'Bistability' experiments and photoperiodic morph determination in the aphid Aphis fabae

Abstract. Newly born larvae (presumptive gynoparae) of the aphid Aphis fabae were exposed to 'symmetrical skeleton' photoperiods, consisting of two equal pulses of light per 24 h. The general form of the photoperiods tested was LDLD 1:10:1:12 h. This regime is open to two 'interpretations', LDLD 1:10:1:12 h (PP_s12) and LDLD 1:12:1:10 h (PP_s14). As both light pulses are close to 12 h apart, it is expected that the interpretations result in two distinct steady state phase relationships, i.e. this regime would show bistability. If this is true and both interpretations could be adopted, PP_s12 is expected to result in a low and PP_s14 in a high percentage of 'apterized' insects.
Experiments undertaken with the aphid showed that bistability did not occur either at 15C or 20C, and the results could be entirely explained on the basis of an 'instantly' damping circadian oscillator clock.     

Does immunization of cucumber against anthracnose by Colletotrichum lagenarium affect host suitability for arthropods?

Restricted infection of a lower leaf of cucumber,Cucumis sativus L., with the anthracnose fungusColletotrichum lagenarium has been previously shown by others to induce persistent, systemic resistance to the same fungus and to at least 12 other diverse plant pathogens. The non-specificity of pathogen-induced resistance has fueled speculation that it might also affect arthropod herbivores. However, we found that immunization of cucumber withC. lagenarium had no effect on population growth of twospotted spider mites,Tetranychus urticae Koch, reared on foliage for which induced resistance to the same pathogen was confirmed. Similarly, immunization withC. lagenarium had no systemic effect on weight gain, duration of development, or pupal weight of fall armyworms, or on progeny production by melon aphids. In reciprocal tests, previous feeding injury from spider mites or fall armyworms did not induce systemic resistance toC. lagenarium. These results indicate that, at least for cucumber, pathogen-activated induced resistance is specific to plant pathogens, suggesting separate mechanisms of induced resistance to pathogens or herbivores.

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Résumé Dans une étude de résistance induite, l'infection basale des feuilles du concombre,Cucumis sativus L., avecColletotrichum lagenarium, le champignon de l'anthracnose, a généré une induction systémique et persistante, non seulement au même champignon, mais aussi et surtout à l'égard de douze autres. La nature non-spécifique de cette résistance a dès lors engendré une hypothèse, celle de savoir si cette non-spécificité pourrait s'entendre au niveau d'arthropodes phytophages. Cependant, nos travaux ont démontré que l'immunité vis-à-vis deC. lagenarium n'affecte ni la population de tétraniques,Tetranychus urticae Koch, élevée sur des feuilles résistantes de concombre; ni le gain pondéral, ni la durée de développement, ni le poids nymphal deSpodoptera frugiperda, ou la fertilité des aphides de melon. De ces résultats, il peut-être déduit que, au moins chez le concombre, l'induction de résistance due àC. lagenarium démeure spécifique aux champignons saprophytes, c'est à dire qu'il existe des mécanismes séparés pour la résistance, soit aux champignons, soit aux arthropodes phytophages.

     

Probing behaviour of Aphis craccivora Koch on host plants of different nutritional quality

Abstract 1. Reciprocal transfer experiments were undertaken using four different hosts to examine the effects of a change in host at adult moult on the probing behaviour of the cowpea aphid Aphis craccivora Koch.
2. The highest intrinsic rate of increase r _m was achieved by aphids born and reared on cowpea, followed by those born and reared on broad bean, then Larissa, and finally Huia.
3. Mean probing time, time to first probe, number of probes per time allocation, and non-probing time were all examined. Host quality r _m did not affect the instantaneous probability of probing, and aphids that did not find a probing site within 200 s did not settle and did not probe further on that leaf/stem.     

