Known Pests: Whiteflies, Aphids and Spider Mites

In short: Whiteflies, aphids and spider mites damage plants by sucking sap from young parts and quickly form large colonies. Spider mites, about 0.5 mm in size, leave fine webs and yellowing leaves, while aphids deform shoots. Protect natural enemies such as ladybugs and predatory mites, and rotate insecticides against whiteflies, adding a wetting agent.

Common Pests: Whiteflies, Aphids and Spider Mites

These plant pests are commonly characterized by causing damage through sucking on young plant parts. They are relatively small, an individual does not cause major direct damage, however their reproduction capacity is enormous. They form colonies of many individuals and together they are capable of severely damaging the infested plant. Apart from the direct damage they can cause, they are often vectors of viral diseases, some of which are quite serious. Aphids and whiteflies suck sap from plants from which they obtain proteins, however they do not utilize sugars and excrete them from their body onto the plant surface. Here they are overgrown by sooty molds, which are fungal diseases causing blackening of plant surfaces, or in the case of aphids, the excreted sugars are a sought-after food for ants and other insects. Using non-selective (killing a wide range of living organisms) products is not a long-term solution for outdoor areas, quite the opposite, by eliminating beneficial organisms together with the pest we will achieve that the next occurrence will be more massive and resistant to the used chemicals.

Whiteflies

They are persistent heat-loving pests especially in greenhouses, however in recent years they regularly occur also on field crops. Whiteflies go through development stages egg-larva-puparium-adult. In our conditions we can encounter several species of whiteflies.

• Greenhouse whitefly – Trialeurodes vaporariorum
• Cotton whitefly – Bemisia tabaci

The first two species are found mainly in greenhouses, they parasitize mostly on cucumbers, tomatoes, and eggplants. Whiteflies transmit viruses, with the Cotton whitefly transmitting more than 25 viruses. Both species overwinter in our conditions only in enclosed spaces.

• Cabbage whitefly – Aleyrodes proletella

It is a pest of brassica vegetables, it mostly attacks Brussels sprouts, cauliflower, broccoli.

Protection Against Whiteflies

Greenhouses are a suitable refuge for whiteflies, especially because the same plants are grown continuously in them. The importance here lies in removing and burning plant residues at the end of the growing season and disinsection in winter. Natural enemies of whiteflies have difficulty entering the greenhouse from the surrounding environment, however the application of commercially produced bioagents is important here.

Outdoor areas are usually populated by a more diverse species composition of natural enemies.

For greenhouses and enclosed areas, more aggressive products with longer-lasting effects can be used.

Recommendations for Insecticide Application for Whiteflies

Whiteflies are persistent pests especially because they are quickly able to develop a resistant population to the used insecticide. Therefore it is necessary to rotate the used products.

Due to the fact that their skin contains a large amount of wax, it is necessary to use a wetting agent together with the chemical preparation.

Aphids

There are two perspectives on aphids. On one hand, they are a persistent pest that can transmit numerous viral diseases, and on the other hand, they are beneficial, as the honeydew secreted by aphids is sought after by bees, serves as their food, and forms a substantial part of forest honey. Of course, there are many species of aphids, which are divided in their development into monocyclic (populations of these species develop on only one host throughout the year), dicyclic (two), and polycyclic (more). This classification is particularly important for revealing their development and possible transmission of viruses.

Aphid development is relatively fast, their reproduction varies - young individuals either hatch from eggs or are born directly as adults, sometimes even without prior mating, occurring through so-called parthenogenesis. In some aphid species, all of this can be combined, in others it cannot. Some generations of aphids are winged and serve to spread to the surrounding area. For most aphids in our conditions, thick-walled eggs overwinter. Practically every fruit species has a specialized aphid species, some even two - for example, on plums you can find the Plum aphid and the Damson-hop aphid. Most aphid species attack multiple plant species.

Most Common Aphid Species

Apple aphid, Plum aphid, Cabbage aphid, Poppy aphid

Symptoms of Aphid Infestation

As a rule, due to toxic substances secreted by aphids that are harmful to plants, after aphid feeding there are shape deformations of leaves or plant shoots, leaves may change color. Infested plant parts lag in growth, bloom less, and with stronger and longer-lasting infestation, the affected part may die. The honeydew secreted by aphids then grows sooty mold on plant surfaces during dry periods.

What About Aphids and Ants

As mentioned earlier, aphids cannot process sugars, which they then excrete in the form of honeydew on their abdomen. This sugar-rich excrement is sought after by bees, ants, and other species as food. While other insects only collect honeydew, ants went further and protect aphid colonies from their enemies. Ants are even capable of spreading aphids around plants and essentially farm them similarly to how humans keep cows. It is precisely ants that play a role in the natural regulation of aphids by ladybugs, as ants actively defend aphids and attack ladybug larvae. In such cases, it is sufficient to prevent ants from accessing by applying glue to the tree trunk.

How to Deal with Aphids

Natural enemies of aphids are very abundant and usually sufficient in the wild to suppress aphids that do not form large populations (woolly aphids, plantain aphids). Other species are capable of completely infesting even a mature tree (apple aphids, plum aphids). Aphids serve as food for ladybugs, ground beetles, predatory bugs, lacewings, spiders and many others. In many cases, humans are responsible for the repeated occurrence of aphids by using non-selective insecticides that eliminate both aphids and their natural enemies, which have a development cycle several times longer than aphids.

For woody plants, spring treatment with oil-based products is essential. During the growing season, it's better to choose products based on natural substances for outdoor areas. In enclosed spaces, spraying with a more potent insecticide with long-lasting effects is worthwhile, for example here.

Spider Mites

Together with woolly aphids and gall mites, they are tiny mites, barely visible to the naked eye. Overwintering females lay eggs from which males hatch, and together with the overwintering females, they form the basis for the next generation. Spider mites go through the development stages: egg-larva-nymph-adult. Nymphs have five developmental stages, and spider mites have 6-8 generations per year in total.

Symptoms of infestation are characterized by interveinal yellowing of leaves, followed by their drying and falling off. On the underside of leaves, there are fine webs covering tiny eggs and small mites approximately 0.5mm in size.

Plant Protection Against Spider Mites

Products for spider mites are called acaricides. They work on spider mites as well as other mites, such as woolly aphids (currant woolly aphid, cherry woolly aphid) and gall mites (grape erineum mite). Calamitous outbreaks of spider mites occur in outdoor areas when their natural antagonists die out. This is usually caused by the application of broad-spectrum insecticides. Natural enemies of spider mites include predatory mites, ladybugs, lacewings and predatory thrips. Currently available is the predatory mite Typhlodromus pyri, whose application is useful in greenhouses and outdoor areas. For indoor plants and in justified cases also for outdoor areas, more potent products like Karate can be used, but it is always better to use products based on natural substances.