Hypera postica (Gyllenhal, 1813)

Skuhrovec, Jiří, Štys, Pavel & Exnerová, Alice, 2014, Intraspecific larval aggression in two species of Hyperini (Coleoptera: Curculionidae), Journal of Natural History 49 (19), pp. 1131-1146 : 1136-1138

Hypera postica ( Figures 1–3 )

Single-instar groups

The number of fights recorded during the experiments was influenced by instar (Wald Statistic (Wald Stat.) = 27.11; df = 3, 153; p <0.01) and by the presence of food (Wald Stat. = 19.27; df = 1, 153; p <0.01). The interaction of the factors (instar age and presence of food) was not significant (Wald Stat. = 3.19; df = 3, 153; p = 0.36). Attacks were more frequent among larvae of L2, L3 and L4 and in the absence of food ( Figure 1 ).

Experiment (p1). See Figure 1 . Larvae of all instars started immediately to bob around, and when they touched any kind of object (leaves of Medicago sativa , another larva), they moved closer to it. If the object was a leaf, the larva would sample it and, eventually, eat it. When the larva touched another individual, it continued to bob, and for some time repeatedly touched the other larva; then the attack followed ( Figure 5 ). The mode of attack was constant: the offensive larva twisted itself around the defensive one and pressed upon it ( Figure 5 ). The defensive larva rotated and jerked at the ground, and tried to release itself. If the offensive larva relaxed its grip after a while, the defensive larva could extricate itself. The fight was then resumed, or the offensive larva started bobbing again and tried to find another object in its vicinity ( Figure 5 ). Later, the offensive larva might attack again (the same or another defensive larva) or move in another direction. The roles of the offensive larva and the defensive sometimes changed during the fighting.

The fighting larvae sometimes came close to alfalfa leaves or upon the leaves, and if they stopped the fight, they could taste the leaves. Both offensive and defensive larva preferred the food to the fight on almost all occasions. Nearly all of the larvae were eating leaves after 10 minutes (see Figure 3 ).

If several larvae were clumped and formed a combat ball (see Figure 5 ), the fights might have continued longer since the larvae had been joining the ball by turns. However, they always stopped fighting when food was found. The fights sometimes lasted for only a few seconds, and sometimes for the whole day (particularly in combat balls).

The youngest (L1) and the oldest (L4b) larvae wandered more than the others.

Experiment (p2). See Figure 1 . Two major activities were apparent: wandering and fighting. The youngest (L1) and oldest larvae (L4b) wandered more than the others. L4b larvae did not fight, even when they encountered one another. When the other larvae (L2–L4a) encountered each other, the fighting usually started immediately, though every body contact did not result in a fight. On several occasions, all the larvae joined the fight, formed a combat ball (see Figure 3 ) and continued to fight even over the last regular control (30 minutes). All the larvae forming the combat ball were found dead after 12 hours (or earlier). Their tissues, at sites attacked by other larvae, were black owing to necrosis.

Mixed-instar groups

In mixed-instar groups, attacks were more frequent when food was absent (Wald Stat. = 7.84; df = 3, 76; p <0.01, Figure 2 ).

Experiment (p3). See Figure 2 . The results were the same as in (p1). The older larvae (L3/L4) reversed the attacks of the small larvae (L1/L2) by employing their size and body mass. Finally, all the larvae were eating the leaves, and no fights were taking place.

Experiment (p4). See Figure 2 . The results of this experiment combined those of the previous experiments ( Figure 2 ). The fights occasionally resulted in the death of all the larvae. The bodies of dead larvae were twisted into a combat ball.

The possible behaviours of all the instars of Hypera postica (L1, L2, L3, L4a and L4b) are summarized in Figure 3 .

Figures

Figure 1. The number of fights observed in experiments with groups of Hypera postica larvae of various instars (L1–L4), in the presence (plain boxes) or absence (hatched boxes) of food (leaves of Medicago sativa). Filled circles and squares indicate the mean, boxes indicate confidence intervals, whiskers show the non-outlier range and empty circles represent outliers.

Figure 2. The number of fights observed in experiments with mixed-instar groups of Hypera postica larvae in the presence or absence of food (leaves of Medicago sativa). Filled circles indicate the mean value, boxes show the confidence intervals, whiskers represent the nonoutlier range and empty circles represent outliers.

Figure 3. A summary of the possible behaviours of each instar of Hypera postica (L1, L2, L3, L4a and L4b).

Figure 5. Intraspecific larval aggression of Hypera postica. Twisted fighting larvae formed a cluster named a ‘combat ball’. All the fights took place only after contacts between or amongst the larvae searching for food, not among those looking for potential competitors. Fighting was observed only in cultures, never in the field.