Hypera postica
Creators: name: Skuhrovec, Jiří
name: Štys, Pavel
name: Exnerová, Alice
Publisher: name: Zenodo
Year: 2014
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Insecta
Coleoptera
Curculionidae
Hypera
Hypera postica

description: 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.
description: Published as part of Skuhrovec, Jiří, Štys, Pavel & Exnerová, Alice, 2014, Intraspecific larval aggression in two species of Hyperini (Coleoptera: Curculionidae), pp. 1131-1146 in Journal of Natural History 49 (19) on pages 1136-1138, DOI: 10.1080/00222933.2014.974704, http://zenodo.org/record/4005897