By Brian Finch
We do not have the abundance of species or individuals every year, far from it. There are several factors that lead to a “BUTTERFLY YEAR.” They include plentiful supply of the often species-specific food plants, which are entirely dependent on the rains to bring on this abundant sustenance. There is also a direct effect from the predators, but the numerous parasitic species that attack the caterpillars are a major culling force. These parasitic wasps that lay their eggs inside the caterpillars, hatch and consume the unfortunate victim’s interior, then break out through the body to pupate, leading to the next generation of parasitic wasps to start the process all over again.
After good seasons with numerous caterpillars for prey, wasp numbers increase, and correspondingly fewer caterpillars reach maturity. This leads to fewer adult butterflies breeding to produce more caterpillars to complete their own cycle. As butterfly populations crash, there’s less for the parasites to infest. This results in them also suffering a drastic reduction in numbers.
A greater percentage of the now fewer butterfly caterpillars are not affected by the previously numerous parasites, and over several generations of small populations, they multiply whilst the wasps are going through a low ebb in their own numbers.
The whole cycle is quite protracted and covers several years; butterfly numbers recover before the wasps and with several good seasons of ideal conditions for breeding soon greatly outnumber them, as one female butterfly produces more eggs that, without the parasites, are raised to maturity and start the next generation with an exponential increase. Eventually the parasitic wasps catch up and their numbers increase to affect the butterfly numbers, bringing on a rapid decline in their populations and the complex cycle starts over again in this rather macabre see-sawing of numbers. This is a rather simplistic explanation and there are other external factors that affect this.
This year following good rains, abundant food availability immediately after successive years of low numbers of both butterflies and wasps is now manifesting itself, and could last for several seasons before the wasp numbers catch up again and do their damage.
Local abundances cause sudden mass movements and one such movement on 11th February of Brown-veined White Belenois aurota was most impressive. I am sure we had thousands per minute going through the garden. They were all moving directly west towards the Great Rift, intent on searching out the Caper foodplants (Capparis spp). Three weeks ago there was a major movement of Yellow Pansy Junonia hierta flying north-east.
Additionally over 150 other species are frequenting the garden, which is an impressive figure anywhere for such a small area. Many of these make irregular appearances or appear in small numbers following the wet seasons unless it is too dry a period, others only ever seem to appear in these “butterfly years,” and on top of all this, there are the unexpected wanderers from the dry country, the coast and the west, that would not normally be expected in this area.
Many butterflies are transient, some probably include the paddock as part of a wide range that may be trap-lining (a route followed on consecutive days). But this is all hypothetical, and I am not aware of any study that has ever considered the subject, but it would not surprise me if there has been. Hummingbirds are the best examples of trap-liners, and several studies have demonstrated this. But what explains the regular appearances of what looks like the same Noble Swallowtail for example, that appears daily in the same area but continues its passage onwards and only ever staying for a very short while. Other species take up a territory.
Finally there are species that may spend some time outside of the paddock, but in the late afternoon they return to a communal roost in a patch of longer grass which is used nightly. Most interestingly this roost is multi-species not only at generic levels but also familial. The majority consists of species of the family commonly called the Whites (Pieridae), especially the genus Colotis (Orange-Tips and allies), but also other Pierids, then in different families, Commodores (Nymphalids), Blues (Lycaenids), Browns (Satyrids), and Citrus Swallowtail Papilio demodocus (Papilionids). What the advantage of this multi-species roosting remains uncertain, but the same roost has been used for several seasons. Again I don’t know of any studies made on this phenomenon.
So with all of these strange habits that butterflies demonstrate, anyone interested now has the right year in which to document the behaviour, and with the recent unseasonal deluges more and more butterflies can be expected.
It may be just a small paddock, but the concentrated effort into recording its wildlife has led to so many questions being asked, and it really is a case of “so much to learn and so little time,” especially with the Standard Gauge Railway baying at the heels!