It walks and quacks like a mallard, but does it look like one?

This is a mallard (Anas platyrhynchos). It is your basic duck, familiar from park wetlands. A mallard quack is also the classic duck sound.

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A wintering mallard flock is quite colourful: males have green heads with yellow beaks and both sexes have blue wing spots.

Age and season affect plumages

But mallards do not always look like those in the picture above. Males do not always have green heads, nor are females always brownish grey. Depending on the season, and the age and genes of an individual, mallards can look a little different. Downy ducklings resemble the ducklings of all other dabbling duck species. However, they rapidly develop species-specific characters, and young drakes for example develop a hint of green on their head even before all the down has disappeared. In the summertime males briefly change into summer (eclipse) plumage that looks like female plumage. Except that a male beak is still yellow.

Wetland ecology group_University of Helsinki_duck_mallard male_sinisorsa

A young male mallard still has down on his back, while some green is already glittering on his head. Both female and male mallards are brown during summer and autumn. The yellow beak reveals that this individual is a male. © Sari Holopainen

Beak reveals sex

In addition to normal changes in plumages caused by seasons or growth, weird looking mallards can also be found. Their plumages might be different due to changes in their genes or hormones.

Wetland ecology group_University of Helsinki_duck_mallard female_sinisorsa

Light female mallard.

Various phenotypes are rather typical among animal species. These variations are common in mallards, and peculiar individuals can be found especially in cities. For example, females might be light due to mutations. Mutations can work in several ways causing changes in pigment production or in its appearance traits. Lightly coloured mallards produce pigments, but their colour appearance has changed. If an individual does not produce melanin pigments at all, it becomes a completely white albino.

Colour variations are thought to be typical in mallards in city environments, where predator pressure is lower and thus exceptional individuals survive better. On the other hand, mallard farming has potentially produced weird-looking individuals that have escaped and spread their genes to natural populations.

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Wetland ecology group_University of Helsinki_duck_mallard_intersexual_male_female

These peculiar mallard males in wintering flocks are actually females. The pictures show intersexual females together with two normal males and a female. Moulting males changing their eclipse plumage into nuptial plumage can look similar, but their beak colour once again reveals the actual sex. These pictures were also taken in the middle of winter, when males have already changed to their nuptial plumage.

The beak has an important role in identifying mallard sexes because males have yellow beaks and females have orange-spotted beaks around the year. The beak can also reveal intersexual females. They are individuals that express both female and male outfit. This can be caused by disturbances in female hormone production, or then an individual has both female and male features. Hormones regulate the outfit, and if large quantities of testosterone are produced, male plumage may result. Beak colouration is not as sensitive to hormonal changes and even though a female displays male characteristics, it will still have a female beak.

Hybrid ducks

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This common teal x mallard hybrid male was coupled with a normal mallard female and defended it against clearly larger mallard males.

Mallard flocks may also have hybrid individuals. Duck species are close relatives, and can thus mix rather easily. Various species mixes are known, for example mallards can mix with common teals, Eurasian wigeons, northern pintails and black ducks. However, hybrids are quite rare, because each duck species have specific behaviours and characteristics that prevent hybridization. But sometimes these barriers collapse, and hybrid individuals are born. Hybrid individuals express characteristics from both original species. Their habits and characteristics typically do not interest individuals from the original species and therefore might not breed successfully.

Hybridization can cause several problems, which in the worst-case scenario can lead to the extinction of the original species. The hybridizations of mallard and black ducks in North America is becoming more common after shifts in their distribution. Hybridization is now threating black duck populations. Alien mallards can also cause a serious risk for endemic duck species and to their gene pool. For example, the Hawaiian duck (Anas wyvilliana) is unfortunately going extinct because of non-native mallards. Survival of the species now depends on protection actions that target the extirpation of all mallards and hybrids from the islands

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Four naturally different mallards wintering in southern Finland. The normal type male was coupled with a normal female. An intersexual and a light female are in the upper part of the picture.

It looks like a duck

This white domestic duck is a descendant of a mallard. © Sari Holopainen

This white domestic duck is a descendant of a mallard. © Sari Holopainen

Mallards are commonly farmed, and several different colour variations exist among the domestic breeds. A white duck known by everyone is also a mallard variant. Farmed mallards have sometimes escaped, and now breed with natural mallards. Extraordinary ducks, resembling mallards more or less, are a fairly common sight in Southern and Central European parks. Alien genes in the natural mallard population become more rare in the northern parts of Europe.

