NOTULAE ENTOMOLOGICAE - Helda

s. EKHOLM, THE PEA MOTH (LASI'EYRESIA NICRICAXA STEPII.) 1 1
Discussion
Though only one species, Laspeyresia nigricana L., has been shown to be
a pest of pea pods in Finland, there are some older records in which the species
L. dorsana is mentioned. It seems to me obvious that these old statements
are not founded on investigations but only on an erroneous belief also held,
in several other European countries in the early years of this century (comp.
LAXGENBUCH 1941).
The rather low degree of infestation of some wild leguminous plants around
the investigation area may be due to the fact that peas have been continually
cultivated for long periods. Therefore egg-laying Pea ]Moths seem not having
been obliged to lay their eggs on wild leguminous plants.
As shown above, the Pea Moth is a species that is relatively dependent on
high summer temperatures, especially during the time when the eggs are laid.
In cold years, such as 1962, development was retarded to such a degree that
only a few of the larvae were able to leave the pods before frost destroyed
plants and larvae. In wet years, which are very often cold, too in this country,
the rains beat down the plants so that the pods are in continous contact with
the soil surface, the result being comparatively early cessation of growth,
rapid decay of the pods or germination of the full-grown peas.
No absolute figures are available for the fluctuations in the numbers of
the Pea Moth. In figure 4 the number of records received from growers forms
the basis for the frequency investigation. Many factors may affect the sending
in of reports, such as changing interest, number of reporters, and difficult
external conditions for the growers, such as wars. Consequently, the results
must be interpreted with caution. Yet, when the mean temperatures for the
summers of 1911 — 1960 and those of July during the same years were
drawn in this diagram, it was evident that one of the main factors affecting
the fluctuations of the Pea Moth in Finland is high July temperature. The
fluctuations in the numbers of the Pea Moth show a fairly good correlation
with the temperature changes. Many other factors may also affect the size
of the populations of the species. According to STEI^LWAAG (1939, as cited by
ScHwERDTFEGER 1941), the biotic factors decrease in monocultures and the
importance of weather factors increases. Because the larva lives inside the pod,
where relative humidity is 100 per cent, except during the short period when
it creeps from the remains of the egg shell to the host pod, temperature seems
to affect both the egg-laying adult and the larva inside the pod to such an
extent that in cold years, such as 1962, nearly the entire population
is wiped out.
As emerges from NORDMAX'S hitherto unpublished observation of a migration
of the Pea Moth, these minute insects fly through forests in order to reach