Insect aggregations

Insects disappear from our planet at a rhythm of around one species every quarter of an hour. Deforestation, fertilizers, insecticides, urbanisation and construction of roads are the main causes for this fast rate of extinction. However, our activities help certain populations to grow to the size of millions of individuals at some places. Those which disappear and move towards extinction, are insects linked to a specific host-plant, as well as apterous or brachypterous forms, such as Timarcha, incapable of flying to populate another suitable area. Those which fly have a thousand chances more to escape extinction. Among surviving insects, many show formation of groups of a considerable size or aggregations.

Cycloalexy (see the chapter "Round defense") is also an instance of aggregation. It is the resting period of insect larvae, or rarely of adults, generally forming a circular group, after a day or a night of activity. This gives a protection against predators and parasites. But here we shall discuss mainly long period aggregation, which is an assemblage of many individuals of one species of insects in a place, at certain time of the year, for hibernation, diapause or any other reason, understood or ununderstood. These aggregations are for a considerable length of time, up to some months. Some aggregations are annual, like the case of the coccinellids (see the chapter on "Insect migrations"), or the Panamean Stenotarsus, or there can be in a cycle of a longer period linked with climatic reasons, crowding or any uknown reason.

Large aggregations of certain cicadas in the US are formed every 13 years, or every 17 years, as for the Magicicada septemdecim in the USA. In that case the cicad aggregations are related to the simultaneous emergence of a large number from their pupal skin. This happens also in Europe, every 4 years with the cockchafer. In New-Guinea, PJ has witnessed the mass hatching of the local cockchafer (Lepidiota vogeli) in the highlands in Goroka. The Papuans, with pieces of cloth, collected them to eat. This sudden mass emergence is linked with the first rains after a long period of drought, and, as good rains come after a gap of several years, the phenomenon of group emergence also shows a cycle repeated after a period of several years. It is a festive day in New-Guinea, when there is such mass emergence of adults, and thank God it is a better way than cannibalism to get proteins. PJ tried one day to eat the beetle also by frying them with butter on a stove. He removed the elytra, the head, the hairy and spiny legs, but not much was left and the taste was not very attractive; it tasted terebenthine or something similar. In the Sahel zone, just north of the Sahara, in Sudan, around Khartoum, and in the North-East of Thailand, Gonocephalum spp., tiny greyish tenebrionid, eclode en mass, by millions, immediately after the first heavy rains. They also come to the light and aggregate under the lamp posts. In the Sahel zone also, in Senegal, once a year, following heavy rains, Cahsoma senegalensis emerge in large numbers, and this nice carabid, normally aphid hunter, fills the streets of St Louis on several days, and then disappears. Similar aggregations of Caosomafrigidum have been described in Ontario in Canada. They are all linked with special meteorological conditions. All these cases are not long period aggregations. They are just a result of mass eclosions. Mass emergence may cause temporary aggregations, and we wish to concentrate here on a long time accumulation of an insect species in a given place. The butterfly Danaus plexippus makes an aggregate when hibernating in Mexico, in winter, on trees. The neotropical saturniid, Arsenura armida, in Costa Rica and in Brazil, is somewhat cycloalexic and gregarious in the larval instars. PJ has observed frequently in Ilha de Maracca, in Brazil, near Boa Vista, the last instar caterpillars feeding during the night on their tree and forming aggregations on the trunk in an oval grouping during the day. They seem to be faithful to their chosen site. The caterpillar skin is soft like silk in the last instar. They are known to use a trail pheromone in the young instars and pheromone certainly helps to keep the group together (Costa et al, 2003). Another saturniid, Hylesia spp., which is highly venomous, also has gregarious caterpillars, producing a trail phero-mone, in America, but do not show cycloalexy at rest (Fitzgerald and Pescador-Rubio, 2002).

Among insects, Crowson (1981) mentions the aggregations of many aposomatic forms, such as Danaiinae adults, various Zygaenidae and Arctiidae among Lepidoptera, and in Coleoptera the hibernating aggregations of various Coccinellidae. The breeding aggregations of many Lycidae probably belong to this category. It is evident that the small aggregations of Gerridae, among the Hemiptera, and of Gyrinidae, among the beetles, on water respond to different needs, such as food, sex, protection.

