The potato beetle, scientifically known as Leptinotarsa decemlineata, is a widespread beetle in our country. It is a highly destructive insect and challenging to control. The extensive use of chemical pesticides in traditional agriculture is one of the main reasons for its increasing resistance. Nowadays, containing its infestations is not at all simple. However, using organic remedies and avoiding pesticides is a choice that every potato grower should make.
In this article, we aim to understand how to intervene using products allowed in organic agriculture and suitable agronomic techniques. Before that, let’s get to know this beetle better, its origins, its biological cycle, and the damages it can cause to crops (not just potatoes).
Origins and Spread of the Potato Beetle
The potato beetle belongs to the order Coleoptera, suborder Polyphaga, and family Chrysomelidae.
This insect is of North American origin. The first specimens were observed in the Rocky Mountains of the southwestern United States in the early 1800s. It was immediately associated with potato cultivation and was seen in Nebraska in 1859.
The current Anglo-Saxon name, Colorado potato beetle, refers to a severe infestation recorded in potato fields in Colorado in 1865. This leads to the belief that the insect originated from there.
Since then, the potato beetle has spread rapidly throughout the United States. In some unfavorable years, it has become a real scourge.
The first form of biological defense used by American farmers, when there were no synthetic products yet, was to release poultry in the fields, as they are fond of this beetle.
In Europe, the potato beetle arrived first in England in 1876. However, it was eradicated there and is not present today.
On the continent, the beetle landed in France a few years later, then spread almost everywhere. In Italy, it arrived during the Second World War, in 1943, in Piedmont.
The potato beetle is easily recognizable in its adult stage. It takes the form of a typical beetle, oval and convex. The length ranges from 8 to 12 mm. On the dorsal side, there are evident elytra, light yellow in color, marked longitudinally by 10 prominent black stripes (hence the name decemlineata). The head has a triangular black spot, while the legs are orange with black hues.
The lower part, the prothorax, is of a conspicuous brown-orange color, with the edges marked by black punctuations and a sort of black “scarf” that wraps around the head.
Male and female specimens are very similar, and distinguishing them is not easy. In the larval stage, the coloration is reddish-orange, with a double row of black tubercles on the sides of the body.
They have the typical shape of Chrysomelidae larvae, with a curved body and a more expanded abdomen.
The eggs are also easy to recognize, with an elliptical-cylindrical shape and reddish-yellow color. They are usually laid in groups of 20-40, typically on the underside of the leaf blade.
Let’s start the description of the potato beetle’s biological cycle from the winter months. During this time, adult specimens manage to burrow up to 40 cm deep into the soil, where they overwinter peacefully. In spring, when the soil temperature reaches 14 °C, adults emerge from the ground and begin both their trophic activity on plants and mating and oviposition. A female, whose lifespan is about 4 to 5 weeks, can lay between 300 to 1,000 eggs. Moreover, she can emit an adhesive secretion to attach them to the leaf blade, usually on the lower side. The secretion also protects the eggs from sunlight and weather agents.
The small eggs hatch in 4-15 days, depending on environmental conditions. They generate larvae that undergo three molts in 20 days and then return to hide in the soil, though not too deep.
After a few more days, the larvae transform into pupae and give rise to the first generation of the year. The first-generation potato beetle resumes the life cycle, giving birth to the second generation of the year, which is usually the overwintering one. However, in favorable environmental conditions, such as in the southern regions, there can be a faster cycle, resulting in a third generation.
Damages and Affected Crops
The damages caused by the potato beetle to crops are due to the trophic activity of both adult specimens and larvae. The potato beetle is a defoliating insect, meaning it eats all the leaves without mercy. This, even if the plant reacts and produces new shoots, leads to stunted growth. In more severe situations, it can cause the death of the plant. Specifically for potato cultivation, the damage typically results in small-sized and low in number tubers.
Besides potatoes, other crops can be affected by attacks from this small beetle. In particular, solanaceous crops such as eggplant and tomato can be affected, especially if grown in proximity to potatoes or in agronomic succession with potatoes themselves.
