August Dorn

Effects of azadirachtin on the moulting cycle, endocrine system, and ovaries in last-instar larvae of the milkweed bug, Oncopeltus fasciatus

Abstract Azadirachtin, injected into newly moulted last-instar larvae of Oncopeltus fasciatus , induces a variety of effects, which are dose-dependent. It can be used as a tool for studying some processes of the moulting cycle and their endocrine control, as well as regulation of egg maturation, since azadirachtin-induced permanent larvae exhibit adult ovarian development. Low doses of azadirachtin merely prolong the intermoult stage, apparently due to a delayed ecdysteroid peak. Medium and high doses suppress adult ecdysis, and the larvae become permanent larvae, the longevity of which increases with rising doses. Although medium doses prevent ecdysis, apolysis and secretion of adult cuticle takes place. The ecdysteroid peak is further delayed in these larvae and is somewhat lower than in controls. Permanent larvae induced by high azadirachtin doses show neither ecdysis nor apolysis. However, the epidermis engages in secretory activity which may correspond to adult procuticle secretion. These larvae also show an ecdysteroid peak, which is considerably delayed and distinctly lower than in the controls. Thus, treatment with different azadirachtin doses allows some dissection of the moulting cycle into different steps, in which the hormonal regulation can be studied independently. Adult ovarian development begins in a number of female permanent larvae induced with high azadirachtin doses, in some cases leading to chorionated eggs. The corpora allata are enlarged in a number of permanent larvae. It is suggested that the last-larval ecdysteroid peak sets a clock for activation of the corpora allata, i.e. its gonadotropic function, regardless of whether the adult moult takes place or not. Neurosecretion appears to be affected by azadirachtin, however, the exact mode of action remains to be determined.

Structure and function of prothoracic glands and oenocytes in embryos and last larval instars of Oncopeltus fasciatus Dallas (Insecta, Heteroptera)

Summary1.Active prothoracic glands and oenocytes of last larval stage are both characteristized by well-developed smooth and rough endoplasmic reticulum (ER). Prothoracic glands also show plasma membrane infoldings, but not oenocytes which contain a large number of pleomorphic vesicles.2.The fine structure of embryonic oenocytes corresponds after blastokinesis with that of active larval and adult cells. Thus, an activity in the late embryo can be assumed. Embryonic prothoracic glands reveal no signs of activity: smooth and rough ER are absent. The subcellular structure resembles that of organ anlagen, i.e. not yet fully differentiated tissue. Hormone synthesis is not likely.3.Ecdysone titer was determined throughout embryonic development and in mature adults. Although prothoracic glands break down during adult ecdysis, imagines contain in the Calliphora-bioassay active factors: females 0.9 CU/g and males 0.5 CU/g. As sites of synthesis the oenocytes are suggested.4.A relatively high ecdysone titer of 7 CU/g is measured in newly deposited eggs. The hormone is presumably of maternal origin. Subsequent to blastokinesis the hormone content increases dramatically up to about 180 CU/g, apparently due to endocrine function of the embryo. Oenocytes are proposed as the source of ecdysone during late embryonic development.5.The function of ecdysone during early and advanced embryogenesis, especially in view of “embryonic molts”, is discussed.

An Incremental Analysis of the Embryonic Development of the Tobacco Hornworm, Manduca sexta

Summary A timetable of the embryonic development of Manduca sexta has been compiled to serve as a basis for physiological studies. Embryogenesis is complete within 117 hr at 24° C and was analyzed by examining specimens representing 20 stages, i.e. intervals of 5% of the total developmental time. Stage 0 denotes the newly deposited egg. Germ band formation, gastrulation, differentiation of tissues and organs, and blastokinesis are described in detail. By stage 1 (6 hr post-oviposition) the cellular blastoderm is evident and the presumptive serosal and embryonic cells are distinguishable. At stage 2 the germ band has separated from the blastoderm, and by stage 3 it has elongated enormously, segregated the inner layer and is completely segmented. Subsequently, the segmental appendages differentiate: at first in the head, then in the thorax, and finally in the abdomen (prolegs). Stage 5 marks the onset of organogenesis while blastokinesis takes place at stage 8. A provisional but incomplete dorsal closure oc...

