Vol. 13, No. 4. SINGAPORE MEDICAL JOURNAL 178 August, 1972. THE HORNETS OF SINGAPORE: THEIR IDENTIFICATION, BIOLOGY AND CONTROL By K. L. Chan SYNOPSIS This is the first report on the identification, biology and control of hornets in Singapore. There are 3 hornet species in Singapore - Vespa affinis indosinensis Perez, Vespa tropica leefmansi van der Vecht, and Vespa analis analis Fabricius. Their identification is presented for the first time by a pictorial key and a simplified dichotomous key. The stinging apparatus of hornets is presented in detail for the first time and those aspects of hornet biology related to stinging and association with man are briefly mentioned. These include the relative prevalence and aggressiveness of the 3 species, their distribution, location of nests, types of nest sup- ports, the chemical composition of the sting venom, and the stinging incidence in Singapore in recent years. The technique of hornet control and precautionary measures taken during control are given. INTRODUCTION stings. They are thus able to sting their victims repeatedly without having to lose their stings and Wasp and bee stings occur in Singapore with die. Wasps are thus generally more deadly killers sufficient frequency as to constitute a medical pro- than bees. Because this in blem (Wong, 1970). Records show many cases of of difference the anatomy stinging with several deaths in recent years. Al- of the stings, it is possible to differentiate between the wound by a hornet though 3 recent fatal cases reported in Singapore made sting and that by a were due to systemic toxic reactions (Tan et al, bee sting. The bee sting always includes the sting 1966; Wong, 1970), fatality from hymenoptera and poison gland in the wound unless brushed off, stings appears to result mainly from anaphylactic while the hornet sting leaves only a wound mark, shock and rarely from systemic toxic effects based which is usually larger than that made by the bee on the study of 50 fatal cases by Barnard (1967) sting. in America. Death from anaphylaxis can occur from The word "hornet" is interchangeable with a few stings, even one if the victim is hypersensitive, the word "wasp" but in its proper usage it refers but death from systemic toxic effects normally to "large wasps" (Encyclopaedia Britannica, 1970 results after multiple stings are received coinci- edition). Wasps thus include hornets and many dentally. No mention of the reaction to stings and other smaller stinging species of the same order, the treatment of stings is made here as these have Hymenoptera. been treated in detail by Wong (1970). Only the hornets of Singapore are presented Wasps are easily distinguished from bees by here. their simple, unmodified legs. The highly developed are so modified for the col- (swollen) legs of bees IDENTIFICATION OF SINGAPORE lection of pollen. Wasps also differ morphologically HORNETS from bees in their stings. The bee sting, because it is barbed at the end like a fish hook, is invariably There are only three hornet species in Singapore. left behind in the wound, and results in the sub- All belong to the genus Vespa of the family Ves- sequent death of the bee. Bees thus can sting only pidae in the order Hymenoptera which include once. Wasps, on the other hand, have unbarbed ants and bees. The identification of the 3 hornet species are presented pictorially in Fig. 1. Female hornets have Vector Control & Research Branch, Ministry of Health, 12 segments in the antenna and 6 visible segments Singapore. in the abdomen. Males have 13 segments in the K L CHAN, M.Sc., Ph.D.,Chief Vector Control Officer. antenna and 7 visible segments in the abdomen. 179 SINGAPORE MEDICAL JOURNAL PICTORIAL KEY TO SINGAPORE HORNETS HEAD CLYPEUS I. 2 TRIANGULAR NARROW ROUNDED BROAD ROUNDED LOBES LOBES LOBES \»"e:i7 Vespa tropica Vespa analis Vespa aff inis 1 t \.ò°7'/ o ' .af,y / 2ND SEGMENT YELLOW LAST SEGMENT YELLOW IST d 2ND SEGMENTS YELLOW I ABDOMEN] - Antenna 12 - segmented abdomen 6 - segmented SEX j? ld- Antenna 13 -segmented, abdomen 7 -segmented Fig. I. Simplified pictorial key for the identification of Singapore hornets, A simplified identification key 3 to the hornet recent being in 1911. According to Roche (1970), species is as follows: this sub -species had not been taken in Singapore Key to Hornets of Singapore since then, having been replaced by the typical 1. Clypeus with narrow rounded lob- and commoner form, V. analis analis. Roche (1970) es. 6th abdominal segment yellow- Vespa analis analis collected a worker Fabricius tyrannica near Lombang, some Not so 2 8 miles north of Kota Tinggi in Johore on Novem- 2. Clypeus with blunt triangular lob- ber 19, 1967. The records listed in van der Vecht es. 2nd abdominal segment yellow - Vespa tropica L. (1957, 1959) must now be leefmansi van der amended. Alfred's (1970) Vecht record of V. analis tyrannica as the sub -species Clypeus with short broadly round- in Singapore must also be corrected. ed lobes. 