Extraordinary ducks in European parks are probably related to mallards: Switzerland, Germany and Sweden. © Sari Holopainen

Extraordinary ducks in European parks are probably related to mallards: Switzerland, Germany and Sweden. © Sari Holopainen

Read more:

Pär Söderquist: Large-Scale Releases of Native Species: the Mallard as a Predictive Model System

Pictures by Harry J. Lehto, intersexual mallards

Pictures by Pekka Sarvela, colour variations

Ducks Unlimited: Waterfowl Hybrids


David and Goliath – a story of bark beetles

Bark beetles (Scolytinae) are small beetles a few millimeters in size. Their larva develop under tree bark eating the phloem, xylem, and cambium layers. Certain species cause extensive forest damage by killing healthy trees, while others only impact weakened individuals. The eating patterns (called galleries) and the trees’ defensive reactions cause disturbances in the nutrient and water cycling within the trunks. The trees literally dry to death.

Bark beetles can be detected by the gallery patterns they leave on tree trunks. These patterns are species-specific, and often very beautiful. The patterns can be used to recognize infestations and begin warding off the worst damage. Then again, the gallery patterns cannot be seen until the tree bark falls off.

Bark beetles also have a secret weapon: wood-staining fungi. This group of fungi includes several species that damage wood or cause serious diseases to trees. Bark beetles and wood-staining fungi have developed various relationships such as the ambrosia beetles that spread certain fungi species into their galleries to farm them for food. Wood-staining fungi benefit from the bark beetles transporting them to new trees, and have developed exceptionally sticky spores that attach to adult beetles as they are preparing to disperse. Bark beetles also benefit: the fungi weaken new tree individuals, giving adult bark beetles the opportunity to infest and lay their eggs in these trees.


A possible wood-staining fungus is spreading beneath the bark of a birch infested by the birch bark beetle. ©Stella Thompson

It’s hard to believe that tiny beetles and even more minuscule fungi can kill gigantic trees. Situations where a bark beetle or fungi spreads to a new geographical region among lumber are particularly devastating. The new host trees have no immunity or defense mechanisms against this new organism and the alien species spreads like wildfire.

Dutch elm disease is a prime example of this. Ophiostoma ulmi, a fungus killing elm shoots spread from Asia initially to Europe and then, fueled by the post-World War I reconstruction boom, from Europe to North America in lumber. European elm species coped with the disease slightly better than their North American cousins. European elms also died, but the spread of the disease around Europe took several decades and finally the outbreak waned. 10–40% of the elms died, depending on the country in question. The situation was very different in North America. The American elm (Ulmus americana), a very popular urban and ornamental tree, formed large forests in the eastern areas of the continent. It narrowly escaped extinction through active eradication and education measures such as campaigns forbidding the transportation of firewood outside infected states. Unfortunately, a new, much more virulent fungus (Ophiostoma nova-ulmi) causing Dutch elm disease spread to Europe and North America during the 1940s. This fungus has caused the near annihilation of elms from several European countries. As of yet Finland has mostly been spared by the disease, but this may change with a warming climate that allows beetles belonging to the Scolytus genus that carry Dutch elm disease to overwinter in more northern regions. These beetles are already found on the northern coast of Estonia and in the Stockholm area of Sweden. The birch bark beetle (Scolytus ratzeburgi), commonly found in Finland, does not spread Dutch elm disease as it has specialized in solely utilizing birch trees.

However, the birch bark beetle spreads the Ophiostoma karelicum -fungus. Trappings


The presence of birch bark beetles can be detected by their unique eating patterns. ©Stella Thompson

conducted during 2008 and 2009 for a study carried out in Norway, Finland, and Russia revealed the prevalence of O. karelicum: every single birch bark beetle individual carried the fungus, which was also found in each of the beetle’s galleries that were searched. The life cycle and ecology of O. karelicum is very similar to the fungi spreading Dutch elm disease, and the commonness of the fungus and the birch bark beetle means a very high risk of the disease spreading to e.g. North America. The birch species native to North America would most probably have no resistance to the disease.