Biological Defense, Premise
As mentioned in the introduction, biological defense against the potato beetle is proving to be the only way for all farmers.
Over time, it has been observed that this insect can develop resistance that makes the action of synthetic pesticides ineffective. Currently, the potato beetle can resist up to 25 active ingredients contained in the most widely used insecticides in agriculture.
This compels farmers to increase dosages and frequently change active ingredients. In short, it creates a vicious cycle that makes the insect stronger and more resistant.
Another intrinsic difficulty in fighting the potato beetle is its self-defense system, secreting toxic substances to ward off potential predators.
Now, let’s see the alternatives provided by organic agriculture.
One of the most effective biological products against the potato beetle is Bacillus thuringiensis. It acts on larvae through ingestion, using a protein crystal that, once ingested, activates an enzymatic mechanism in the insect’s intestine. The larvae stop feeding, and death occurs after 4-5 days, interrupting the biological cycle in this way.
It is essential to intervene against the potato beetle in the larval stage since adult insects are essentially immune to the action of bacillus.
Among the various existing varieties of thuringiensis, those most effective against the potato beetle are:
- Bacillus thuringiensis ssp. tenebrionis;
- Bacillus thuringiensis ssp. kurstaki strain EG 2424.
Unfortunately, these two versions are the most challenging to find on the market.
Less effective but still worth trying is the classic Bacillus thuringiensis var. kurstaki, which is more readily available online.
It is always recommended to apply the product in the evening, with abundant wetting of the foliage. This way, you will give the bacillus a longer time to act.
Another biological product to control potato beetle infestations is azadirachtin, which is the active ingredient found in neem tree seeds. One of these products can be purchased here.
The main action of this active ingredient is to arrest the development of insects in their larval stages (juvenilizing action).
Azadirachtin can interfere with the hormonal system of insects, causing a chitin-inhibiting effect based on the blockage of ecdysone. This causes an alteration in the beetle’s molting, preventing the larva from forming the external cuticle of the adult insect.
Azadirachtin can act both through ingestion and contact.
To defeat the potato beetle, you can use a bioinsecticide based on the spores of the pathogenic fungus Beauveria bassiana.
This is a highly effective natural agent against this type of insect. It acts by contact, penetrating the hard cuticle of the beetle and spreading the mycelium inside the insect.
The beetle’s death is due to the mechanical action of mycelium penetration, resulting in water and nutrient loss.
Online, with proper research, you can find bio products of this type, without GMOs and authorized for use in organic farming. More information on this product can be found here.
In the 1980s, the hymenopteran Edovum puttleri, a parasitoid of potato beetle eggs, was discovered in South America.
This is a small hymenopteran with a black color, with the dorsal part of the abdomen reddish in males and greenish in females. The female hymenopteran lays eggs inside the potato beetle eggs or feeds on the interior of the eggs itself.
Agronomic Defense Techniques
To limit the damage of potato beetle infestations, in addition to the above-mentioned biological defense methods, agronomic practices should be implemented. This mainly involves crop rotation, meaning not repeating the same crop family in the same plot of land in sequence.
Let’s take an example. If in one season, we had the presence of the insect in our potato field, we can rest assured that the problem will reoccur in the next season. Therefore, it is absolutely unwise to replant potatoes in the same place; doing so will only favor the proliferation of the insect. Moreover, be cautious because potatoes are part of the solanaceous family; hence, other crops belonging to this family should be rotated and excluded from cultivation. This applies especially to eggplants and tomatoes.
Once this is fixed, in the winter months, when adults are underground for overwintering, a good idea is to perform deep plowing of the soil. This cultivation technique is now in disuse, but it is advisable in this case.
Another agronomic measure is the use of natural mulching, which hinders the potato beetle’s ascent from the soil.
Finally, another technique is the manual removal of eggs (when they can be identified), larvae, and adult insects. This remains the simplest and most effective method of elimination.