506 research outputs found
Ecological and physiological aspects of aestivation-diapause in the larvae of two Pyralid stalk borers of maize in Kenya
No full textStalk borers are highly destructive to a large number of important graminaceous crops all over the world. Some examples of economically important stalk borers and a general description of their life-cycle are mentioned in chapter 1. In the same chapter difficulties in controlling the insects are described. The crucial role of aestivation- diapause in the life history of tropical stalk borers is elucidated and the importance of further research on this subject is demonstrated.Aestivation-diapause in two Pyralid stalk borers, Chilopartellus (Swinhoe) and Chilo orichalcociliella (Strand) was investigated under field and laboratory conditions.The relation between diapause and climate during three consecutive years is described in chapter 2. Yearly and seasonal fluctuations in the larval and pupal populations of the two stalk borers in maize appeared to be considerable. As long as the water conditions for plant growth were suitable, insects had a continuous development. Under these conditions larvae had pigmented spots and could not survive dry conditions. Soon after cessation of the rains (or irrigation) rates of pupation decreased. At that time larvae lost their cuticular pigmentation and became resistant to drought. Comparison of the incidence of aestivation in the field with the prevailing climatic conditions showed that only lack of rain could be associated with the arrested larval development. No effects of temperature, relative humidity or photoperiod could be found. These results indicate that the host plant may be involved in the induction of diapause.Chapter 3 is concerned with characteristics of pre-diapause and diapause larvae. Evidence was obtained that under natural conditions larvae do not feed during diapause as long as they are not disturbed. The physiological condition of field-collected stem borer larvae changed considerably upon entering diapause: a decreased rate of oxygen consumption, rate of heart beat and water content, an increased fat content, and arrested development of the testes were found. These changes normally occurred before larvae were turning unspotted and/or were becoming resistant to drought.The condition of the host plant in relation to diapause induction is described in chapter 4. Diapause could be induced inside maize plants of different developmental stages. It was shown that the first (physiological) signs of the diapause syndrome appear in larvae feeding in stems containing 70-80% water and very little (Marked differences were found in the consumption and utilization of stems of maize plants in different developmental stages. Its possible relevance to diapause is discussed.In chapter 5 experiments are described on the induction of aestivation-diapause by varying the food condition. Most early 5th instar larvae of C. partellus entered diapause after being introduced into aged maize stems containing 75% water, 8% sugar and 1.3% protein (fresh wt.). Pupation rate, cuticular pigmentation, QO 2 and water content of these larvae werecomparable to values obtained from field-collected aestivating larvae. Larvae which had developed beyond the early 5th instar were less sensitive to the diapause inducing factors of the aged maize stem : most of them pupated.Test of 30 different diets with varying protein, sugar and water contents, indicated that diets containing 0.9-1.1% protein and 70% water were best in inducing diapause. Early 5th instar larvae on diets with the above mentioned protein content grew slowly (but reached normal weights), moulted into the unspotted form and had a retarded rate of pupation. Larvae on diets with lower protein contents hardly developed at all, whereas on diets with higher protein contents larvae pupated normally. Larvae reared on diets in which the water content had been reduced from the normal level of 86% to 70%, resulted in areduction of the larval water content and respiratory rate, close to values normal for field-collected diapause larvae. Evidence was obtained that larvae reared on diapause inducing diets attained a certain degree of drought resistance. Early 5th instar or younger larvae were the most sensitive stages to diapause induction by diet.Many larvae on aged maize stems and artificial diets turned unspotted even though pupating soon thereafter. The relevance of the cuticular pigmentation as a criterion for aestivation-diapause is discussed. The endocrine involvement in the aestivation- diapause is described in chapter 6. From juvenile hormone titre determinations and ligation experiments evidence was obtained that the diapause is regulated by an intermediate level of JH. Application of JH to non-diapause larvae prevented pupation of these larvae but did not evoke other aspects of the diapause syndrome
Genetic variability in Cotesia flavipes and its importance in biological control of lepidopteran stemborers
