Frequently Asked Questions about the Colorado Beetle

1. What is a Colorado beetle and where does it live?

The Colorado beetle  (Leptinotarsa decemlineata) is a beetle with a convex back. It belongs to the family of leaf beetles (Chrysomelidae). Originally from North America, the beetle first spread to France in the 1920s. Since then, it has continued to spread eastwards and northwards, and now occurs throughout Europe. It became a persistent pest in the southern and southwestern parts of Lake Ladoga in the late 1970s. Scandinavia, Great Britain and Ireland are still free of the beetle.

2. On which plants does the Colorado beetle live?

The Colorado beetle feeds on solanaceous plants (Solanaceae). The most important cultivated plant is the potato, other plants including the tomato and eggplant. The beetle may also live on uncultivated plants such as black nightshade and bitter nightshade.

3. How does the Colorado beetle damage potatoes – does it also feed on potato tubers?

Fully grown beetles and their larvae begin by eating holes in potato leaves. The actual damage is caused by the larvae. If present in large numbers, they almost completely consume the leaves, with only the leaf stalks remaining. The potato haulms on which larvae have fed form colonies, which are easily distinguished from other potato plants. Both mature beetles and larvae also badly stain the potato foliage with their dark excrement. The Colorado beetle does not feed on potato tubers.

4. Potato leaves have been eaten. Is this a definite sign of Colorado beetles?

No, it is not. Other leaf-eating insects live in potato fields. However, the damage they cause is different and on a smaller scale than that caused by the Colorado beetle. The most common such insect is the beet carrion beetle ( Aclypea opaca), which measures approximately 1 centimetre and whose shiny black larvae resemble isopods. Both fully grown beetles and the larvae occasionally feed on potato leaves, leaving behind tatters and lace-like damage, but they never eat entire leaves. Neither do they leave excrement where they feed.

5. Why is the Colorado beetle such a harmful pest?

If the field is home to a large number of beetles, they quickly eat potato haulms, to such an extent that the potato crop is significantly reduced. A few beetles carried by the wind will not cause such damage, but if the beetles are not eradicated and are allowed to hibernate over the winter, reproducing during the following summer, they will begin to have an impact.

The Colorado beetle quickly grows resistant to plant protection products, making it more difficult to eradicate. Beetles carried in the wind to Finland may have already become resistant in their country of origin. Only a few effective plant protection products exist, making it difficult to entirely eliminate beetle populations.

6. How to recognise a Colorado beetle?

An adult Colorado beetle is about one centimetre long. It has 10 black longitudinal stripes on its yellow back and its pronotum is orange with black spots. The beetle’s 1.5-millimetre-long yellowy-orange eggs are found on the underside of leaves, in tight clusters.

A newly hatched larva is approximately 1-2 millimetres long and darkish in colour. Larvae grow rapidly, their convex backs soon turning a reddish orange colour. Each larva has a black head and two rows of black spots along its sides. A full-grown larva is just over a centimetre long.

Colorado beetle pupae burrow into the soil beneath the potato plant. They are oval, orange, immobile and measure slightly over one centimetre.

7. I have spotted red-yellow insects with a convex back and black spots. Are these Colorado beetle larvae?

No. These are lady beetle pupae. They are reddish yellow in colour and have black spots all over their bodies. Colorado beetle larvae only have black spots along their sides. Lady beetle pupae are immobile and attached to the leaf. The lady beetle and its larvae are beneficial insects. They do not feed on potato leaves but on the aphids which live off the leaves.

8. How do Colorado beetles develop after the wind has carried them to Finland?

If the wind carries a fully developed female beetle to Finland, the insect will probably have already mated in its country of origin, or will go on to do so in Finland. The female begins to lay eggs immediately after mating, in batches of around 20 on the underside of potato leaves. Each female can lay up to 200–500 eggs. Depending on the temperature, the eggs hatch into larvae in a week or week and a half. Because the female only eats what it needs to survive, it only causes minor damage. At first, the larvae remain in batches, growing rapidly. They are true gluttons. Since there is a large number on each haulm, they cause serious damage to potato leaves, to the point where only the stalks remain.

Depending on the temperature, the larvae become fully developed in approximately two to four weeks, after which they burrow into the soil to pupate near the surface. The pupa stage takes two to three weeks, after which the adult emerges from the ground to live on the potato. High temperatures and heat can speed up their development phases considerably.

The new adult beetle begins to prepare for the winter by feeding. Relatively early in the autumn, it burrows deep into the ground to spend the winter in diapause. In the following summer, when the temperature of the ground is sufficiently high and the potatoes are already growing, the beetle digs its way to the surface, eats until it has recovered from the winter and then begins to mate. The same cycle continues.

9. How might the Colorado beetle spread to Finland?

The most effective and the fastest way for Colorado beetle to spread is through strong southeasterly winds from Russia. The Colorado beetle also lives in the Baltic countries, but not on the coastline of the Gulf of Finland. Strong winds could carry the beetle to Finland from the South, but this has not happened so far. Storm fronts usually move east from the Baltics, then turn northeast and move round the Gulf of Finland through Russia, finally arriving in Finland.

It is also possible for Colorado beetles to occasionally arrive in Finland as stowaways by truck and train traffic from Russia.

10. When should I look for Colorado beetles in my potato field?

In Russia, Colorado beetles usually rise to the surface when their food source, potatoes, are already growing. A short while afterwards, they instinctively climb to the top of the potato plant and will migrate elsewhere if the population becomes large. If there is a suitable storm wind blowing towards Finland, the risk of their spreading is at its worst. Carried by a strong wind, the beetles may travel up to hundreds of kilometres. This usually occurs after Midsummer, in early July. You should start checking for beetles at Midsummer, especially if the wind is blowing from the Southeast. We recommend that you check your fields once a week.

11. How can I eradicate the Colorado beetle if I suspect it is living in my field?

You should not try to eradicate the Colorado beetle on your own. In Finland, the Colorado beetle is a quarantine pest and plant protection authorities therefore decide case-specifically on the required measures. On small private fields, they also take care of eradication in practice. On larger farms, the authorities eradicate beetle colonies and instruct the farmer on other measures, such as treating plants with a plant-protective agent. This ensures that eradication is effective throughout the country.

12. What should I do if I suspect that there are Colorado beetles in my field?

If you think you have found Colorado beetles, immediately take a sample, which can be used later to verify your sighting. If possible, use a sealable glass jar or a hard plastic container with a tight lid. We recommend that you make small holes on the lid to allow air circulation. The beetle can eat its way out of a plastic bag. Place some potato leaves in the container, to feed the beetle and keep it alive longer.

If you can use a cell phone or digital camera to take pictures of the insect to send to Evira, your sighting can be verified sooner. You will be given the correct address when you call our service number to report your sighting.

If you sight beetles on a large field, mark the spot e.g. with a stick, in order to make it easier to find later.

Following this, immediately contact Evira's service number to report your sightings. The person who takes your call will give you instructions on what to do next.