On the other hand, pitch canker (Gibberella circinata) is a fungus spread by bark beetles, originating in North America, which has now spread to Europe where it causes pine mortality. The Scots pine (Pinus sylvestris), native to e.g. Finland, is especially susceptible, but the disease has not spread as far north as Scandinavia yet.

To make these dynamics even more complicated, several mite species have also been shown to transport or act as the primary hosts of wood-staining fungi. These mites are in turn spread by bark beetles. The relationships and interactions between these three organisms are still poorly understood.

The disease resistance of tree species can be increased through cultivation. American elm cultivars more resistant to Dutch elm disease have been found, and their disease resistance has been further enhanced through cultivation. These cultivars are most probably the reason that elm forests still exist today in North America, although the age and size composition of these forests has changed considerably with the death of the old and large trees. Biological and chemical disease control is also a possibility: fungicides can be injected into live trees to stop the spread of specific diseases. Six fungicides combating Dutch elm disease are currently on the market in the US.

Similar control measures can most probably be developed against O. karelicum. However, widespread injection campaigns are difficult to implement. In the US, Dutch elm disease is mainly controlled by injecting individual ornamental or urban trees. Injection control as an effective eradication measure requires more development before it becomes a feasible tool for preventing damage caused by alien species.

Vanishing wigeons and fading horsetails

Over 20 years ago Finnish and Swedish duck researchers began the “Northern Project” and conducted vegetation measurements on 60 Finnish and Swedish lakes while also counting their duck populations. The study lakes were located from southern Sweden and Finland to Lapland in both countries. Researchers found that the water horsetail (Equisetum fluviatile) grew abundantly on many of the study lakes. Breeding Eurasian wigeons (Anas penelope) were also abundant according to the study.

The water horsetail prefers eutrophic lakes and wetlands. Horsetails are an ancient plant group that has existed for over 100 million years. They are thus living fossils.

Wigeons also utilize eutrophic lakes during the breeding season. Adults are vegetarians, but wigeon ducklings also consume invertebrates, a common trait in young birds.

Wigeon brood foraging within water horsetails at Lofoten. © Sari Holopainen

Wigeon brood foraging within water horsetails at Lofoten. © Sari Holopainen

The vegetation mappings and duck surveys connected to the Northern Project were repeated in 2013–2014. The researchers wished to find reasons for the deep decline in breeding wigeon numbers. They observed that wigeons had disappeared from several lakes where they were found on 20 years ago. When the habitat use of wigeon pairs was studied, the pairs were observed to particularly prefer lakes with water horsetails. In Evo, southern Finland, the feeding habitats of wigeon broods were followed over a period of 20 years. Broods were found to forage significantly more often within water horsetails than in other vegetation.

Wigeons therefore prefer lakes with water horsetail present throughout their breeding season. However, the long-term research by the Northern Project has shown that water horsetail has declined and even disappeared from many lakes in Sweden and Finland: this is a large-scale phenomenon. The wigeon is suspected to suffer due to vanishing water horsetail populations. Also, Finnish pair surveys in addition to reproduction monitoring show negative trends for the wigeon.

Health water horsetail at Lofoten © Sari Holopainen

Health water horsetail at Lofoten © Sari Holopainen

The reasons behind diminishing water horsetail numbers are not known. Impact from alien species can be suspected locally. Glyceria maxima, an alien species in Finland, appears to be growing in areas were water horsetail has traditionally grown. Grazing by the muskrat (Ondatra zibethicus) could also be a reason, but the species does not occur in southern Sweden. The whooper swan (Cygnus cygnus) could be another potential grazer, and the species’ populations have rapidly increased during the last decades. But these species can only have local effects, which do no not apply to the whole study area. Researchers cannot exclude other possible explanations, for example diseases or changes in water ecosystems. Despite water horsetail having commonly existed in boreal lakes, their influence in the water ecosystem is poorly understood. This study suggests that the water horsetail has an important role, and its disappearance will be reflected in the food web.


Read more: Pöysä, H., Elmberg, J., Gunnarsson, G., Holopainen, S., Nummi, P. & Sjöberg, K. Habitat associations and habitat change: seeking explanation for population decline in breeding wigeon Anas penelope. Hydrobiologia.  