No full textLepidoperan stemborers are a major constraint to increasing the production of maize and sorghum under subsistence farming conditions in sub-Saharan Africa. Classical biological control is considered as the most cost-effective form of pest management but it has not attained the desired success rate. It has been postulated that one major reason for the failures in classical biological control is related to the genetic diversity of released individuals of natural enemies. The aim of this study was to examine the importance of genetic variability in the establishment and performance of the parasitoid Cotesia flavipes (Hymenoptera: Braconidae) as a classical biological control agent of lepidopteran stemborers. Field surveys carried out on Unguja and Pemba islands of Zanzibar showed that the introduced stemborer Chilo partellus (Crambidae) was the most abundant and widespread species in the stemborer complex. Although a number of indigenous natural enemies were recorded, they had a low impact on stemborer populations and hence, classical biological control was considered as an option. The endoparasitoid C. flavipes , an old association natural enemy of C. partellus , was collected from central India and imported in Kenya for laboratory and field studies. Olfactometer studies to examine the effect of duration of laboratory rearing on the parasitoid's responses to host- and host plant-associated odours showed that the overall behavioural response was stable over many generations regardless of the genetic diversity of the population. A reduced genetic variability at the sex locus of some Hymenoptera can result in biocontrol failures due to the production of diploid males from fertilized eggs as a consequence of single locus complementary sex determination mechanism (sl-CSD). In species with sl-CSD, inbreeding that may occur during rearing and release in the field will result in the production of diploid males and the associated reduction in population growth rate. Models were developed to predict the frequency of matched matings in populations with different frequencies of sib mating and egg fertilization. The models showed that sl-CSD could be detected from brood sex ratios if the diploid male offspring survives. Analysis of sex ratios from field-collected data showed that sex ratio was highly female-biased. Sex ratio frequency followed a unimodal distribution instead bimodal distribution that is expected if sl-CSD with diploid male survival exists in this parasitoid species. However, the results suggested that the presence of sl-CSD with diploid male mortality could not be excluded . Inbreeding crosses to deduce further evidence for the existence of sl-CSD in C. flavipes revealed that brood sizes resulting from matched matings were not smaller than those from crosses among unmatched matings, suggesting that sl-CSD with diploid male mortality did not occur in crosses involving matched matings. Moreover, inbreeding of populations for several generations did not result in male-biased sex ratios. The importance of genetic variability to the colonization and establishment of C. flavipes was investigated through the release of three genetically impoverished populations (isofemale lines) and one genetically diverse (mixed) population on two islands of Zanzibar. The mixed population showed higher colonization of stemborers than one of the isofemale lines but it was not different from the other two isofemale lines. This suggests that genetic variability may not have been an important factor in the colonization of C. flavipes , possibly due the similarity in climate of the area of origin in central India and the release areas in Zanzibar. Suggestions for future research to improve the biological control of lepidopteran stemborers in maize and sorghum are presented. </p
Les émissions de L. Flaminius Chilo
No full textRevue Numismatique, 6e série, XXIII, 1981, pp. 68-73. Leslaw Morawiecki, Les Émissions de L. Flaminius Chilo. — ■ En raison d'une nouvelle vision du personnage de L. Flaminius Chilo comme un fonctionnaire soumis à la politique du Sénat il faut changer l'interprétation et la chronologie de ses émissions : elles apparurent entre le 1er janvier et le 2 février 43 av. J.-C. comme un geste de conciliation fait vers Antoine par le parti pacifique du Sénat. Cependant les émissions de P. Accoleius Lariscolus et de Petillius Capitolinus ne furent battues qu'après le commencement de la guerre contre Antoine, mais avant l'entrée d'Octave à Rome, donc entre le mois de février et le mois d'août de 43 av. J.-C.Morawiecki Lesla. Les émissions de L. Flaminius Chilo. In: Revue numismatique, 6e série - Tome 23, année 1981 pp. 68-73
Behaviour and biology of Chilo partellus on maize landraces
