Many thrips are pests of commercial crops due to the damage caused by feeding on developing flowers or vegetables, causing discolouration, deformities, and reduced marketability of the crop. Thrips may also serve as vectors for plant diseases, such as Tospoviruses. Over 20 plant-infecting viruses are known to be transmitted by thrips. Due to their small sizes and high rates of reproduction, thrips are difficult to control using classical biological control. All predators must be small and slender enough to penetrate the crevices where thrips hide while feeding, and then prey extensively on eggs and larvae. For this reason, many growers are occasionally forced to make limited use of pesticides to control thrips populations in the field and in greenhouses.
Since thrips feed on plants and damage fruits, leaves, buds and flowers, they affect their cosmetic appearance, which lowers their marketing quality. Ornamental plants and certain vegetable crops are more susceptible to serious damage caused by thrips, especially when the plants are young. Thrips can attack vegetables (tomatoes, peppers, cucumbers, onions, eggplant), fruits (citrus, blueberries, avocados, plums, apricots, peaches, melons), ornamental flowers (gerberas, geraniums, carnations, chrysanthemums, violets from Parma, roses), cereals, tobacco, trees and shrubs.
The fact that thrips feed on plants can hinder their growth, affecting them by transmitting viruses and by reducing their productivity. Attacked leaves are damaged, may fall prematurely and present, along the ribs, small yellow-grey or dark brown spot. It is very important to distinguish between species of thrips where integrated pest management methods are used. Under appropriate conditions, such as greenhouses, solariums or various confined spaces for plant growth, many species of thrips can multiply exponentially due to the lack of natural predators. They form large swarms, becoming a nuisance for gardeners and growers.
Although damage caused by thrips is usually aesthetic, that does not necessarily justify the use of biological insecticides. The damage of the plants is usually visible only after the tissues grow and develop and by then, the pests might not even be present anymore. Any type of insecticide application will not be able to repair the damaged tissues. And when the problem becomes the viruses that can be transmit by the thrips, insecticides are not fast enough to kill these pests, and thus to stop the transfer of these viruses in time.
Therefore, it is better to prevent than cure such an infestation. Below you can see several ways of keeping thrips away from the plants in your garden or greenhouse, thus avoiding their infestation with viruses and the subsequent problems that this entails.
Preventive methods for the garden
The main measures you can apply to control this pest are primarily a matter of proper maintenance of the garden. Here’s what you can do to keep away the plants in your garden from a thrip infestation:
- take care of your garden soil, water it regularly and apply mulch on it. It is known that thrips are attracted to plants that are already ill or unhealthy and thus, they can often be seen on plants growing in harsh conditions, such as in dry land, not irrigated and without mulch on it;
- eliminate all vegetal residues, such as dry or wilted flowers;
- do not fertilize the soil more than necessary, as this will encourage the growth of other juicy plants, that will attract other pests, such as aphides;
- cultivate plants that are adapted to the environment from the area; if you cultivate plants that prefer sun, in a shady and chilly area, they will become more predisposed to thrip infestation.
- be vigilant and periodically inspect the plants to see if they have or have not been infested. You can see signs of thrips infestation in the inferior sides of the leaves and in the nip between the leaves and stems. Do not hesitate to remove the already infested plants and to cut the grass and weeds around your home to exclude alternative hosts where thrips could relocate.
- do not cut the leaves off of the plants that thrips prefer with a scissor, as this will determine a younger and softer leaf to grow, thus pleasing these pests;
- inspecting all the plants you buy before you bring and plant them into your garden;
- provide good, constant irrigation to plants;
- using row covers, hot caps, or other kinds of cages with a fine mesh in order to keep thrips (and other insects) away from vulnerable vegetables or young plants; once the latter become larger, though, it is important to remove the covers, allowing them enough growing space.
Sometimes chemical treatments for thrips are necessary when populations in lawns, grassy areas or on ornamental and landscaping plants are large. However, most of the time, thrip treatments can be accomplished using non-chemical techniques. Some examples of non-chemical treatments you can employ include removing infested plants, excluding thrips from getting inside the structure of your greenhouses and using vacuums to physically remove dead or living insects.
