Group size in a gregarious tortoise beetle: patterns of oviposition vs. larval behaviour

In laboratory and garden experiments, we tested for the existence of adaptive patterns of oviposition and larval behaviour regarding group size in the gregarious tortoise beetle Chelymorpha varians Blanchard (Coleoptera: Chrysomelidae: Cassidinae) on its host plant Calystegia sepium L. (Convolvulaceae). Specifically, we addressed the following questions: (i) Which is the more frequent egg cluster size? (ii) Does cluster size fully predict larval group size? (iii) Are newborn larvae attracted or repelled to conspecific groupings? and (iv) Which is the group size associated with enhanced larval development and adult mass? We found that the mean cluster size was 21.4 eggs. Egg hatch time was significantly shorter in larger clusters. A regression analysis of larval group size against cluster size showed non‐significant results. Thus, original cluster size did not totally determine the larval group size. The mean larval group size was 17.1. Choice tests in an experimental arena showed that larvae clearly preferred leaves of a host plant rather than moistened papers, and that larvae preferred a small group of conspecifics (four larvae per leaf) over larger groups (12 or 20 larvae). Empty leaves of the host plant showed an intermediate level of preference. Development time and beetle performance (adult mass) were affected by larval group size. Larvae in the smallest group (one per leaf) took four more days to attain adulthood than larvae in the larger groups (12 and 20 larvae). Adult C. varians reared in the 12‐larvae group were significantly larger than those reared at the other densities. Comparison of patterns across experimental groups, excluding the 12‐larvae group, showed a tendency for a greater final mass with slower developmental rate.     

Facultative gregarious development in a solitary parasitoid wasp,Dendrocerus carpenteri: larvae may share nutritional resources

Solitary parasitoids generally produce only one offspring per host. Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae) develops as an idiobiont ectoparasitoid on prepupae and pupae of primary aphid parasitoids inside the mummified aphid host. Females normally deposit a single egg but superparasitize when suitable hosts are scarce. We show that facultative gregarious development may occur but is constrained by resource competition between larvae. The probability of more than one offspring surviving increased with the intensity of parasitism; an age difference of ≤9 h between older and younger first instars did not promote gregarious development. Two female parasitoids and, rarely, up to three male parasitoids could develop together. Average body size in terms of dry mass did not differ between singly developing females and the combined mass of two females sharing host resources, but the combined mass of gregarious males was greater than that of their singly developing counterparts. Females were 3× more likely to eclose from single than gregarious mummies. The amount of host resources available per larva declines with increasing clutch size, in turn causing a corresponding reduction of adult size and size‐dependent fitness attributes. We suggest that competition for limiting host supplies may influence the transition from solitary to gregarious development and should be considered in models of clutch size evolution in parasitoid wasps.     

Gregarious development of the solitary endo-parasitoid, Microplitis rufiventris in its habitual host, Spodoptera littoralis

Abstract:  Earlier research has shown that the koinobiont parasitoid, Microplitis rufiventris , attacks and can develop on early instars of Spodoptera littoralis larvae with preference to third instars. However, the present study was carried out using the newly moulted sixth instar larvae at two different temperatures (20 ± 1 and 27 ± 1°C) to study the developmental interaction between the parasitoid and the last instar host larvae. Parasitoid eggs laid in singly parasitized host larvae invariably died. As the number of parasitoid eggs/host larvae increased, the proportion of eggs that hatched and number of viable parasitoid larvae successfully reached to their final instar increased. The effect of superparasitization seems to be dose (no. of eggs + parasitoid factors)-temperature-dependent. The results demonstrate a kind of 'Allee effect' suggesting that superparasitized last instar S. littoralis larvae provide a better host environment than singly parasitized hosts for the parasitoid, M. rufiventris . This may be due to host's hormone and/or low dose of factors injected with parasitoid eggs. The supernumerary individuals of wasp larvae developed normally to the point of emergence but most did not successfully emerge from the host. The improvement of both hatchability and post-embryonic development of parasitoid wasp was significantly (P < 0.01) greater at 20°C than at 27°C. The results of the present study are useful in understanding the evolution of life-history strategies and host range in parasitic hymenoptera.     

群居型和散居型东亚飞蝗雌成虫飞行肌的超微结构

应用电子显微镜对群居型和散居型东亚飞蝗Locusta migratoria manilensis（Meyen）雌成虫背纵肌进行了比较观察。结果表明：群居型和散居型成虫背纵肌具有类似的亚细胞结构,飞行肌的肌原纤维具有1∶3粗细丝比例,每根粗丝由6根细丝环绕排列成六角形结构。飞行肌的发育和线粒体的形成均是渐进的过程,在不同日龄成虫间存在差异。肌节长度为2.1～3.4 μm;7和10日龄时群居型成虫肌节长度小于散居型;7日龄群居型肌原纤维直径显著大于散居型。背纵肌内线粒体含量约占肌纤维的20%～43%,两型飞蝗之间存在着显著的差异,7日龄时群居型线粒体占肌原纤维的比例高达42.96%,而散居型的只有22.45%;10日龄群居型线粒体含量为41.32%,散居型线粒体29.98%。上述差异可能是东亚飞蝗群居型成虫飞行能力显著强于散居型成虫的重要原因之一。     

Survival advantage of sluggish individuals in aggregations of aposematic prey, during encounters with ambush predators

