Seasonal Anomaly in Monthly Moth Totals.
It is clear from the monthly trapping totals over
several years that overall catches do not increase steady as the year
progresses to a summer peak. Following the rise from February to March there is
always either a plateau or more often an actual fall in the catch in April. The
May, June and July totals then rise steadily.
This anomaly was investigated by Derrick Kilsby for
the 2002 report and shown to be statistically valid. He then offered a possible
explanation for the effect, see 2002 report pages 10-11.
“I was intrigued by the article on pages 10 and 11
of the annual report for 2002. I have used the moth-trap records on pages 19 to
28 of the 2002 Report to offer an alternative reasoning for the shape of the curve for the monthly catch totals.
I have slightly simplified the data, using only
those species where the number trapped was at least 10 moths in one trap for at
least one month of the year. This uses the most significant data, while greatly
reducing the input work. The records total 17,125 moths for the year, while my
version uses 15,978 moths, or about 93% of the annual total.
The moth population breaks down into a number of
groupings, based upon life-history differences. I attach the data used, and a
chart showing the monthly totals for each group.
The Spring moths are the group that
have a peak flight-time of March and April, the commonest being Hebrew
Character and Common Quaker. The Over-wintering moths are those
that appear in the autumn and are found through to March or April. Only the
Chestnut and Satellite occur in my chart. The Autumn/Winter moths
appear from September, and are mostly finished by the end of the year.
This leaves
the main group, the moths occurring during the Summer months. The
numbers involved far outweigh all the other groups, giving monthly totals in
the thousands, rather than hundreds. I have separated the moths which appear in
the data to be Bivolent, having two peaks of appearance, generally early
and late summer.
For a few species, my placing in a given group is a
bit arbitrary – Mottled Umber could be Autumn/Winter or Over-wintering. This
does not affect the monthly totals in my chart.
Because of the much greater total in the summer
months, I have plotted the Y-axis to a logarithmic scale.
From January to March the moth numbers increase,
made up of Spring and Over-wintering moths. Both
groups are at or past their peak numbers by April, depending on the year. This
is why the total moth numbers are static (or fall) in April compared with
March.
In May the Over-wintering and Spring
moths are almost finished, and the Bivolent and Summer moths
are only just appearing. Total numbers for May are therefore only slightly up
on the April figure.
From June to August
the large number of Summer moths peak, and the Bivolent
species peak in May.June and again in August/September.
As the Summer and Bivolent
numbers decline in October, the Autumn/Winter and Overwintering
species provide a small counter-balance to the decline in overall numbers
The above timings of emergence and decline of
species in these different groups gives the characteristic pattern of monthly
totals, which are basically the same every year. The large summering numbers,
peaking usually in July, show a smooth curve. This is bounded by the smaller
peaks being the species which emerge in Spring and Autumn. I don’t have all the
data for earlier years, but I am sure that a patter similar to that for 2002
would be shown to be normal.
The first chart on page 10 of the 2002 report
appears to show for 1999 a strong Spring emergence, followed by its decline
before the Summer moths appear. The spring numbers are less in 2001 and 2002.
The latter shows the chart that I have attached, although to a different
vertical scale.”
Data from
Note Logarithmic Scale.
Poplar
Hawk Moth laothoe populi
Courtesy of English Nature