No full textPlants defend themselves against herbivores through activation of both constitutive and induced defences. Previous studies reported that egg deposition on maize landraces induces the release of volatiles that attract parasitoids, but little is known on the effect of these volatiles on subsequent herbivore oviposition. In addition, larval preference and development on these maize landraces is unknown. We evaluated six landraces and one hybrid maize variety for their resistance to Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). Larval orientation, settling, arrest and dispersal, feeding, development, survival, and subsequent oviposition of moths were determined for individuals reared on each of these varieties under laboratory and screen house conditions. For oviposition preference studies, all treatments were initially exposed to egg deposition whereas the control treatments were not. Larval preference was generally higher for hybrid maize, compared to the landraces. Similarly, first-instar feeding on maize leaves was more intense in hybrid maize than in four of the six landraces. The amount of food consumed and assimilated by third instars over a 24-h period was, however, not different among the maize varieties. Larval survival was significantly lower in maize landraces (32%) compared to hybrid maize (54%). However, there was no difference in the larval development period between any of the treatments. Two-choice oviposition assays showed that moths preferred non-exposed maize landraces for subsequent oviposition, whereas in the hybrid, there was no difference in oviposition preference between exposed and non-exposed plants. Although the mechanism of larval suppression was beyond the scope of this study, it was inferred that the landraces showed some resistance to C.partellus feeding and that initial egg deposition on these landraces deters further colonisation by the herbivore
Comparative development and population growth of an exotic stemborer, Chilo partellus (Swinhoe), and an ecologically similar congener, C. orichalcociliellus (Strand) (Lepidoptera: Crambidae)
No full textStudies were conducted to determine the effect of temperature and diet on the development of Chilo partellus (Swinhoe) and Chilo orichalcociliellus (Strand) (Lepidoptera: Crambidae). Developmental times of C. partellus and C. orichalcociliellus for the egg, larval and egg - adult life stages were inversely related to temperature. The larval developmental period of C. orichalcociliellus reared on artificial diet was longer than the developmental period on natural diet. Chilo partellus had a higher intrinsic rate of natural increase than C. orichalcociliellus at all diet/temperature combinations except natural diet at 31ºC. Des études ont été menées afin de déterminer l'effet de la température et de l'alimentation sur le développement de Chilo partellus (Swinhoe) et Chilo orichalcociliellus (Strand) (Lepidoptera: Crambidae). Les durées de développement des oeufs, des stades larvaires et du développement total sont inversement corrélées avec la température. La durée du développement larvaire de C. orichalcociliellus élevé sur milieu artificiel est plus longue que celle observée avec une alimentation naturelle. Chilo partellus a un taux intrinsèque d'accroissement naturel plus élevé que C. orichalcociliellus pour toutes les combinaisons d'alimentation et de température étudiées à l'exception de la combinaison associant une alimentation naturelle et 31 ºC
Behaviour and biology of Chilo partellus (Swinhoe) on Striga hermonthica (Del.) Benth. infested and uninfested maize plants
No full textStemborers and Striga hermonthica co-exist in cereal fields in sub-Saharan Africa where together they cause crop damage that sometimes results in total crop loss. Striga alters both the chemistry and morphology of cereals and this may influence behaviour and performance of stemborers on the host plants. Studies were undertaken to compare the effects of various levels of Striga infestation of maize on oviposition preference and survival of Chilo partellus. Potted maize plants were exposed to different densities of Striga seeds (0, 1000, 2000 and 3000/pot) and subsequent effects on C. partellus oviposition evaluated in no-choice and choice-tests. In addition, larval arrestment, settlement, feeding, growth and development were assessed. Results showed that Striga infestation and plant height had non-significant influence on oviposition preference of C. partellus, except in 2-choice assays involving uninfested maize and one under high Striga infestation, in which case they preferred to oviposit on the latter. Similarly, Striga infestation had no influence on larval arrest and settlement. The larvae, however, consumed significantly more of the leaves and stems from uninfested than infested maize plants, with a progressive decline in the consumed proportions with increasing level of Striga infestation. Larval food assimilation was not influenced by Striga infestation, but larval period, percentage of pupation and growth rate were significantly higher on uninfested plants as compared to medium and high Striga infested plants. Possible reasons underlying these observations and their implications are discussed
