Guest blog post by Jean-Claude Grégoire
The bark beetle Dendroctonus micans Kug. (Coleoptera, Curculionidae, Scolytinae) attacks spruce (Picea spp.) and occasionally pine (Pinus spp.). It has spread westward from eastern Siberia, and reached the limits of the natural and planted range of spruce in Europe by the beginning of the 21st century, causing substantial damage along the way.
Distribution range of Dendroctonus micans (EPPO), and an adult beetle (photo: Gilles San Martin)
Its natural enemy, a specialised predatory beetle, Rhizophagus grandis has partly followed it west and keeps the pest in check wherever it has become established. However, landscape fragmentation and geographic barriers have slowed or prevented the expansion of R. grandis into areas where the pest had been introduced via timber transport.
To deal with this problem, biological control was implemented through mass rearing and releases of R. grandis, beginning in Georgia (1963), France (1978), Great Britain (1983) and Türkiye (1985). This method worked and led to lasting control.
Why is this case unique?
This case results from the interaction of two uncommon features: the bark beetle is parasitic and the predator is highly specific.
Parasitic bark beetles
Development of Dendroctonus micans. Egg gallery with adult female (A1); eggs (A2); young larvae (A3); larvae feeding gregariously in the phloem (B); pupae in individual niches dispersed in the frass (C). Photos: J.-C. Grégoire
Contrasting with the numerous saprophagous bark beetle species (colonising dead trees) and tree-killing species (mass-attacking and killing living trees), the parasitic bark beetles, represented by only a few species (D. micans, D. punctatus, D. murrayanae), do not need to kill their living hosts because they are tolerant to the tree’s chemical and physical defences.
One or a few solitary females, each fertilised by a brother in their natal nest, settle alone on a living tree. Their gregarious larvae feed on the living phloem (the inner bark), probably to build a fast feeding front that outruns the tree’s defensive reactions. Because these parasitic bark beetles do not attack en masse and are not regularly associated with tree-killing fungi, the tree stays alive. This allows for a slow life cycle of up to three years, and several generations can succeed one another on the same tree. Because attacks are solitary, competition between broods is rare or absent.
Furthermore, the hostile environment of a fully defended tree also protects parasitic bark beetles against competition from other phloeophages (organisms that feed on the pholem) and against most bark-beetle natural enemies.
So, the causal chain [sib-mating + resistance to tree defences => no mass attack + no pathogenic fungi => tree survives => long life cycle] characterises the parasitic bark beetles.
A highly specific predator
Only one specific predator, Rhizophagus grandis, has adapted to D. micans‘ peculiar ecology.
It tracks its prey very efficiently, by following chemical cues (oxygenated monoterpenes emitted by the larval frass), even where prey is scarce. The female R. grandis, lays her eggs right inside the prey’s brood chambers, and both adults and larvae feed on every immature stage of the prey.
Like D. micans and unlike more generalist natural enemies of bark beetles, it is highly resistant to the chemical defences of the host trees. The protracted life cycle of D. micans makes it vulnerable to discovery by R. grandis over an extended time window, so that the proportion of prey broods discovered and colonised increases over time, reaching 90% even in Belgian forest stands that have low densities of D. micans. Once in a brood chamber, the predator finds the clustered prey larvae an easy meal.
Highly specific predatory insects are uncommon
Such narrow prey specificity is rare, found in only a few ladybirds (coccinellids), lacewings (chrysopids) and tooth-necked fungus beetles (derodontids), in addition to R. grandis. These species display one common trait: the adults generally secure resources for their larvae, behaving very much like parasitoids (insects whose young develop on a host and eventually kill it), by depositing their eggs inside the prey’s concealed, defended microhabitat.
Mass-producing natural enemies: a constant challenge
In theory, a specialised predator should need only a small release: a few individuals would spread through the forest and find their prey on their own (also known as inoculative biocontrol).
In practice, however, large numbers of individuals must be released in many sites and, when possible, several times in each site, to compensate for forest fragmentation, unpredictable weather and possible bad timing relative to the prey (for example, when the prey are pupae dispersed in the frass, or pre-emergent adults, which are harder to attack).
In addition, some rearing batches can be contaminated by insect-killing fungi, such as Beauveria bassiana, or show unexplained prolonged dormant phase before pupation or an inability to fly.
In total, 4,000,000 R. grandis were produced and released in Georgia, 800,000 in France, 250,000 in Britain and 10,880,000 in Türkiye.
Predator mass production relies either on D. micans-infested spruce logs in all four countries or, later, on a semi-artificial technique in plastic boxes in France and Britain.
Oviposition in polystyrene boxes. Prey larvae are enclosed with pieces of fresh phloem and rehydrated phloem powder. Photos: J.-C. Grégoire 
Larval rearing and pupation boxes. Photos: J.-C. Grégoire. A. Three feeding boxes in a pupation container; B. Rhizophagus grandis larvae feeding on artificial medium; C. R. grandis prepupae in the sand in a pupation container; D. The feeding and pupation containers can be easily stored and inspected.
How successful were the releases?
In all four countries, after six to eight years, the releases resulted in a significant drop of the pest populations, as measured by the numbers of infested trees in the treated stands, by the proportions of brood chambers colonised by the predators and by the predator/prey ratios in the brood chambers. Dendroctonus micans did not totally disappear from the treated stands but remained at very low densities, with more than 90% of the broods colonised by R. grandis as observed in France. Temporary pest upsurges are occasionally observed, after a dry year or on severely stressed trees.
Why has biocontrol largely failed against tree-killing bark beetles?
In a global review, Goundar et al. (2026) highlight the partial or total failure of biocontrol attempts against other bark beetle species. This may be related to the much more eruptive population dynamics of these pests, which are very much influenced by bottom-up forces such as host stress linked to site and climate. In addition, most species targeted so far complete one or more generations per year, leaving natural enemies little time to find and exploit their broods.
The take-home message here may be that a careful preliminary assessment of the nature of a target pest and of its known or likely relationship to a candidate biocontrol agent could spare time and effort.
Original source:
Grégoire J-C, Alkan Akıncı H, Burjanadze M, Evans HF, Fielding N, Mayer F (2026) Successful bark-beetle biocontrol. A review of the only case in history Releasing Rhizophagus grandis in Georgia, France, Britain and Türkiye against Dendroctonus micans. NeoBiota 109: 271-313. https://doi.org/10.3897/neobiota.109.205672