Scale insects are one of the most common pests on house and garden plants. The animals sometimes cause great damage, especially to hard-leaved plants as well as fruit and ornamental trees. How to effectively combat plant suckers and ensure that your plants are never attacked in the first place.
- Scale insects are one of the most common pests on both house and garden plants.
- There are various species that primarily attack sclerophyll and evergreen plants as well as fruit and ornamental trees.
- They are often difficult to detect in the early stages and can cause great damage.
- The best way to combat scale insects is with a soap and alcohol mixture or with nettle manure.
Identifying scale insects
Scale insects (lat. Coccoidea) form a very species-rich family within the plant lice (lat. Sternorrhyncha). There are around 4,000 different species worldwide, but only around 90 are native to Central Europe. The different varieties specialize in quite different plants. So-called scale insects such as the comma scale insect or the introduced San Jose scale insect, which typically do not secrete honeydew, are mainly found on fruit trees.
White scale insects love houseplants, but are also often found on fruit and other deciduous trees. These species in turn secrete honeydew, which is usually colonized by sooty mold fungi. But whatever the type, the countermeasures described here are equally effective for all of them.
What do scale insects look like?
Scale insects can be green, black or light
Scale insects are generally very small: depending on the species, the animals reach a length of between 0.6 and six millimeters. What is characteristic is the striking protective shell that only the adult females develop. The pests use this to attach themselves to a suitable feeding place, where they also lay their eggs and raise their larvae - the shell not only protects the female, but also her eggs and larvae. Typically, scale insects do not settle individually, but in real colonies on leaves and shoots as well as trunks, branches and twigs.
Incidentally, the male scale insects are completely harmless to the plants because as adults they do not eat any food and have an extremely short lifespan anyway. You might mistake them for small flies, but they don't always appear. Males are not absolutely necessary for the reproduction of scale insects, as the animals can also reproduce through virgin production.
Lifestyle
A single female scale insect lays up to 2000 eggs a year, which she keeps under her protective shield. The newly hatched larvae are also well protected here; they only leave the cocoon in a later nymph stage. These young animals are very mobile and can easily migrate to other plants. Since the light scale insects can also be carried by the wind outdoors, further infection does not necessarily have to be limited to neighboring plants.
As soon as a suitable place is found, the female animals attach themselves and form the characteristic shield shell. Since this sticks and the animal also loses all external organs with the exception of its proboscis, the female scale insects are usually immobile. Due to the rapid reproduction method and the high risk of infection, timely control measures are so important - especially since scale insects are quite stubborn and difficult to control.
Damage caused by scale insect infestation
Scale insects leave holes in the leaves
Unfortunately, a scale insect infestation is usually only noticed very late, as both the adults and their larvae initially hide very well and therefore remain inconspicuous. If the first typical symptoms become noticeable, the infestation is usually already very advanced. To prevent this from happening in the first place, you should check endangered plants regularly and thoroughly. The following damage pattern is typical for a scale insect infestation:
- scattered or clustered shield shells
- appear as green or brown bumps
- wool or wax-like webs on the plant
- Plant parts and surroundings feel sticky (honeydew)
- encrusted shoots and twigs
- Leaf spots, usually light brown, yellow or reddish in color
- Buds and flowers do not open
- Leaves become crippled, are thrown off
Symptoms such as leaf spots and shedding as well as short stature and deformed buds and flowers can not only indicate scale insects, but can also occur as a result of other pests or plant diseases. The only characteristic features are the bumps or webs (depending on the type of scale insect), which can appear on almost all parts of the plant.
Particularly endangered plants
Scale insects prefer to settle on sclerophyll, evergreen plants, which is why popular houseplants such as rubber and lemon trees, holly and orchids are particularly at risk. In the garden, the pests particularly like to settle on fruit and ornamental trees, where they can be found not only on the leaves and soft shoots, but also on the trunk, branches and twigs. You can recognize an infestation by the scale-like formations that often cover the various parts of the plant in large quantities. You can see which plant species are particularly at risk in the table.
| Houseplants | Garden Plants |
|---|---|
| Asparagus (various) | Apple tree (Malus domestica) |
| Ferns (various) | Pear tree (Pyrus communis) |
| Window leaf (Monstera) | Berry bushes (various) |
| Ficus (various) | Cherry trees (Prunus) |
| Flamingo flower (Anthurium) | Peach trees (Prunus persica) |
| Cacti (various) | Plum trees (Prunus domestica) |
| Orchids (various) | Oleander (Nerium oleander) |
| Rubber tree (Ficus elastica) | Ivy (Hedera helix) |
| Palm trees (various) | Ferns (various) |
Excursus
Mealybugs and mealybugs are also scale insects
By the way, mealybugs and mealybugs also belong to the group of scale insects. These pests, which are up to five millimeters long and mostly pink, white or light brown, are not protected by a shield, but rather by a web of wax ora powder-like, greasy dust. In these species, the females, otherwise atypical for scale insects, remain mobile and cleverly hide themselves and their clutches in hard-to-reach places such as the leaf axils, the bracts or even in the substrate.