A model of temporal distribution of Aphis gossypii (Glover) (Hem., Aphididae) on cotton

The spatio‐temporal distribution of Aphis gossypii (Glover) was studied for a 4‐year period in a plot of Gossypium hirsutum located in Bangui, Central African Republic. A study of the temporal evolution of the number of aphids by means of non‐linear regression indicated a rapid population increase during the first 8 weeks of cotton cultivation. The overdispersion of the aphids was found to be significant during most of the season. We tested three population dispersion models; Iwao's model and Taylor's power law describe the relationship between the mean m and variance σ² for the number of aphids, and the Nachman's model describes the functional dependence between m and the proportion p of non‐infested cotton plants. The results of these statistical tests indicate that only the Taylor's model was not rejected. The aphids’ aggregation increases with population density. The regression parameter confidence intervals for each of the 4 years of observation prove the stability of the Taylor's model. The model σ² = 45m^1.3 is recommended when the environmental and cultural conditions are similar to those of our study.     

Juvenile Hormone Stimulation of Oocyte Development and Embryogenesis in the Parthenogenetic Ovaries of an Aphid,Aphis fabae

Summary

The parthenogenetic ovaries of the black bean aphid, Aphis fabae, contain developing embryos. When reared at 15°C in long days (LD 16:8) oocyte development begins within the ovaries of the largest embryos of a fourth instar mother 24–48 hr after her ecdysis from the third instar. Starvation, decapitation and precocene III treatment inhibit embryonic oocyte development; juvenile hormone treatment reverses this inhibition. A method for the in vitro culture of embryos is described and under these conditions juvenile hormone again stimulates oogenesis. Embryogénie growth in vivo, as measured by the increase in length of the oldest daughter embryos, is also stimulated by juvenile hormone treatment. The results are discussed in relation to other roles proposed for juvenile hormone in aphid development.     

Photoperiodic induction of winged females in the black bean aphid. Aphis fabae

Abstract. The Photoperiodic of winged females (alatae) in the black bean aphid, Aphis fabae Scop. (Homopetera: Aphididae), is investigated in detail with emphasis on the interaction of the maternal and embryonic/young larval photoperiodic clocks. Previous work had shown that in uncrowded conditions the induction of gynoparae (winged females that produce sexual females) requires both prenatal and postnatal exposure to long-night (12 h) Photoperidic cycles: present results show that sole postantal exposure to long nights of any lenght does not induce wing formation in early-born aphids.
When aphids were exposed to experimental light-dark cycles postanatally only, their daughters developed as alate in long nights and as apterae in short nights: the critical night lenght (CNL) was 11:1 h. Additional prenatal exposure to experimental regimes resulted in a significantly shorter CNL (10.6 h). This difference could be accounted for by the fact that more experimental light-dark cycles were experienced in the latter case.
Apterous aphids transferred from LD 16:8 h to LD 12:12 h as either third-or fourth-stadium larvae, or young adults, switched for aptera-production to alata-production. The transition form aptera- to alata-production was rather abrupt in third-stadium transfers but more gradual when transfers occurred as fourth-stadium larvae and adults. Moreover, s the number of days required for 50% of the aphids to become alata-producers increased from 7–8 in third-stadium transfers, to 9–10 and 11–12 in the later transfers.     

Morph-specific differences in metabolism related to flight in the wing-dimorphic Aphis gossypii

Abstract The cotton aphid, Aphis gossypii Glover, is a wing-dimorphic species, which causes globally important agricultural losses. In this present study, we compared the biochemical basis of wing polymorphism in A. gossypii with respect to trade-off of energy resources, including glycogen, trehalose, lipids (total lipid, triglyceride and phospholipid), free fatty acids, and soluble protein between dispersal and reproduction morphs during the wing-bud nymph and adulthood. Total lipid, triglyceride and free fatty acids were significantly higher in winged versus wingless morphs at 12 h of adulthood, the period during which alates are able to fly. By contrast, the wingless morph contained more glycogen than the winged morph from the 4th nymphal stage to adulthood. Trehalose content in the wingless morph was also higher than that in the winged morph during the 3rd and 4th nymphal stages, but vice versa at 12 h of adulthood. Finally, soluble protein content increased from nymphs to adults and was higher during adulthood in aptera versus alate. Whole-body water content in 12-h adults was significantly higher in apterae than that in alatae. These results indicate significant physiological differences between morphs related to specialization for flight.