Dave Rushworth Insect life has been more prolific this year in reaction to the relatively good rainfall. In our area, the large 'emperor moths' (Family Saturniidae) appear to have been comparatively less abundant than in previous years. Maybe the late and concentrated rainfall has had some influence.
What Eats What
It was with interest, therefore, that I noted the article, with excellent photos, by Guin Zambatis of Skukuza, reporting moth eruptions in the Kruger National Park. (Page 9 of the Kruger Park Times 13th April 2006).
Moths And CaterpillarsOn the escarpment, I have noticed a good number of the Achaea catella species (shown in the article referred to) and more green, long-tailed 'lunar moths' (Argema mimosae) than normal. We have had none of the Imbrasia or Gonimbrasia belina (mopane moth) species attracted to the outside lights.
The week after Guin's article appeared, I found the same large, black, red and white caterpillars, shown in her photos of Bunaea alcinoe. They were on a cape ash (Ekebergia capensis) outside the Community Church opposite the Hoedspruit library.Patches of white moth eggs were noticed on leaves of the same tree and the developing caterpillars had what appeared to be white, oblong eggs attached to their backs between the white spines. On closer inspection these 'oblong eggs' were noted to be small cocoons, many of which were neatly opened at the top. The caterpillars appeared to be lethargic and slightly shrivelled.
An investigation, with the assistance of Skaife's African Insect Life (John Ledger & Anthony Bannister) and Insects of South Africa (Alan Weaving, Picker and Griffiths), indicated that the cocoons belonged to one of the many, small parasitic wasps. The open cocoons indicated that the pupae had matured and emerged as adults.
Parasitic WaspsOn a previous occasion I mentioned the order Hymenoptera, which includes the wasps, ants and bees. There are estimated to be hundreds of thousands of species worldwide of which less than 40% have been described. There are over 100,000 species in South Africa.
This large array is divided into sub-orders - super families, -families, sub-families and so on. There are far too many to even start discussing here and I want to restrict this to the context of the parasitised caterpillars mentioned above. Remember that most of the existing insects have not yet been described. What we term 'wasps' are classified under several suborders and super families. They vary from large to almost microscopic species, the majority of which are very beneficial to humanity. Although many adults pollinate plants and feed themselves and their larvae on plant matter, the majority are predatory or parasitic on just about every other form of 'insect', including their own families.
The predatory wasps have retained a 'tail' with which they 'sting' to kill or paralyse their prey. In the parasitic wasps the 'tail' is developed into an egg-laying 'ovipostor' with which they can penetrate the host. Wasps control more insects within a given area than any application of pesticides. Note that I use the term 'control' and not 'destroy'. When we destroy a species (that may not have even been described) we destroy the diversity which maintains the ecological balance. Predators kill and consume their prey. Parasites live off another living organism. Most parasitic wasps are neither parasitic nor predatory.
Unlike true parasites, the larval stages invariably kill the host on which they feed, and unlike predators they require only one host ('prey') individual for their complete development. Species of the division Parasitica are therefore more accurately referred to as 'Parasitoides'. Endoparasites develop within their host / prey white ectoparasites develop outside the host.
Hyper-ParasitismThe Bunaea alcinoe (common emperor) caterpillars mentioned above, had been discovered by a tiny specie of the large family of parasitoid Braconid wasps (Braconidae). The adult wasp had penetrated the live caterpillar(s) with her ovipositor and laid eggs inside the caterpillar. The eggs had hatched into larvae which fed within the caterpillar.
The larvae, on reaching full size, cut their way out of the caterpillar and formed tiny, white cocoons, within which they pupated, on the outside of the caterpillar. Within a few days the mature wasps cut their way out of the cocoons to repeat the cycle. The caterpillars, denuded of their nutrients and depending on their rate of leaf consumption, slowly shrivel and die. What was even more interesting to me was that, while I was watching the caterpillars, I noticed a small (two mm) wasp settle on one of the 'parasitised' individuals. It showed interest in the unopened cocoons, which it penetrated with its tiny ovipositor. Investigations led me to discover that this wasp was a member of another large family of parasitoid Chalcid wasps (Chalcidoidae) and that it was, in turn, parasitising the Braconid cocoons. Its egg would hatch within the ready-made cocoon and its larva would consume the Braconid pupa. It would then pupate and emerge as an adult from its 'hijacked' cocoon. The occurrence of parasitism on other parasites has been termed 'hyper-parasitism'. I haven't had an opportunity to observe the sequel of the wasps and caterpillars but it makes one aware of the unseen dramas taking place in the natural world around us. I am sure that the flycatchers would be eating the little wasps and cuckoos certainly eat caterpillars. How very like our business world where 'dog eats dog' ! Without all this diverse insect control we would soon be overrun by fast breeding species. All species have been designed to perform a particular role in nature.
If you destroy the species you destroy their function. Take care where and how you use destructive devices like pellet guns and insecticides so that you don't upset the delicate balance of natural controls. We are supposed to be 'keepers' of this earth with the intelligence to differentiate between 'need' and 'greed'.