Causes and Prevention
Basically, scale insects themselves are only a symptom, as they only attack previously weakened or diseased plants. These are weakened in their defenses - for example due to an unsuitable location or poor care - and have nothing to counteract the pests. For this reason, scale insects (and many other pests) appear, especially after overwintering, when houseplants are affected by dry heating air, lack of light and/or overwintering that is too warm.
You should also avoid these care mistakes if possible:
- Lack of light / location too dark
- low humidity / dry air
- Nutrient deficiency / insufficient fertilization
- Overfertilization, especially with nitrogen
- Water shortage
- Waterlogging
In addition, many houseplants should not overwinter in the warm living room, but should be placed in a bright and cool place. This is especially true for Mediterranean plants such as citrus plants, olive trees or oleanders, all of which are often attacked by scale insects.
Can you do something preventatively against scale insects?
“Prevention is better than cure!”
You can prevent scale insect infestation by following these care instructions, especially for your houseplants:
- as much light as possible, install plant lamps if necessary
- Don't overwinter too warm - many Mediterranean plants can only be kept cool up to a maximum of 10 to 12 °C
- balanced supply of water - do not allow the substrate to dry out, remove excess water from the saucer or planter as soon as possible
- Ensure high humidity, especially when it is warm (summer months, heating period), spray plants regularly or install indoor fountains
- fertilize regularly, but not excessively
- be economical, especially with nitrogen-containing fertilizers
- use high-quality substrate, tailored to the respective plant species
In addition, many houseplants not only need a lot of light in the summer months, but also air. If possible, place lemon and olive trees as well as oleanders etc. in a suitable location outside, such as on the balcony or terrace.
What helps? Fight scale insects effectively
Since scale insects only cause damage to the affected plants, multiply rapidly and quickly infect other plants in the area, immediate and sustainable control measures are immensely important. Otherwise, the affected plants will die over time. In addition, the larger the population, the more difficult scale insects are to eliminate. In this section we will introduce you to the most effective methods against scale insects.
First aid measures
Showering will remove most of the scale insects
Aphids are often driven away with a strong jet of water. This simple method doesn't work with scale insects because their stuck-on shields keep them firmly attached to the plant. Instead, you can fight individual animals - for example on house plants - in this way:
- Isolate the infected plant immediately.
- There must not be any other plants nearby.
- Place it in a location that is as bright and cool as possible.
- Spray them with water and keep the humidity high.
- If necessary, cut back heavily affected plant parts.
- Soak a cotton swab with rubbing alcohol.
- Press the cotton swab onto each scale insect individually.
- Spread the spirit carefully over it.
- Carefully wipe off any honeydew with a soapy solution.
If the infestation is too severe, you can spray the plant instead, for example with a soap and alcohol solution. This procedure is easy to handle, especially with individual houseplants, but requires a lot of time. Do not scratch or scrape the scale insects under any circumstances, as this will keep the eggs and larvae alive and spread them over the entire plant.
Proven home remedies
If you have a scale insect infestation, you don't have to run to the gardening store straight away, because fortunately, various home remedies have a very good effect against the pests. The best way to apply this to the plants is to use a spray bottle, making sure not to miss a spot, no matter how small. Please note that some scale insects also like to hide in the leaf axils and bracts. In hard-to-reach areas, simply apply the decoction with a brush or a cotton swab. Repeat the application until the pests disappear.
For houseplants: soap-spirit-lye
A mixture of (liquid) curd soap and spirit has proven to be practical, especially for houseplants. To do this, carefully mix 15 milliliters of soap and spirit with one liter of water and fill the mixture into a spray bottle. If possible, use low-lime water such as rainwater or well-stagnant tap water. The liquid should also be at room temperature and never cold or hot - this will harm your plants.
First test in an inconspicuous area to see whether the plant in question can tolerate the mixture at all. Many sensitive plants also react to such home remedies with hanging and/or shed leaves. However, if it is normal after the test - just wait an hour or two for any reactions - you can finally treat it.
For garden plants: nettle or wormwood manure
Of course you can also treat your garden plants with a soap and spirit solution, but the very versatile nettle manure is more effective here. This has the advantage of being much gentler on the plants treated with it and also providing a balanced nutrient cocktail. This also gives the plant a powerful boost to its own defense system. Instead of nettle, you can also prepare and use wormwood manure.
And this is how you prepare the nettle manure:
- Collect one kilogram of fresh leaves and stems.