Organic preventive measures
You can also use fine nets that will keep back thrips and other plant pests. They can be installed once you plant your seedlings, or before the plants grow out of the soil, and keep them over the plants as a means of protection, while they are young and sensitive. After the develop well enough and temperature rises, you can remove the nest in order for the plants to have room to grow, and also to avoid overheating. When installing such nets, you need to take into consideration the fact that they need to allow light and sun to enter, and the type of the net also depends on the plant you are trying to protect. Wooden frames, or made from plastic or wire, covered with muslin, nylon or other fine materials can be used on long term. Irrigating furrows or rows of seedlings is required when using this method.
This method of using reflective sheeting is also a common measure of keeping thrips and other pests away from young plants. These foils reflect light so that it blocks UV rays and affects the ability of insects to locate plants (as thrips see in the UV spectrum). They need to be installed before the plants come out of the ground or when planting seedlings, but leaving holes large enough to allow the plant to get outside when it grows. Where flowers and vegetables are particularly susceptible to viruses transmitted by these insects, cost and effort to use reflective sheeting is justified since this are significantly more effective than insecticides when it comes to preventing or delay infection in small plants. It must however be noted that these sheets cease to be effective as an insect repellent when the plant canopy covers more than half of the surface of the ground below.
Silver or grey sheets are the most effective, but also the white ones work quite well. Typically used are the polythene metallic grey ones. If handled carefully, they can be used for more than one season. Aluminum foil is also effective and is more suitable for a small garden, but it is an expensive option and disposable as foils can not be reused.
Another advantage that you can see by using these reflective foils, is that besides the fact that they temporary reject thrips and other flying insects, they also augment the growth of certain crops because they make a contribution of increased light, keeping the soil warmer overnight, reducing weeds and maintain the soil moisture. However, reflective foils can also have negative effects. One is the emergence of disease susceptibility at the roots of the plants, because by avoiding to water the plants from above, irrigation is done at ground level without being able to adequately control its humidity.
You can also use glue traps, preferably blue or yellow ones. This method is usually applied in greenhouses or tunnels and is indicated in order to establish the presence of adult thrips, so you’ll know for sure what is attacking your greenhouse plants and you will be able to properly channel your efforts for the prevention and control of this pest. Hanging the strip over the plants so that the top is at a height of 60 cm, and the lower one at around 25 mm above the highest leaves. Putting traps out in numbers can be effective in reducing airborne adult populations of certain pests, such as the thrips.
Preventive measures using natural predators
Several species of parasites are known, such as Eulophidae and Trichogrammatidae, that can parasitize eggs and larvae of thrips. Other natural agents, consumers of thrips larvae and adults individuals are aphid wasps, beetles from the Orius family, some mites and ladybirds. To naturally encourage the emergence of such predator populations, it’s better to avoid dust deposits on plants, the persistent systematic use of insecticides and you can also plant a diversity of plant species. However, the most efficient predator belongs to a species of mites, Hypoaspis miles, which lives in the soil and feeds on thrips larvae in the ground, that are preparing for their pupa stage. To be effective at maximum capacity, the soil where they are introduced should be moist and warm, so it’s better to use them especially in the summer and to make sure that plants are properly irrigated. Since they feed on thrips only at the pupa stage of their development, it is good to know that their use can have a big impact on the population of thrips next year because they interrupt the life cycle of these pests. Thus, you can better them as a preventive measure rather than as a solution against thrips that have already attacked the plants in your garden.
Thrips can be difficult to control effectively with insecticides, partly because of their mobility, feeding behaviour, and protected egg and pupal stages. Improper timing of application, failure to treat the proper plant parts, and inadequate insecticide coverage when using contact materials are common mistakes that can prevent potentially effective insecticides from actually providing control. Keep in mind that an effective prevention can happen only when you take into consideration all the factors involved. Should you not properly take care of your garden and keep applying the preventive measures listed above, pests such as thrips can easily return and cause damage to your crops, making it necessary to find ways of getting rid of a thrip infestation.