Screening of medicinal and ornamental plants for insecticidal and growth regulating activity

Extracts of 121 medicinal and ornamental plants were screened for insecticidal and growth regulating activity to milkweed bug, Oncopeltus fasciatus Dallas. The most effective extracts stemmed from Inula helenium L., Rumex crispus L., R. acetosa L., Asarum europaeum L., and Calendula officinalis L. All these extracts exerted growth inhibiting activities and moderate or low acute toxicity. Most promising were extracts of dried roots of I. helenium. Besides a moderate chronic toxicity which reduced the fitness of the milkweed bug, the treatment produced overaged nymphs, supernumerary nymphs, disturbances of the moulting process, morphological defects, delay of adult maturation and reduction of fecundity. In addition, I. helenium extracts had a strong antifeedant effect when topically applied and very strong repellent effect when applied to the food (peeled sunflower kernels). It is obvious that the extract contains several active constituents. Such a cocktail may reduce the threat of rapid resistance development.

Effects of JH III and JH analogues on phase-related growth, egg maturation and lipid metabolism in Schistocerca gregaria females

Abstract Adult growth, egg maturation, fat body development and the strength of the adipokinetic reaction triggered by flight have been determined in laboratory-bred gregarious and solitary female Schistocerca gregaria . The body weight in young, maturing solitary females was higher than in gregarious ones. Egg maturation, i.e. vitellogenin synthesis, vitellogenin incorporation into the terminal oocytes and oocyte growth, started earlier in solitary than in gregarious females. Fecundity was also higher in solitary females. However, the proportion of fat body content and strength of the adipokinetic reaction were much higher in gregarious females, which is seen as a prerequisite for long-distance migratory flights. JH III and JH analogues were applied to newly emerged gregarious females. JH III and JH analogues induced egg maturation soon after application; in the case of fenoxycarb and BASF 228743 even much earlier than in control solitary females. The latter JH analogues also increased fecundity. JH III and JH analogues inhibited fat body development and suppressed the adipokinetic reaction. Thus they may suppress migratory flights. A possible employment of JH analogues in alternative locust control is discussed. JH III and JH analogues had an effect on the fatty acid composition of the haemolymph 1,2-diacylglycerols. The treatment increased the proportion of the polyunsaturated fatty acids. The effects of JH III and JH analogues on gregarious locusts are interpreted as solitarization effects. The role of JH in phase-related lipid metabolism and egg maturation is discussed. Our results are consistent with the hypothesis proposed for migratory insects, that migration and the underlying physiological processes are facilitated by low and female reproduction by high JH titres.

EFFECTS OF JUVENILE HORMONE AND JUVENILE HORMONE ANALOGUES ON THE PHASE BEHAVIOUR OF SCHISTOCERCA GREGARIA AND LOCUSTA MIGRATORIA

Phase behaviour of Locusta migratoria and Schistocerca gregaria has been studied using animals kept isolated (representing the solitary phase) and reared under crowded conditions (representing the gregarious phase). Social aggregation (grouping), marching activity and aggression/flight behaviour (reactions to confrontation) were monitored separately in adults and/or hoppers of different larval stages. All three qualities of behaviour revealed characteristic and highly significant differences between isolated and crowded locusts. Effects of JH III and three JH analogues on the phase behaviour of crowded locusts have been analysed. Aggregation was significantly reduced by all compounds in a dose-dependent manner suggesting a “solitarization” effect and a rather direct implication of endogenous JH in this behavioural parameter. JH analogues had, surprisingly, a stimulating effect on the marching activity of crowded locusts. This might be caused by pharmacological side-effects of the JH analogues. Crowded locusts showed almost exclusively “weak” reactions to confrontation with decoy animals. After treatment with JH analogues, the test animals exerted more “strong” and less “weak” reactions, thus indicating a tendency towards “solitarization”. Generally, L. migratoria was slightly less responsive than S. gregaria to the tested compounds. JH III was less effective than the JH analogues. The varying strength of impact which the compounds had upon the three behavioural qualities could reflect a differential involvement of endogenous JH in these parameters.