PJ was the witness in 1970 of an enormous aggregation of millions of a small tenebrionid beetle over various trees, in one arboretum (Jolivet, 1971). That was in Phu Kae, in the North-East of Bangkok. At the foot of a single tree (Dipterocarpus elatus), of 30 m height and with 6 m of circumference, there were 15 millon of these beetles, with an approximate total weight of 170 kg. On all the trees of the arboretum there were hundred millions of insects, all Mesomorphus, one species the most common among the three, found there. How to explain that extraordinary accumulation of a beetle? It was probably a sudden massive eclosion, but according to the Thai entomologists, the fact had never been observed before and did not seem to be related to special meteorological conditions.

The case of Coccinellidae, which enter into hibernation, grouped by millions, in a given place, often on an elevated site, is well known. The migrations of those beetles are linked with the density of their population. Cassidinae, leaf-beetles, can also suddenly migrate, and accumulate on a beach, as once seen in Holland. Colorado potato beetles also migrate and can appear by thousands on potato fields. Both migrations are linked with multiplication of the insects with specially favourable climatic conditions. Mosquitoes (Anopheles, Culex) hibernate in caves and houses, with their blood transformed into fat bodies. Once in Iran, PJ was witness to thousands and thousands of Anopheles on the roof of caves in the Elbourz mountains, near Caspian sea.

A strange case of aggregation is the one of the Endomychid, Stenotarsus subtilis (ex rotundus) in Panama (Denlinger, 1994; Roubik and Skelley, 2001). PJ saw the beetle for the first time in Barro Colorado island in 1990 on the palm tree, and all the entomologists of the area were there taking pictures. There are two diapause groups in the island with up to 200,000 individuals in each aggregation. Sex ratios varied from 1:1 to 1:4, females often being much more numerous, and one of those groups persisted for at least 20 years. The beetles diapause more than ten months each year. The trees used as rest sites by the beetles were Oenocarpuspanamanus (Palmae) and Tetragastis panamensis (Burseraceae). Fungi do not seem to attract the beetles in the resting sites. Bark and wood substrata do not seem also to be involved in the attraction. The beetles live one year or more and return to diapause sites after completing mating, feeding and reproductive cycle. The reason for this strange behaviour and the reason why the beetles come always to the same site remains a mystery, and perhaps will be solved one day when we will understand better the rest of the cycle.

In most cases long period aggregations are for diapause or hibernation. Reasons for other cases remain to be discovered.

References

Costa, J. T., Gotzek, D. A. and Janzen, D. H. 2003. Late-instar shift in foraging strategy and trail pheromone use in caterpillars of the neotropical moth Arsenura armida (Cramer) (Saturnidae: Arsenurinae). Journal of the Lepidopterists' Society 57 (3): 220-229.

Crowson, R. A. 1981. The Biology of Coleoptera. Academic Press, London: 802 pp.

Denlinger, D. L. 1994. The beetle tree. American Entomologist: 168-171.

Fitzgerald, T. D. and Pescador-Rubio, A., 2002. The role of tactile stmuli in the formation and maintenance of the processions of the social caterpillar Hylesia lineata (Lepidoptera, Saturniidae). Journal of Insect Behavior 15 (5): 659-674.

Jolivet, P. 1971. Une extraordinaire concentration de Ténébrionides dans l'arboretum de Phu Kae, près Sara Buri, Thailande. Bull. Ann. Soc. Entom. Belg. 106 (10-11): 323-326.

Roubik, D. W and Skelley, P. 2001. Stenotarsus subtilis Arrow, the aggregating fungus beetle of Barro Colorado Island Nature Monument, Panama (Coleoptera, Endomychidae). Coleopterists Bulletin 55 (3): 249-263.

— Fig. 12.1. Mesomorphus vitalisi Chatanay (Col. Tenebrionidae). Millions of them were accumulated over the trees of the — Fig. 12.3. Dipterocarpus tree with the Phu Kae arboretum, in Thailand (photo trunk darkening with beetles (photo P. P. Jolivet). Jolivet).

— Fig. 12.2. Aggregation of 15 millions of M. vitalisi over a trunk of Dipterocarpus elatus (Dipterocarpaceae). Probably over 50 trees of the arboretum, they were nearly 7 billions beetles (photo P. Jolivet).

Oenocarpuspanamanus, a palm tree, of Stenotarsus subtilis (Endomychidae) (photo P. Jolivet).

— Fig. 12.5. The aggregation in Fig. 12.4, magnified (photo P. Jolivet).

— Fig. 12.6. Arsenura near armida (Saturnidae) resting in a quasi-cycloalexy position in the morning. Maracca island, Amazonia (photo P. Jolivet).

Dicrochelesphalaenodectes in the tympanic area of Leucania commoida male, a moth (after Treat, 1975).

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