Movement is an important element of prey defense ensembles. The adaptive advantages of either remaining motionless or rapid escape are clear. In contrast, putative benefits are unclear for sluggish movement of aposematic prey that are neither fleeing nor avoiding detection of predators. Nonetheless, sluggish movement is common in aposematic insects. Our central hypothesis is that sluggish movement evolved in part by motion-oriented predators culling the fastest-moving insects from an aggregation. This would be particularly likely in chemically defended prey, which would deter continued predation. We test predictions of our hypothesis with feeding experiments using the sluggish, gregarious Eastern lubber grasshopper and a motion-oriented predator, the Northern leopard frog. Sluggish-moving (i.e., control) grasshoppers were significantly less likely to be eaten than fast-moving (i.e., motion-induced) grasshoppers (p = 0.0098). Next, non-moving grasshoppers were used as extreme sluggish-moving prey. Non-moving prey were significantly less likely to be eaten than sluggish-moving grasshoppers (p = 0.05). In addition, and most importantly, sluggish-moving grasshoppers in an aggregation were significantly less likely to be attacked than fast-moving grasshoppers in the aggregation (p = 0.0156). Finally, the survivorship of sluggish-moving grasshoppers in pairings vs. aggregations was not significantly different (p = 0.33). Our results demonstrate that the fastest-moving individual in an aggregation of aposematic insects is more likely than sluggish cohorts to be attacked by motion-oriented predators. This survival disadvantage for fast-moving, gregarious prey could create a selection pressure for the evolution of sluggish movement as a defense mechanism in aposematic, gregarious prey.     

Facultative gregarious development in a solitary koinobiont parasitoid,Ephedrus californicus: implications of larval ecology and host constraints

In solitary parasitoids, the mandibulate first instars behave aggressively towards potential competitors so that generally only one larva survives per host. A ‘failure of competition’ may result in facultative gregarious development, however. We used Ephedrus californicus Baker (Hymenoptera: Braconidae: Aphidiinae), a solitary koinobiont parasitoid of aphids, to test two hypotheses in the laboratory that could explain facultative gregarious development. Gregarious development increased with the intensity of parasitism, with two (rarely three) parasitoids successfully developing in a single aphid. In heavily superparasitized hosts, interference between surviving larvae often caused abnormal pupation behaviour and inability to emerge from the mummy. The hypothesis that the survival of more than one larva per host is dependent on differences in larval age was not supported. The total body size in terms of dry mass of two males or two females developing together in the same host was higher than that of same‐sex counterparts developing singly. Females were larger than males with which they shared a host. Hypotheses about the evolutionary transition from a solitary to a gregarious lifestyle in parasitoid Hymenoptera have focused on lethal fighting between first instars but have ignored other constraints including immature mortality during later development and limiting host resources. Especially in species that pupate inside the dead host, specific requirements for pupation and emergence may determine whether one or several offspring per host can develop to adult.     

An Addition to the Dinosaur Freeway Megatracksite,Dakota Group (Upper Cretaceous), Bent County,Colorado

A newly discovered dinosaur tracksite in the Upper Cretaceous Dakota Group of southeastern Colorado preserves tracks attributable to ornithopod, theropod, and possibly ankylosaurid dinosaurs. Minimally 79 tracks occur at the site, and are comprised predominantly of ornithopod prints preserved as natural sandstone casts. Nine ornithopod tracks are preserved in situ, and all but one of these trends in a northeasterly direction. The high density of ornithopod tracks coupled with the similar trend of those in situ suggest gregarious behavior. Size distribution of the ornithopod tracks indicates that some juveniles are present, with sub-adults most abundant and lesser numbers of mature adults. Considerable size variation exists within the nine in situ tracks with similar trends. Thus, the site could record the passage of a mixed herd composed of several age classes. This may be the only reported Dakota tracksite where ornithopod, theropod, and ankylosaurid prints occur together.     

群居型、散居型意大利蝗形态特征的数量分析

意大利蝗Calliptamus italicus(L.)是新疆草原的主要危害害虫之一,为了从形态上精确区分群居型和散居型意大利蝗,本文用数值分类学的方法对两型成虫的10个形态指标和5个形态指标比值进行了数量分析.方差分析结果表明:两型成虫在所测量的10个形态指标上都存在着显著性差异(P＜0.05);形态指标比值中前翅长...     

Fecal volatile components elicit aggregation in the oriental migratory locust,Locusta migratoria manilensis (Orthoptera: Acrididae)

Abstract The aggregation components from fecal volatiles of the oriental migratory locust, Locusta migratoria manilensis were identified with gas chromatographic/electroantennographic detector (GC‐EAD), GC‐MS (mass spectrometry) analyses and behavioral bioassays. Both last instar nymphs and adults of the oriental migratory locust have similar aggregation pheromones in their volatiles. A total of 11 electrophysiologically active compounds, namely, hexanal, cyclohexanol, heptanal, phenol, 2,5‐dimethyl‐pyrazine, benzyl alcohol, benzaldehyde, guaiacol, nonanal, 2,6,6‐trimethyl‐2‐cyclohexene‐1,4‐dione and decyl aldehyde were identified in the fecal volatiles of 2‐day‐old immature adult male locusts. Only hexanal, nonanal, benzaldehyde, cyclohexanol and 2,5‐dimethyl‐pyrazine elicited significant aggregation responses in immature 2‐day‐old adult males. However, adult males had significantly lower behavioral responses to synthetics of five single compounds than the blend of cyclohexanol, 2,5‐dimethyl‐pyrazine, benzaldehyde, nonanal, hexanal in ratios of 100: 100: 2: 60: 30 in the range of 30–60 μg/mL. We propose that it is the blend of these five compounds that plays a key role in eliciting and sustaining aggregation in gregarious oriental migratory locusts. These results also showed that the aggregation pheromones of the oriental migratory locust are significantly different from those found in the desert locust.