Ent. exp. & appl. 15 (1972) 191t---194. North-Holland Publishing Co., Amsterdam THE ICHNEUMON AND WASP VENTURIA AVOIDANCE CANESCENS: OVIPOSITION OF SUPERPARASITISM BY DAVID ROGERS Hope Department of Entomology, University Museum, University of Oxford, England The parthenogenetic ichneumon wasp Venturia ( = Nemeritis( canescens (Grav.) carries a single egg in readiness for oviposition in a cavity right at the tip of the long ovipositor. The egg is passed into the cavity during a flexing or "cocking" movement of the abdomen, and this behaviour can be used to determine whether or not Venturia has deposited an egg into its host. The cocking movement shows that Venturia tends to avoid superparasitism. Venturia is a solitary internal parasite of the caterpillars of various species of phycifid moths, and has often been used in studies of superparasitism (Salt, 1961) and multiparasitism (Fisher, 1961). In the laboratory it is usually reared on larvae of Anagasta ( = Ephestia) kiihniella (Zeller) or E. cautella (Walker). Oviposition by the parasite takes only a fraction of a second: the egg is laid in the haemocoel of the host caterpillar, which continues development up to the last larval instar before finally succumbing to the parasite. Only one parasite completes development on one host and, in the event of superparasitism, the oldest parasite larva almost always wins in competition with younger larvae (Fisher, 1963). Superparasitism commonly occurs in the laboratory only when the host supply is limited. The tip of the Venturia ovipositor, with the left first valvula removed, is shown in Figure 1. The second valvulae are joined together for most of their length by a dorsal hinge (Figure 1; section AA'), and provide support for the separate first valvulae. These slide independently on the second valvulae and are kept in position by an interlocking tongue and groove arrangement clearly visible in transverse sections of the ovipositor (Figure 1). At the tip of the ovipositor the first valvulae are enlarged, and together with the second valvulae form an approximately spindleshaped cavity. The cavity is partly occluded by a delicate projection from each first valvula which gently grips the egg in position within the cavity. The egg is eventually released from the cavity by relative movement of the first and second valvulae, when the ovipositor penetrates a host larva. The egg passes down the ovipositor and into the cavity at the tip during a very characteristic cocking movement, which must always be performed before Venturia begins to stab host larvae. The cocking movement is compared with the stabbing movement in Figure 2. 191 OVIPOSITION OF AN ICHNEUMON WASP EGG CANAL SECTION AA' VALVULA 1 VALVULA 2 SECTION BB' PROJECT ION "~'~ ~ ~ ~ VALVULA 2 EGG CAVITY .~~VALVU LA 1 SECTION CC' ! 0.1 ram. ,J Fig. 1. Sketch of V e n t u r i a canescens female in the stabbing position (above). Tip of the V e n t u r i a ovipositor with the left first valvula removed, showing the egg cavity and the projection from the right first valvula (middle). Sections through the ovipositor and the egg cavity (right). 192 DAVID ROGERS RESTING STABBING Fig. 2. Sketches comparing the stabbing and cocking movements of Venturia. The position of the next egg to be laid is also indicated. Before stabbing (lower half of figure) the ovipositor is flexed down and forward on the hinge of the large second valvifer. This valvifer is then in turn rotated away from the ventral surface of the abdomen during stabbing. During cocking (upper half of figure) the second valvifer and the ovipositor flex at the same time, and the ovipositor remains approximately horizontal. At this stage in the cocking movement an egg is passed into the ovipositor and down towards the cavity at the tip. If the tip of the Venturia ovipositor is cut off just after a healthy host has been stabbed, and before the next cocking movement, the egg cavity is always empty. But if the tip of the ovipositor is cut off just after the cocking movement, the egg cavity always contains a single egg. Venturia will often continue to cock after such 193 OVIPOSITION OF AN ICHNEUMON WASP an operation and, each time, an egg emerges from the cut end of the ovipositor. Thus the cocking movement normally shows that an egg has been laid in the last host stabbed. Using the cocking movement to indicate oviposition it is now clear that Venturia tends to avoid superparasitism by detecting the presence of parasite eggs already in a host. Individual Venturia females in 7.5 cm ;< 2.5 cm glass tubes were allowed to parasitize one healthy Ephestia cautella caterpillar