Precocene-induced effects and possible role of juvenile hormone during embryogenesis of the milkweed bug Oncopeltus fasciatus.

Abstract Precocene 2 was used as a tool for uncovering possible juvenile hormone functions during embryogenesis of the milkweed bug Oncopeltus fasciatus . The most conspicuous effect, which was observed after precocene treatment of freshly laid eggs, was the secondary dorsal closure defect (DC-defect). In these embryos growth processes of the ectoderm leading to dorsal closure were disturbed, i.e., the ectoderm (resp. epidermis) failed to replace the amnion in the abdominal dorsum. The critical period for triggering the DC-defect is around 72 hr, a few hours after blastokinesis. This is the time when the endogenous juvenile hormone titer rises and the fine structure of the corpus allatum indicates a synthetic activity in nontreated embryos ( A. Dorn, 1975 , Verh. Deutsch. Zool. Ges , 67 , 85–89). Thus. also during embryogenesis an antiallatotropic action of precocene 2 seems probable. The fact that the DC-defect can be overcome when precocene treatment is followed by application of juvenile hormone 1 supports this assumption. On the grounds of the described experiments it is suggested that endogenous juvenile hormone controls ectodermal (resp. epidermal) growth processes leading to secondary dorsal closure.

20-Hydroxyecdysone-induced differentiation and apoptosis in the Drosophila cell line, l(2)mbn

20-Hydroxyecdysone has an inhibitory effect on the proliferation of l(2)mbn cells, causes vacuolization and fragmentation of cells, and promotes a strong phagocytotic activity. From several lines of evidence, it can be concluded that 20-hydroxyecdysone induces apoptosis. Long-term video observations following the fate of individual cells, scanning and transmission electron microscopy reveal the typical characteristics of apoptosis: sequestration of small cellular protuberances or larger parts of the cell with nuclear fragments (apoptotic bodies), chromatin condensation, condensation and vesiculation of cytoplasm, whereas the mitochondria retain their normal appearance. The induction of apoptosis by 20-hydroxyecdysone was confirmed by the TUNEL reaction and quantitatively determined by a method based on this reaction. Onset of apoptosis precedes phagocytotic activity. JH III alone has no clear-cut effect on l(2)mbn cells. In double treatments, the inhibitory effect of 20-hydroxyecdysone on cell proliferation is significantly reduced by the addition of JH III. Whether or not JH III also reduces apoptotic activity is not yet clear. It is shown that the l(2)mbn cell line is an advantageous model system for the exploration of steroid-induced apoptosis.

Die endokrinen drüsen im embryo von Oncopeltus fasciatus Dallas (Insecta, Heteroptera)

The differentiation of the endocrine glands in the embryo of Oncopeltus fasciatus is described. The function of these glands can be correlated with the embryonic moults. The nuclei of some tissues already become polyploid in the embryo. It is discussed whether the endomitotic growth is dependent upon the function of the endocrine glands.

Effects of azadirachtin on the regulation of midgut peristalsis by the stomatogastric nervous system inLocusta migratoria

A single injection of azadirachtin into nymphal and adult locusts results in a decrease of food intake but not in reduced food utilization. Feeding deterrency is correlated with a gradual inhibition of midgut peristalsis. Gut movement is under the control of the stomatogastric nervous system, especially the ingluvial ganglion, as demonstrated in anin vitro system. Serotonin is the most potent peristalsis-stimulating drugin vitro. Concurrent with the cessation of midgut peristalsis after azadirachtin injection, a depletion of the serotoninergic cell bodies in the frontal ganglion can be observed. Midgut preparations which show strongly inhibited contractions due to previous azadirachtin treatmentin vivo are minimally, or not at all, stimulatedin vitro by serotonin, depending on the amount of time that has elapsed since treatment. When midgut preparations of untreated locusts are challenged with azadirachtinin vitro, peristalsis is suppressed in a dose-dependent manner. Simultaneous application of azadirachtin and serotonin results in a slight stimulation of midgut peristalsis. We propose the hypothesis that azadirachtin interferes with the serotoninergic system of the stomatogastric ganglia and thus inhibits midgut peristalsis. The molecular mechanism of the action of azadirachtin remains to be determined.