Host preference of the bean weevil Zabrotes subfasciatus

It is largely known that the range of an insect diet is mostly determined by oviposition behavior, mainly in species with endophytic larvae such as Zabrotes subfasciatus. However, the proximate factors determining host choice and the subsequent steps leading to the expansion or reduction of the host number and occasional host shifts are largely unknown. We analyzed various factors determining host preference of Z. subfasciatus through the evaluation of: (i) oviposition preference of a wild population of Z. subfasciatus on the usual host (bean) and unusual hosts (lentil, chickpea and soy), and the performance of the offspring; (ii) artificial selection for increasing preference for hosts initially less frequently chosen; (iii) comparison of oviposition behavior between two different populations (reared for ∼30 generations in beans or chickpeas, respectively); (iv) oviposition timing on usual and unusual hosts; and (v) identification of preference hierarchies. We found that when using unusual hosts, there is no correlation between performance and preference and that the preference hierarchy changes only slightly when the population passes through several generations on the less frequently accepted host. We also found a positive response to artificial selection for increasing oviposition on the less preferred host; however, when the host-choice experiment involved two varieties of the usual host, the response was faster than when the choice involved usual and unusual hosts. Finally, beetles reared on an unusual host (chickpea) for 26 generations showed similar good fitness on both usual and unusual hosts, indicating that the use of a new host does not necessarily result in the loss of performance on the original host. Nevertheless, this population showed lower fitness on the usual host than that of the original population, suggesting an underlying partial trade-off phenomenon which may contribute to a broadening of diet of this insect species.     

Egg-laying in response to prostaglandin injections in the Australian field cricket, Teleogryllus commodus

ABSTRACT. We report the effects of prostaglandin (PG) injection on egg-laying by sexually mature virgin crickets, Teleogryllus commodus Walker. High egg-laying activity is associated with PGE₁ PGE₂, 6-keto-PGE₁ and 15-keto-PGE₂, compounds that share the basic prostanoid backbone (C20 substituted fatty acid, 5 membered ring, a 7- and an 8-carbon aliphatic chain), and 9-keto, 11-hydroxyl ring substitutions. PGs without these ring features and other compounds that lack the prostanoid backbone have no or only intermediate egg-laying effects. Adding oxygen functionalities or an aliphatic double bond tends to increase egg-laying activity. Hence, 15-keto-PGF₂ and TxB₂, both of which have an additional backbone oxygen compared with their less active analogues, are highly active compounds. Two of the most active PGs, 15-keto-PGE₂ and 15-keto-PGE_2α, are the products of inactivating metabolism in mammalian systems, describing a fundamental difference in PG biochemistry with respect to mammals and insects.     

Temporal variation in the availability of an ecdysial motor programme during the last instar and early adult stages of the cricket Teleogryllus oceanicus

Abdominal peristalsis is a highly sterotyped motor programme which is normally performed only during ecdysis in the cricket. Peristaltic abdominal waves can be released in non-ecdysing crickets by providing them with sensory feedback similar to that which they would normally experience only during ecdysis, and transecting the ventral nerve cord between suboesophageal and prothoracic ganglia. The intensity of the behaviour released by these treatments increases significantly just before the expected time of the imaginal ecdysis, and decreases greatly afterwards.     

State-dependent life-history theory and its implications for optimal clutch size

Summary Life-history theory is usually based on an animal's age or size. McNamara describes a general technique for finding the optimal life-history when an organism's strategy is allowed to depend on other aspects of its state. In this paper we describe the technique in the context of previous work in life-history theory and discuss how it can be used to look at decisions on a finer time scale than the usual annual decisions. We show how it can be used to model optimal clutch size when there is a trade-off between number and quality of offspring. It is shown that the optimal clutch size is typically less than the most productive clutch size. Measuring the value of a clutch in terms of the number of offspring that survive to breed or even the number of grandchildren that survive to breed may give misleading results.     

A look into the invisible: ultraviolet-B sensitivity in an insect (Caliothrips phaseoli) revealed through a behavioural action spectrum

Caliothrips phaseoli, a phytophagous insect, detects and responds to solar ultraviolet-B radiation (UV-B; λ ≤ 315 nm) under field conditions. A highly specific mechanism must be present in the thrips visual system in order to detect this narrow band of solar radiation, which is at least 30 times less abundant than the UV-A (315–400 nm), to which many insects are sensitive. We constructed an action spectrum of thrips responses to light by studying their behavioural reactions to monochromatic irradiation under confinement conditions. Thrips were maximally sensitive to wavelengths between 290 and 330 nm; human-visible wavelengths (λ ≥ 400 nm) failed to elicit any response. All but six ommatidia of the thrips compound eye were highly fluorescent when exposed to UV-A of wavelengths longer than 330 nm. We hypothesized that the fluorescent compound acts as an internal filter, preventing radiation with λ > 330 nm from reaching the photoreceptor cells. Calculations based on the putative filter transmittance and a visual pigment template of λ_max = 360 nm produced a sensitivity spectrum that was strikingly similar to the action spectrum of UV-induced behavioural response. These results suggest that specific UV-B vision in thrips is achieved by a standard UV-A photoreceptor and a sharp cut-off internal filter that blocks longer UV wavelengths in the majority of the ommatidia.     

Egg laying decisions by the bean weevil Callosobruchus maculatus

ABSTRACT.

• 1 Previous work has shown that female Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) can accurately assess the number of eggs on a seed and use this information to produce a near uniform distribution of egg-loads, so minimizing larval competition within hosts.
• 2 The decision to lay on a particular seed may be based on one of two possible rules: (i) a‘relative’rule, which compares the number of eggs on the present seed with that on previously encountered seeds; and (ii) an‘absolute’rule, which takes into account the current egg-load alone.
• 3 This paper describes an experimental study of oviposition behaviour which concludes that an absolute measure of egg-load probably determines egg-laying decisions. The implications of this result are discussed.