Problems in paradise: the destruction of Hawaiian species

A few months ago I wrote a post on invasive species in Finland, and in particular on the North American beaver (Castor canadensis). I received a comment on how it is bold (or maybe the commenter meant reckless) to say that almost all invasive species are threatening the native species of the region. I began thinking of this comment, and tried to find some studies that proved that invasive species are beneficial for the subject ecosystem. Unfortunately, I only came up with sad tales. One very devastating example of invasive species is the Hawaiian Islands.

The Hawaiian Islands in the center of the Pacific Ocean are one of the most isolated islands in the world. Their endemic terrestrial species originate from some hundred species that migrated thousands of kilometers over the Pacific Ocean during several millions of years. Because of the immigration bottleneck and isolated evolution, the Hawaiian Islands have become a place for numerous distinctive and fascinating species. But it has also made the fauna and flora of the islands very vulnerable to various disturbances, such as human invasion and human-mediated invasions.

Nowadays almost a quarter of Hawaiian terrestrial species are non-native. Birds have probably suffered the most. Previously there were 11 native goose species in the Hawaiian Islands, but nowadays only one species is left: the nene (Branta sandvicensis), also known as the Hawaiian goose. The same has also happened to the native duck species; just two duck species are left (the Hawaiian duck, Anas wyvilliana and the Laysan duck, Anas laysanensis).

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The nene, also known as the Hawaiian goose (Branta sandvicensis), is the only endemic goose species left in the Hawaiian Islands. © Sari Holopainen

The main reasons for these extinctions are introduced predators (e.g. the feral cat and mongoose), and feral and game species (e.g. the mouflon, Axis deer and feral pig). There are almost 60 studies on domestic ungulates, but none have demonstrated any positive effects of them on native species. Ungulates stimulate the growth of grass among other things, leading to more grasses and less forest. And all this changes the light regime and fire resistance of an ecosystem. Grazing is therefore destructive to Hawaiian forests and to every native organism living in them. It has also been proven that the invasive vertebrate species of Hawaii have facilitated at least 33 invasive plant species. In addition to damages caused by grazing, feral pigs alter nutrient cycling and accelerate soil erosion.

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

The main problems caused by feral pigs are alteration to nutrient cycling and acceleration of soil erosion. © Sari Holopainen

There is still some light at the end of the tunnel, although it might be rather dim. The public has come to aid in the eradication of many species. Scientists and wildlife managers have concurrently begun multi-scale population monitoring, which includes aerial and ground-based visual surveys as well as trail cameras. To intensify and simplify the eradications even further, several hundred kilometers of management fences have been constructed. As an outcome of this some success stories have emerged; the eradication of rabbits and feral goats. Furthermore, the midway islands of Hawaii are now rat and non-native mammal free!

Unfortunately, it has been too late for some Hawaiian ecosystems. A key threshold has been crossed in some regions, and recovery of certain ecosystems may not be possible any longer. The populations of illegally introduced axis deer (Axis axis) have been reduced to some dozens, but their eventual eradication has been problematic, because assessing the number of remaining deers on private properties has proved difficult. The axis deer was introduced to provide game, so private properties owned by hunters act as reservoirs for the deer, from where they can be disperse to clean areas.

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

The main feral goat eradication was performed in 1980s, and nowadays the Hawaiian Islands are goat free. © Sari Holopainen

To conclude, I still dare say that almost all invasive species threaten native species. Even though some invasive species don’t harm all native species, we are always looking at nature as a complex ecosystem consisting of several species and functions. When introducing an alien species, we will always alter the pristine ecosystem.

Eradicating species – an occasional necessity

If Finland is to obey the EU strategy on Invasive Alien Species (IAS), 10 000 North American beavers (Castor Canadensis) are to come under the trigger. Why is this eradication necessary?

Although invasive alien species, e.g. the American mink (Neovison vison), the ruddy duck (Oxyura jamaicensis) and the Himalayan balsam (Impatiens glandulifera), may seem adorable and interesting novelties, they nearly always threaten the survival of native species. Invasive alien species are harmful to agriculture and forestry, and at their worst can even threaten human health. Sometimes we face the inevitable: the eradication of an animal or plant species.

The most efficient way to minimize the risks is to prevent it spreading to an area in the first place. Australia is probably the most famous example of preventing the spread of alien species, as they even clean the shoes of tourists at the airport before allowing them into the country. Unfortunately no nation has been successful in averting the spread of IAS.