Assessment of the potential of Napier grass (Pennisetum purpureum) varieties as trap plants for management of Chilo partellus
No full textTen Napier grass [ Pennisetum purpureum Schumach ( Poaceae)] varieties, used in various parts of Kenya as animal fodder, were tested for their potential role as a trap crop in the management of the gramineous spotted stemborer, Chilo partellus ( Swinhoe) ( Lepidoptera: Crambidae), through habitat manipulation. Oviposition preference and larval survival and development were determined for each of these varieties under laboratory and screen-house conditions. Two-choice tests revealed that seven of the varieties tested were preferentially chosen by gravid female moths for oviposition over a susceptible maize cv. Inbred A. Larval survival was significantly lower on the majority of the Napier grass varieties ( about 10%) than on maize cv. Inbred A ( about 60%). Similarly, larval development was about 2 weeks longer on the majority of the Napier grass varieties. It was concluded that four of the varieties tested ( Bana, Ex-Machakos, Gold Coast, and Ex-Nyanza-2) had potential for use as trap crops in C. partellus management because they were more preferred by the borer moths for oviposition and allowed minimal survival of the larvae. This minimal larval survival is desirable in the conservation of C. partellus natural enemies when the cultivated host plant is out of season
Suitability of brachiaria grass as a trap crop for management of Chilo partellus
No full textAbstract The cereal stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) is a major insect pest of sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) in Africa. Trap cropping systems have been shown to be a valuable tool in management of this pest. To optimize trap cropping strategies, an understanding of host-plant preference for moth oviposition and host suitability for larval survival on potential trap plants is a prerequisite. Therefore, we assessed seven brachiaria accessions (Poaceae) for preference by C. partellus moths and subsequent larval performance. In two-choice tests with a local open-pollinated maize variety (cv. Nyamula), significantly higher numbers of eggs were deposited on brachiaria accessions Marandu, Piata, and Xaraes than on maize, whereas fewer eggs were recorded on plants of Mulato II, Mulato I, and Cayman. There was a significant and negative correlation between the trichome density on plant leaves and C. partellus oviposition preference for brachiaria. In addition to poor larval performance on brachiaria, there was no clear ranking in the accessions regarding larval orientation, settling, arrest, and food ingestion and assimilation. First instars did not consume leaf tissues of brachiaria plants but consumed those of maize, which also suffered more stem damage than brachiaria plants. No larvae survived on brachiaria plant tissue for longer than 5 days, whereas 79.2% of the larvae survived on maize. This study highlights the preferential oviposition of C. partellus on brachiaria plants over maize and the negative effects that these accessions have on subsequent larval survival and development. Our findings support the use of brachiaria as a trap crop for management of C. partellus through a push-pull technology
Responses of Parasitoids to Volatiles Induced by Chilo partellus Oviposition on Teosinte, a Wild Ancestor of Maize
No full textAbstract Maize, a genetically diverse crop, is the domesti-cated descendent of its wild ancestor, teosinte. Recently, we have shown that certain maize landraces possess a valuable indirect defense trait not present in commercial hybrids. Plants of these landraces release herbivore-induced plant volatiles (HIPVs) that attract both egg [Trichogramma bournieri Pintureau & Babault (Hymenoptera: Trichogrammatidae)] and larval [Cotesia sesamiae Cameron (Hymenoptera: Braconidae)] parasitoids in response to stemborer egg deposi-tion. In this study, we tested whether this trait also exists in the germplasm of wild Zea species. Headspace samples were col-lected from plants exposed to egg deposition by Chilo partellus Swinhoe (Lepidoptera: Crambidae) moths and un-exposed control plants. Four-arm olfactometer bioassays with parasitic wasps, T. bournieri and C. sesamiae, indicated that both egg and larval parasitoids preferred HIPVs from plants with eggs in four of the five teosinte species sampled. Headspace samples from oviposited plants released higher amounts of EAG-active compounds such as (E)-4,8-dimeth-yl-1,3,7-nonatriene. In oviposition choice bioassays, plants without eggs were significantly preferred for subsequent ovi-position by moths compared to plants with prior oviposition. These results suggest that this induced indirect defence trait is not limited to landraces but occurs in wild Zea species and appears to be an ancestral trait. Hence, these species possess a valuable trait that could be introgressed into domesticated maize lines to provide indirect defense mechanisms against stemborers
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