Mesopsocus unipunctatus: an Intriguing Barklouse

I've maintained before that barklice or Psocoptera/Psocodea are the cutest of all insects, an opinion that I still stand by. Nevertheless, their small size and inoffensive habits mean that they don't get the attention that they deserve.

Female Mesopsocus unipunctatus, copyright Tom Murray.

Mesopsocus unipunctatus is a widespread barklouse species in Europe and North America (and possibly in Asia as well where a lack of records may reflect a lack of people looking). It is a relatively large species as barklice go, growing up to about half a centimetre in length. Mature males are fully winged but females have the wings reduced to rudiments and are flightless. Mesopsocus unipunctatus are found living on the bark of trees, primarily on branches rather than on the trunk, and their diet is predominantly made up of the micro-alga Pleurococcus and fungal spores. They are active in early summer: populations in Yorkshire had the first nymphs hatching during April and numbers of individuals reached a peak in late June to early July. The population survived over winter as eggs, laid in clusters of five to eight and covered with a protective layer of hard faecal matter (Broadhead & Wapshere 1966).

Mesopsocus unipunctatus shares much of its range with a closely related species, M. immunis, and the two are often found in association (Broadhead & Wapshere 1966). Differences between the two are slight: M. immunis tends to be paler in coloration but the two species are best distinguished by features of their terminalia. They both feed on the same diet and are active around the same time of year (conversely, other ecologically similar barklice species found in Yorkshire by Broadhead & Wapshere, 1966, were active later in the summer). So how do the two manage to persist without one excluding the other? As it turns out, they differ in oviposition behaviour. Mesopsocus unipunctatus prefers to lay its eggs right at the tips of tree branches whereas M. immunis mostly lays about 25 to 50 cm back from the tip. Mesopsocus immunis also covers its egg masses with a layer of silk in addition to the layer of faecal matter used by both species. These behaviours mean that M. immunis egg masses are better protected from one of their major threats, a mymarid wasp that parasitises them. However, M. unipunctatus compensates for its higher vulnerability to parasitoids through a greater resistance to cold, meaning that a higher proportion of its unparasitised eggs survive the winter. The greater cold resistance of M. unipunctatus means that it may also be found at altitudes and latitudes beyond the range of M. immunis.

Male Mesopsocus unipunctatus, copyright Ken Schneider.

Another feature of M. unipunctatus worth mentioning is that it shows variation in coloration attributed to industrial melanism. This phenomenon is better known in Lepidoptera: you may have heard of one of the most famous animals supposed to exhibit it, the peppered moth Biston betularia. Individuals of M. unipunctatus in England vary in the degree of dark markings on the abdomen, from some that are almost entirely dark through those with a mottled pattern of dark patches and stripes to some in which the dark markings are restricted to the primary transverse stripe on the fourth abdominal segment. The head and thorax are also darker in some individuals than others though it is notable that not all individuals with darkened abdomens also have darkened heads and thoraces (Popescu et al. 1978). Industrial melanism is so-called because this variation in colour pattern is supposed to be related to industrial pollution. It is supposed that the original paler, broken coloration provided camouflage on lichen-covered bark but selection came to favour darker color patterns as trees became blackened with soot. Studies on melanism in M. unipunctatus did indeed find a correlation between the number of dark individuals in a population and the degree of pollution in the environment (Popescu 1979). However, aviary studies of predation rates on M. unipunctatus individuals released into simulated habitats were a bit more equivocable: survival rates of light-coloured individuals were better among branches taken from rural locations but neither morph was definitely favoured among branches from urban environments. Also, darker individuals exhibited faster growth rates in polluted environments than lighter individuals, perhaps due to better absorption of heat despite sunlight being blocked by smog. Are there more dark-coloured individuals in industrial locations because they die less, or because they live more? Another question I don't know the answer to: has M. unipunctatus also reflected Biston betularia in seeing a drop in melanistic individuals with the reduction of smog levels in England in recent decades?

REFERENCES

Broadhead, E., & A. J. Wapshere. 1966. Mesopsocus population on larch in England—the distribution and dynamics of two closely-related coexisting species of Psocoptera sharing the same food resource. Ecological Monographs 36 (4): 327–388.

Popescu, C. 1979. Natural selection in the industrial melanic psocid Mesopsocus unipunctatus (Müll.) (Insecta: Psocoptera) in northern England. Heredity 42 (2): 133–142.

Popescu, C., E. Broadhead & B. Shorrocks. 1978. Industrial melanism in Mesopsocus unipunctatus (Müll.) (Psocoptera) in northern England. Ecological Entomology 3: 209–219.