Not that a tough fight isn’t being fought the new EU strategy on invasive alien species took effect earlier this year.

The main aim of the strategy is to aggregate a list of the most pernicious invasive alien species and to repulse them in different ways. Finland has 160 harmful IAS. Whether each of them will be on the EU’s black list remains unclear.

The North American beaver is first in line

The North American beaver is one of the most potential mammal alternatives for the list. The species was brought to Finland in the 1930s to save Finland’s beaver population. Back then the genetic differences between European and North American beavers were unknown, although we now know that the species differ even more than humans and chimpanzees.

The niches of both species are identical. They eat the same nutrition and their damming activities are the same. Both species have identical effects on ecosystems.

There are approximately 12 000 beavers in Finland. Most of them (10 000) are North American beavers, while European beavers comprise only a fifth of the North American beaver population. The North American beaver threatens the existence of the European beaver in Finland. It might eventually competitively exclude the European beaver. Adhering to the precautionary principle and seriously considering eradicating the North American beaver from Finland and Eurasia is essential. An eradication plan has nevertheless been conspicuously absent. An eradication plan for North American beavers would abide to the guidelines of both the IUCN’s and Finland’s National Strategy on Invasive Alien Species.

How to eradicate a species in practice?

A large scale eradication of the North American beaver is possible, at least in theory. Several possible methods could be used simultaneously, such as hunting, live capture, sterilization, reintroduction of the European beaver and population monitoring.

Beaver hunting is also financially tempting. Beaver furs have once again become popular in China, so their markets have a demand for beaver furs. After the sterilization or dead trapping of North American beavers, they should be replaced with European beavers.

But this is not a straightforward process. Although the two species differ genetically, they have a similar effect on the ecosystem. Beavers act as ecosystem engineers and benefit several other species in Finland and elsewhere. The present population size of the North American beaver ameliorates e.g. the green sandpiper (Tringa ochropus), the moor frog (Rana arvalis) and the Daubenton’s bat (Myotis daubentonii). However, if all North American beaver individuals were removed and replaced by European beavers, the eradication would be harmless to Finnish nature. Unfortunately, there is nothing to guarantee the success of the reintroductions.

Finland must begin eradication if the North American beaver is placed on the EU strategy plan on Invasive Alien Species. The activity of citizens and hunters will determine the eradication outcome. The Ministry of Agriculture and Forestry in concert with the Finnish Advisory Board for Invasive Alien Species are in charge of the decisions and eradication procedures.

Genetic pollution – for the benefit or harm of species?

All living organisms carry genes, or hereditary information that they pass onto the next generation. Gene flows occur when this hereditary information is transferred from one population to another. This in itself is a normal spreading of genetic material between populations of the same species, and upholds genetic diversity. The more genetically diverse a population, the healthier and resistant to disease or pests they are.

However, there are situations when this gene flow occurs between species or in non-natural situations. This has been dubbed genetic mixing, or more negatively genetic contamination or pollution. Genetic mixing is not automatically a bad thing. For example, closely related subspecies can sometimes interbreed, producing hybrids. Hybridization diversifies genetic material and hybrids may grow faster or larger, or be less susceptible to disease. Problems arise when species with no previous contact with each other suddenly meet, or when genetically engineered (GE) organisms are able to spread their genetic material unchecked into wild populations. Both are examples of genetic pollution, which can cause serious harm to individual species or entire ecosystems.

Hybridization can be harmful for species with low population numbers. This is particularly the case with invasive native and non-native species, and ferile and domesticated subspecies that slowly smother and degrade the native species. Examples include the highly common mallard (Anas platyrhynchos) interbreeding with several other duck species in Europe and North America and the very endangered spotted owl (Strix occidentalis) native to western United States and Mexico hybridizing with the more common barred owl (Strix varia) that is spreading from the east. In both cases the rare species are becoming even rarer, and many believe that this form of genetic pollution should be prevented. However, these are just cases of normal evolution, and it is difficult to draw a line on what is true conservation and what meddling. If an invasive species begins aggressively dominating native species and hybridizing with them, our first thought is how to stop its spread.