every 3 minutes. Each parasite attacked the host, and then cocked before being ready to attack the next host. After varying periods of time the parasite was offered one of the already parasitized hosts. It was allowed to stab the parasitized host once only, and was then immediately offered a healthy host. The parasite would often attack the healthy host straight away, without performing the cocking movement, showing that it had avoided oviposition in the previous, parasitized host. In Figure 3 the percentage avoidance of superparasitism (i.e. the percentage of stabs not followed by cocks) is plotted against the', number of minutes that the first parasite egg had been in the host larva. The figure shows that Venturia can detect the presence of a parasite egg (and avoid superparasitism) within 5 minutes of oviposition of that egg in a healthy host: the percentage avoidance increases rapidly during the following minutes, and reaches a level of 70 % after only 30 minutes of egg development. It is not yet known what Venturia detects when it stabs a parasitized host, but there are many sensillae at the tip of the ovipositor (Sirnmonds, 1943; Dethier, 1947). Many solitary parasitic Hymenoptera avoid superparasitism (Salt, 1961), with the obvious advantage that few eggs are wasted. Recent work with Venturia has W 100 z 8o ,.Ore O 6o w (_9 < 40 7 /~ z 111 (J 2 O n,bJ 13_ 0 "" ~" 0 Y I I l I I 10 20 30 40 50 MINUTES AFTER FIRST 60 OVIPOSITION Fig. 3. The relationship between the percentage avoidance of superparasitism of E. cautella larvae containing one Venturia egg, and the time since the oviposition of that egg. 194 DAVID ROGERS shown that this parasite tends to leave areas where a large p r o p o r t i o n of the host p o p u l a t i o n has already b e e n parasitized (Rogers, 1970). This change i n b e h a v i o u r lessens the time spent actually attacking hosts, a n d so m a y b e an i m p o r t a n t feature of parasite i n t e r f e r e n c e (HasseU & Varley, 1969). I w o u l d like to t h a n k Professor G. C. V a r l e y for e n c o u r a g e m e n t , a n d the Science R e s e a r c h C o u n c i l for a R e s e a r c h Studentship. R/~SUM~ NEMERITIS CANESCENS : O V I P O S I T I O N E T E V I T E M E N T D E S U P E R P A R A S I T I S M E l'Ichneumonide parthenog6n6tique N e m e r i t i s canescens porte un seul ceuf pr~t ~t ~tre pondu darts une cavit6 fusiforme, juste ~t la pointe de son long ovipositeur. L'eeuf est 6ventuellement 6mis quand l'ovipositeur p6n~tre dans une chenille h6te. L'ceuf chemine le long de l'ovipositeur, et est mis en place darts la cavit6 apicale pendant un mouvement de flexion tr~s caract6ristique de l'abdomen (Cocking Fig. 2) qui est bien distinct du mouvement accompli au moment m~me de la piqfire de l'h6te (Stabbing Fig. 2). Le mouvement dit d'armement (Cocking) est tonjours effectu6 juste apr~s la ponte dans un h6te sain, assurant ainsi la raise en place d'un nonvel oeuf pour une autre piqfire. L'absence du mouvement de <<Cocking>>entre deux actes de piqfire dans deux h6tes successifs indique que la piqfire dans le premier h6te n'a pas 6t6 suivie de ponte. En utilisant le mouvement de <<Cocking>>comme indicateur de ponte on peut d6montrer clue N e m e r i t i s reconnaSt la pr6sence d'un ceuf dans une h6te cinq minutes apr~s clue celui-ei a 6t6 parasit6, et 6rite le superparasitisme en effectuant une piqfire non suivie de ponte. La proportion de piqfires st6riles s'accroSt quand l'h6te h6berge un oeuf dont le d6veloppement est plus avanc6, l'6vitement du superparasitisme atteint sa plus forte proportion 30 minutes seulement apr~s le d6p6t du premier oeuf (Fig. 3). REFERENCES DETHIER, V. G. (1947). The response of hymenopterous parasites to chemicM stimulation of the ovipositor. J. exp. Z o o l . 105: 199--207. FISHER, R. C. (1961). A study in insect multiparasitism II, The mechanism and control of competition for the host. 1. exp. Biol. 38 : 60,5--628. (t963). Oxygen requirements and the physiological suppression of supernumerary insect parasitoids. J. exp. Biol. 40: 531--540. HASSELL, M. P. & VARLEY, G. C. (1969). New inductive population model for insect parasites and its bearing on biological control. N a t u r e , L o n d . 223: 1133--1137. ROGERS, D. J. (1970). Aspects of host-parasite interaction in laboratory populations of insects. D. Phil. thesis, Oxford. SALT, G. (1961). Competition among insect parasitoids. Syrup. Soc. exp. Biol. 15: 96--119. SIMMONDS, F. J. (1943). The occurrence of superparasitism in Nerneritis canescens Gray. R e v u e can. Biol. 2 : 15--58. - - R e c e i v e d f o r publication : July 15, 1971.