1st and 2nd abdominal segments yellow - Vespa affinis L. in- - dosinensis Perez BIOLOGY OF HORNETS One other species of Vespa must be mentioned. Hornets are social insects which display a high Vecht (1957) records V. analis tyrannica Smith as degree of social organization. They have a complex present in Singapore and Johore, Malaysia, from system of communication by which individuals of a few specimens taken in Singapore, the most the same colony recognize one another. This is AUGUST, 1972 180 achieved by social hormones (pheromones) and by (nectar and other smaller insects) during the latter the mutual exchange of food exudates (trophallaxis), period of the year. as well as by other stimuli. Table I also shows that the commonest species Social polymorphism (polyphasy) or the pre- is V. affinis followed closely by V. analis while sence of different castes (queens, workers, males) V. tropica is much less common. exists for the efficient functioning of the colony Species Distribution through division of labour and functional speciali- zation. In each nest there is normally one queen. Fig. 3 shows the distribution of the 3 hornets She is the most important digit as she is the ori- based on nests destroyed in Singapore. No clear ginator and reproducer of the entire colony. Her pattern of distribution is seen, but from observa- main function is the laying of eggs. The workers tions made on all nests destroyed, V. tropica ap- (all females) tend the young and build the nest. pears to be associated most closely with man, The males exist only for fertilization of the females. showing a distribution more or less related to that of houses. The other two species have their nests Species Prevalence built mostly away from human habitations, often The relative prevalence of the 3 hornet species on plantation trees like coconut, mango, rambutan in Singapore is based on the number of nests and rubber and on roadside trees like acacia and destroyed from complaints received by the Vector madras thorn. Control & Research Branch since this Branch took Nest Supports over this function in November 1969 from other Departments (National Museum, Fire Brigade, Table II shows the kinds of supports or struc- Parks & Trees Unit of the Public Works Depart- tures on which the 3 hornets build their nests. ment, and Works & Buildings Department of V. tropica builds its nests mostly on undis- the Public Works Department). turbed portions of buildings e.g. under roofs (Fig. 4) Table I shows 196 nests destroyed in 1970 and and inside other artificial structures like unused 439 in 1971. The number of nests destroyed in both sheds, boxes, planks, tiles, drums, inside or near years appears to show the same monthly distribu- human habitations. It rarely builds its nests on tion, with more nests destroyed in the latter half trees. of the year (Fig. 2). This distribution is probably V. analis has a distinct preference for trees. related to a proportionate increase in hornet food About 86% of its nests are built on trees. The TABLE I NUMBER OF NESTS DESTROYED IN 1970 AND 1971 1 9 7 0 19 7 I Month V. tropica V. analis V. affinis V. tropica V. analis V. affinis January 2 0 0 0 16 7 February 0 1 0 3 6 4 March 2 3 2 3 4 3 April 0 2 0 2 11 8 May 2 1 0 5 22 14 June 0 1 1 3 17 18 July 4 8 17 0 16 23 August 2 13 .23 0 12 38 September 4 8 24 0 13 50 October 3 12 14 4 19 25 November 6 4 13 2 21 26 December 5 12 7 3 19 22 TOTAL 30 65 101 25 176 238 GRAND TOTAL 196 439 181 SINGAPORE MEDICAL JOURNAL 50 O V. tropica 40 D V. analis o II V. affinis tie -0 30 se c 20 o ci 10 di Er Jan Feb Mar Apr May Jun Jul Aug Sep. Oct Nov Dec 1971 Fig. 2. Monthly incidence of hornet nests destroyed by the Vector Control & Research Branch in 1971. 19 71 I e jo 0 ° bulot pansong A ce o -,. oserangeotitr °. « o \0 e 40, ed jurong . % ° . 4%ce,.0° it,(..17 4 .: '1' 3 0.0 o °II Ii k otong. o 1 o 0 l' o - ..., l' 0; . ift921) dp .... ..P6.0,0 o ...-(.. 0: : Oc 09,..kr o ( .? r. o. e 4,40 + w2 \8; e ". % e ° o 0.% i,..,r ?,: );3'llite ecity cpq. Vt rook° .11. o Vonalis V. attinis Fig. 3. Distribution of the hornets of Singapore based on nests destroyed in 1971.