Individual finds across Estonia last year show that yet another alien species is settling down in the country. This species is the is the Asian ladybeetle or the harlequin, whose various negative features include the unpleasant habit of invading living quarters in large numbers in cold weather.
Toomas Esperk, Co-Professor at the Institute of Ecology and Earth Sciences of the University of Tartu, introduced the alien species, announcing that the Asian ladybeetle (Harmonia axyridis) is probably the best known and most studied ladybug; a telling statement considering how noticeable and well-known the ladybugs are.
On the other hand, in a curious manner it is one of the most difficult ladybugs to recognize – the pattern of the cover wings of mature beetles can vary from yellow-red-orange-brown to black, and the spots characteristic of ladybugs can be both black (if the main color of the wings is not black) or red (the main color of the wings is black).
The number of spots also varies greatly – they may be completely absent or there may be a few, a medium amount or many (up to 22). However, despite the variability, recognizing this species is by no means a hopeless task – this species is usually larger than our other ladybugs, and especially the most similar two-spotted ladybug (usually 7-8 mm in length compared with 4-5 mm for two-spotted ladybugs).
The Asian ladybeetle often has an unusual number of dots for ladybugs (i.e., not two, four, five, seven, or ten, but some other number) and the dots are often asymmetrical (with a «wavy» edge and in slightly different locations on the right and left cover wings). The legs of this species are reddish-brown (black on most of our other ladybugs) and the pronotum is light with a variable (often M-shaped) black pattern.
«If there are several Asian harlequins together (for example in winter), it is especially easy to identify them, because then one can observe together ladybugs largely varying patterns and color range,» Esperk said.
As the name suggests, the original habitat of this ladybug is in Asia, specifically in East Asia. Today, however, the species is widely spread throughout the world, both with the help of humans and on its own, so that it has become a symbol of an invasive alien species.
The Asian ladybeetle arrived in Western Europe – France – in the late 1990s, in Eastern Europe and the southern Baltic states in the late 2000s, and in Estonia last year, according to our current knowledge.
It might be of interest that during the same year the species reached Pärnu, Tartu, Jõgeva and Tallinn, so that in a short time it occupied a large part of the entire mainland Estonia.
«We need not actually fear this creature,» Esperk noted. Although ladybirds are typically poisonous, so that they have few natural enemies and can therefore breed in large numbers, their favorite food are the Homoptera insects (aphids, cochineals and psyllids), and the harlequins can harm humans only if swallowed.
This raises the question of whether this species is actually useful (unlike the Spanish slug or the Krynickillus melanocephalus slug) for controlling garden pests.
«The answer is «yes, with reservations» – the species could be really useful for people and it has been introduced to many places (for example, North America) for biological pest control purposes,» Esperk explained.
The Asian ladybeetle has been described in the April issue of Eesti Loodus magazine. The authors of the article, zoologist Merike Linnamägi and Triin Lepik, postgraduate at the University of Tartu, wrote that since the harlequin is indeed a diligent predator both as a larva and an adult, it has been long considered useful for biological pest control outside its original habitat. The species was first introduced to the United States in the early 20th century, and later to Latin America and Africa.
In Europe, the Asian ladybeetle was sold in gardening shops in the 1990s as a non-chemical and organic repellent of aphids and cochineals. But the species began to spread rapidly from the gardens and greenhouses where people had released them. The threat was noticed and around 2004 sales were discontinued across Europe. However, it was too late them and the rapid expansion of the Asian ladybeetle could not be prevented; besides, there were more and more reports of its damage to nature and gardens.
Esperk noted that the results of several studies indicate a rapid decline in the number of local ladybugs (e.g., the two-spotted ladybug, the seven-spotted ladybug, the 14-spotted ladybug) after the arrival of this stronger competitor species, while in case of hunger (due to high population density) the harlequins can atypically also eat plants, including cultivated plants and especially grapes. «And in the autumn they can invade buildings in large numbers to spend the winter there and thus cause inconvenience to the owners,» Esperk mentioned.
The article in Eesti Loodus noted that the Asian ladybeetles aggregate in residential or other buildings in large numbers, sometimes in their thousands. But this pattern of behavior allows them to survive winter in areas where they would otherwise die from the cold. In addition, the article notes that when ladybugs are disturbed, they emit hemolymph, which not only smells bad but also leaves smears, often damaging carpets, curtains, wallpaper and furniture. Ladybugs can also bite painfully and cause allergies. According to some studies, they are becoming a major seasonal allergen in the United States, causing chronic cough, eye inflammation and even asthma. Wintering specimens at home can also get into food and ruin its taste.