It is more difficult to fathom sanitation in the field versus a greenhouse setting, but it is still important. Field sanitation includes removing weeds around and in between crop plants as they can be hosts to unwanted pests too. It means removing crop residue from the previous harvest instead of just letting it decompose in place. Cleaning equipment such as tiller tines and plow blade sin between uses will also help to prevent the inadvertent spread of pests between crops.
Mechanical Insect Controls
The line between mechanical insect control methods and cultural ones is a bit blurred, but mechanical controls are physical devices employed to control the incidences of pest insects. These methods are intended to preclude pests form ever getting to crops. This is infinitely more difficult to achieve in the field since there are no enclosures, field crops are right out there in the open. This does not mean though that there are no tools at the disposal of growers to use.
Row covers can be used on field crops. These are often used to prevent frost damage but are also used to protect some crops from insect damage. Some types of colored plastic mulches have also been shown to prevent certain pest species from coming to pray on your plants. Other methods include the use of plant collars placed around the base of the stalk. This can prevent some pests from climbing up the crop and starting to feed. For larger stalked plants, cardboard bands are sometimes used and frequently collected as many insects will hide in them. Traps are also a form of mechanical insect control.
Some traps attract pests by their color, such as the sticky cards often used in greenhouses, or with scents or pheromones. In the field, earwigs and slugs can be lured away from crops by placing lids filled with beer just under the soil surface. They are attracted to the scent and will fall in and die. Pheromone traps though, should be used with caution as they can sometimes encourage more of the unwanted pest insects to come to the field. Japanese beetle traps are notorious for actually attracting more pests into a yard or field as they are lured by the scent, but then stop off for a feeding.
Organic Insect Controls
There are some distinct differences between organic controls in a greenhouse setting versus those in the field. The use of organic pesticides is largely the same as far as selection and application. Look for formulations that are certified as organic or authorized for use on organic crops. Biological methods for field insect controls are, however, often different.
Crops grown in a field setting have the advantage of naturally occurring beneficial insects. Many of the same species (regionally specific), that are bred to be released in greenhouse settings are naturally present in the outdoors. Conversely, the intentional release of beneficial insects in field settings is often futile as there is nothing to retain them from flying or crawling off. An exception to this is beneficial nematodes, as they are applied to the soil. Milky spore or Bt, along with other types of beneficial bacteria, can also be applied to field crops as a method of biological insect control, just like in the greenhouse setting. Applying diatomaceous earth onto the soil to deter crawling pests is a method that is far more practical in field-grown crops. This material, often known as “DE”, is ground shells and other natural materials which cause insect that crawl over it to cut themselves open, often resulting in death from drying out. Another natural approach to pest control in the field involves the use of other plants.
Plants that either naturally repel insect pests, or those that attract beneficial insect can be planted among crop plants or along the margins. This is not a practical plan for greenhouse crops necessarily due to the limits of space but can be very effective for field insect control. Species like Yarrow (Achillea sp.) naturally attracts beneficial insect predators such as lacewings, ladybugs, hoverflies, and several species of parasitic wasps. Several species of marigolds, conversely, but no less effectively, repel insects both with their scent and because of a secretion by their roots. Another field control is to plant trap or “sacrificial” crops.
Trap crops are crops specifically planted to attract insect pests. The point is that if they focus on the trap crop, then hopefully the desired crop will be relatively unharmed. This can be done for crops within the same species, such as with squashes, or with different species such as planting sunflowers to detract some pests from tomatoes.
By and large, the use of organic insect pest control for unwanted field insect pests offers several other options than for the greenhouse. If none of the aforementioned strategies are effective at controlling field insect pests, then it is probably time to consider chemical insect controls.
Chemical Insect Controls
The use of chemical formulations to control field insect pests should be considered a last resort only when mechanical, cultural, and organic or biological options have been exhausted. Chemical controls are not one-size-fits-all; they come in many forms and options. There are thousands of chemical formulations out there designed to kill or interfere with the functions of pest species. Some have been in use for decades while others have just entered the marketplace.
Where relevant or practical, only those plants or even portions of plants that are showing evidence of insect pest activity should be sprayed with chemical pesticides, unless the one chosen also serves as a preventative for future outbreaks. When more than one type of chemical pesticide is chosen for use, it should have a different primary ingredient and use a different mode of action to kill the targeted pest. As mentioned, populations of pests can and will build up tolerances to chemical and synthetic pesticides, so the same ones should not be used in succession, or for long periods of time. Very rarely will 100 percent of a pest population get killed with a pesticide application. Those that survive may be ones with natural immunity and those are the ones left standing that will pass down that immunity to their offspring. This is part of the reason that the use of chemical pesticides is a merry-go-round that is hard to get off of. The use of these materials begets the use of more and more.
Synthetic insecticides do more than kill the targeted pest. They often kill beneficial insects as well and can kill or disrupt the entire microbiome of plants. As such, it is advisable that after the re-entry interval has elapsed, that plants sprayed with synthetic pesticides, be given some sort of organic, nutrient rich material such as compost or earthworm castings to help to reestablish the microbial life that was inevitably killed by the chemical pesticides.
Not that all organic pesticides are safe to handle, but it should be assumed that chemical pesticides are toxic to humans, so care should be taken while handling, mixing, and applying. As with all other pesticides, whether used inside or out, wear all recommended ppe and follow the instructions on the label explicitly.