- Prefer nettles before flowering as they contain more active ingredients.
- Cut the plant parts as finely as possible.
- Put them in a plastic bucket.
- Fill with ten liters of rain or spring water.
- Cover the mixture with a breathable cloth or mesh.
- This is to prevent animals from falling in and drowning.
- Place the mixture in a partially shaded and warm location.
- Stir vigorously every day.
The manure is ready for use after about eight to ten days. Now filter out the coarse plant parts and fill the liquid into a canister. It will last for about a week as long as you seal the canister airtight. To use it as a spray, you should dilute the nettle manure with water in a ratio of 1:10 - i.e. H. For one part of manure there are nine parts of rain or spring water.
Tip
Plant manure is really only recommended for use in the garden due to its pungent smell. However, you can mitigate the smell somewhat by adding a handful of rock dust to the mixture.
Biological antidotes: voracious beneficial insects against scale insects
If scale insects appear in the garden or greenhouse, so-called beneficial insects are recommended for targeted control. You can buy these from specialist retailers or online and place them directly on the affected plants. The beneficial insects usually eat eggs or larvae of the pests, but do not cause any damage to the plants themselves. Once the pests have been eliminated, the beneficial insects usually disappear again. By the way, you can also plant them specifically in the garden, so the natural balance is maintained and pests have a harder time establishing themselves from the start.
In a garden that is friendly to beneficial insects, however, you are not allowed to use any sprays (not even home remedies!), as these do not differentiate between pests and beneficial insects. They also use soapy water and alcohol to drive away the desired insects. You should also provide plenty of insect food (for example in the form of lots of umbelliferous plants), because in many cases the larvae of the beneficial insects act as pest controllers, while the adult insects live primarily on pollen and nectar. A cleverly placed insect hotel provides a sought-after shelter.
If you want to use beneficial insects against the scale insect plague, you should use one of the following types:
- Parasitic wasps
- Ladybug
- hoverflies
- lacewings
- Gall midges
Be sure to follow the manufacturer's application instructions and dosage recommendations, as using too few or too many beneficial insects will not lead to the desired success.
Chemical agents
If none of the remedies described help, perhaps because the infestation has already progressed too far, there are also special systemic pesticides. However, their use needs to be carefully considered, especially if you are going to use them in the garden or greenhouse. Chemical agents always have negative effects on the environment and thus the ecological balance.
In this case, use special sticks or sprays against scale insects, as contact poisons have no effect - the shield's shell is too strong for them. Simply stick the sticks into the soil while using the spray as a spray.
Frequently asked questions
Are scale insects poisonous to humans?
You can theoretically eat scale insects
Typically, scale insects not only suck out the plant sap, but also inject toxic substances into the plant cells. These in turn cause the leaf spots that are characteristic of a severe infestation and can be brown, red or yellow in color. However, the pests are harmless to humans and pets and are not poisonous.
Are scale insects dangerous?
Scale insects are in no way dangerous to humans or animals. An infestation is only problematic for the affected plants, because the longer and the more pests are on a plant, the more likely it is to be damaged. There is a long-term risk of deformities - for example unsightly spots on the leaves, yellowing flowers and stunted growth - or even the eventual death of the plant. This is why timely and effective control methods are so important.
How do scale insects spread?
If there is an infestation of scale insects on your houseplants, the animals were probably introduced from a new plant. Therefore, when purchasing, look out for tell-tale signs such as shields on hidden parts of the plant or waxy secretions. It's better to leave such plants alone and not take them home! However, if you receive plants as a gift, first quarantine the new addition and examine it carefully for any characteristic signs.
Otherwise - for example in the garden - scale insects spread because the young animals migrate and conquer new terrain. Infestation can also occur through wind transmission or through already infected shoots (for example cuttings from an infected plant in the neighbor's garden).
Can scale insects fly?
Female scale insects have no wings and therefore cannot fly. They don't have any external organs at all, for example the adult animals also lack legs and eyes. The male scale insects, on the other hand, have front wings but no hind wings, so they are able to fly. They look like flies, but they only live for a few hours. However, scale insects are not necessarily dependent on sexual reproduction, as the female animals can also reproduce asexually through so-called virgin procreation.
Does frost help against scale insects?
Some people hope that the scale insect plague will clear itself up in a frosty winter. However, the pests are completely insensitive to cold and can survive even the coldest winter - at least when it comes to eggs. These are usually located under the lid of the female scale insect and are well protected from the weather. For this reason, it is important that infected plants are treated in February and again at the beginning of vegetation so that the eggs do not develop into larvae and thus into adult scale insects.
Tip
No matter which control measures you choose: a single application is not enough! In order to actually catch all the scale insects as well as the last larva and the last egg, you have to use the products several times at intervals of several days and weeks.