Unidentified duck expressing traits of several species. © Sari Holopainen

Unidentified duck expressing traits of several species. © Sari Holopainen

A more concerning form of genetic pollution occurs when GE organisms exhibit gene flow to native wild populations, e.g. when the pollen of certain GE crops end up dispersing into nearby wild populations of the same crop. This spreading can quickly become completely uncontrolled due to wind or insect pollination, and its effects remain unknown in native populations. The damage caused by GE organisms spreading can involve ecosystems in unprecedented ways, e.g. GE corn has been proved to significantly increase the mortality of monarch butterfly (Danaus plexippus) larvae. This in turns disrupts local bird populations because of lessened food resources.

The key difference between hybrids and GE organisms lies in their ability to occur in nature. The hybridization of two subspecies with one and other is perfectly natural, and happens all the time. Organisms spread into new areas, coming into contact with closely related species. Often times this is purely natural spreading. GE organisms on the other hand are completely man-made, and have no precedence in nature.

GE organisms may have less viable offspring, but this is not enough to curb their spreading in the wild if released uncontrolled. A US study modeled a freshwater lake into which a small number of GE fish were released, and concluded that the GE fish would lead the healthy wild fish population to extinction within 40 years, despite the original wild population being thirty times larger than the GE population. This is naturally only one example of a modeled situation, but the reasons behind the overthrow of wild fish (larger size of GE fish leading to increased mating possibilities) occur commonly among other GE and hybrid populations and can therefore occur frequently in similar situations as well.

Our action plans for dealing with arising problems should be case-specific. Often times the populations suffering from hybridization are already experiencing some other form of stress, e.g. freshwater pollution or the disappearance of old-growth forests. These in turn make these species susceptible to incoming invasives or spreading native species. The original populations may just be too stressed to cope with an additional problem, and so conservations schemes may be called for with certain species and uncalled for with others.

GE organisms however are a different story. We have no clear data on how genetic engineering affects species, how long GE populations can remain in the environment, how easily they transfer the added genetic material to wild populations etc. These are large unknowns when considering their effect on ecosystems and sustainable agriculture, both of which we humans are dependent on.

Domestic goose and greylag goose © Sari Holopainen

Domestic goose and greylag goose © Sari Holopainen

Natural enemies – assistants or catastrophes

Guest author: Samuli Karppinen

The aim of biological control is to use natural enemies for suppressing pest populations. Pest populations include both native and alien species. The natural enemies can be parasitoids, predators, pathogens, antagonists or competitor populations. These natural enemies are called biocontrol agents or briefly agents.

The main aim of biological control is not easy to determine in every case. In agriculture, the aim is to use agents that are beneficial and effective in reducing the pest and which function within the environmental, legal and economic constraints. The use of biological control doesn’t necessarily mean the eradication of the pest populations. Rather, it is a balance between the pests and their natural enemies that prevents the pest populations from expanding to harmful or economically injurious levels. Biocontrol agents can be divided into four classes: classical control, augmentative control, conservation and neoclassical biological control.

The idea of classical biological control is to establish a living organism within an area where it has not occurred before. The living organism can be a natural enemy or competitor of the pest species. The aim of the organism in its new area is to provide pest long-term control. In many cases the target pests are not native species.

The aim of augmentative control or inundative control is to reduce pest population by using sufficient numbers of biological control organisms. In augmentative control, the introduction of organisms will normally need to be repeated.

In conservation biological control, control agents already exist in the target area. The idea of the conservation is to attempt to conserve or enrich the agents. The function of neoclassical biological control is to use exotic non-indigenous agents. The target pests are indigenous. Since they were not in the same area before, this is referred to as a new association.

The ribbed pine borer (Rhagium inquisitor) is a forest pest. © Mia Vehkaoja

The ribbed pine borer (Rhagium inquisitor) is a forest pest. © Mia Vehkaoja

Sterilizing insects is a method of biological control. In this technique huge numbers of sterile, generally male insects, are released into the target area. The idea is that sterile males compete with the wild males for female insects. As a result there will be no offspring and pest populations begin reducing.

Selected natural enemies were previously preferred to be generalists, because non-target species provided alternative food when the target pest was absent. However, the likelihood of attacking non-target hosts is bigger with generalist or polyphagous agents. Nowadays the biological control agent species that are released should be highly specialist predators or relatively host-specific parasitoids. Agent species should control target populations effectively, but they should not affect non-target species. The characterization of suitable biological control agents is difficult because e.g. predator-prey –interactions are very complicated.