Publisher Summary
This chapter briefly discusses hormonal control of diapause. Diapause is an arrest in development accompanied by a major shutdown in metabolic activity. The evolution of diapause is arguably one of the most critical events bolstering the success of insects. The capacity to periodically shut down development has enabled insects and their arthropod relatives to invade environments that are seasonally hostile. Indeed, very few environments permit continuous insect development. Even in the tropics, where temperatures throughout the year may be compatible with ectotherm development, seasonal patterns of rainfall drive cycles of plant growth that favor insects with the ability to periodically become dormant. Unlike a simple quiescence that is an immediate response to an unfavorable environmental condition, diapause is a genetically programmed response that occurs at a specific stage for each species. Sometimes, the insect will enter diapause at this particular stage in each generation regardless of the environmental conditions it receives—a developmental program referred to as obligatory diapause. Much more frequently, the decision to enter diapause is determined by environmental factors, usually day length, received by that individual or its mother at an earlier developmental stage. This is referred to as facultative diapause.

10 – Hormonal Control of Diapause

The seasonal occurrence and spatial distribution of the crambine stem borer, Chilo partellus (Swinhoe) were investigated in maize, Zea mays, and sorghum, Sorghum bicolor, fields at the Kenya coast. During the vegetative growth stage, small-sized larvae were typically found behind leaf sheaths and in whorls in maize and sorghum. In reproductive and senescent maize, small-sized larvae were also found feeding in ears. Later immature life stages were located predominantly in stems in sorghum, and in the stems and ears in maize. Density and variance estimates of small, medium-, and large-sized larvae and pupae in maize and sorghum were fitted to Taylor's power law which provided significant regressions in all cases. The between-plant distribution was aggregated in all life stages. In maize, the distribution remained aggregated through the larval and pupal life stages, but in sorghum the distribution became progressively less aggregated as insects aged. The relationship of C. partellus density to the proportion of plants infested also suggested aggregation. Optimal sample sizes were determined for both binomial and enumerative sampling plans at two levels of precision, and a discussion is included on the relationship of optimal sample sizes to economic injury levels. Additionally, a presence/absence sequential sampling plan is proposed.

Distribution and sampling of Chilo partellus (Lepidoptera: Pyralidae) in maize and sorghum on the Kenya coast

Diapause of the southwestern corn borer, Diatraea grandiosella: Further evidence showing juvenile hormone to be the regulator

Three years work at Mtwapa Research Station summarises the fluctuation of Chilo spp. and Sesamia spp. Chilo partellus has been been shown to be just as important as C. orichalcociliellus nearer the coast, but Sesamia calamistis becomes important towards the second growing season. The loss caused by these borers to maize plants in the field has been estimated to be high when no chemical treatment is applied, but such a loss varies with the intensity of infestation. Control of the borers by cultural methods relies on planting maize at the onset of rains. Selected chemical treatment at the appropriate time reduces the loss by about 20%.

Population Incidence And The Control Of Maize Stalk-Borers Chilo Partellus (Swinh.), C. Orichalcociliellus Strand and Sesamia Calamistis HMPS, in Coast Province, Kenya

The larvae of the spotted stalk borer Chilo partellus and the coastal stalk borer Chilo orichalcociliella enter aestivation-diapause in the stalks of maize stems soon after the rains have ceased. In the field prediapause larvae were found in maize plants of different developmental stages, ranging from early vegetative to beyond maturity: the growth stage is not responsible for the induction of aestivation-diapause. The first symptoms of the diapause syndrome appear in larvae feeding in stems containing relatively high water contents and low protein contents. Diapause could be “artificially” induced in non-diapause larvae introduced into aged maize stems. Larvae beyond the fifth instar had developed too far to be sensitive to the factors in these stems which trigger diapause. The larval pigmentation as a criterion for aestivation-diapause is discussed.



RESUME

LA CONDITION DE LA PLANTE-HOTE PENDANT LA DIAPAUSE D'ESTIVATION DES PYRALES CHILO PARTELLUS ET CHILO ORICHALCOCILELLA AU KENYA



Les larves des pyrales Chilo partellus et Chilo orichalcocilella commencent la diapause d'estivation dans les tiges de mais peu apres la cessation des pluies. Des larves a l‘etat de pre-diapause ont ete t'rouvees sur les lieux dans des plantes de mais a differents stades de developpement, s’eehelonnant du premier stade vegetatif a passe maturite: la phase de croissance n'est pas responsable de l'induction de la diapause d'estivation. Les premiers symptomes du syndrome de diapause apparaissent dans les larves se nourrissant de tiges ayant une teneur en eau relativement elevee et une teneur en proteines faible. La diapause pourrait etre provoquee “artificiellement” dans les larves n‘etant pas en etat de diapause introduites dans des tiges de mais âgees. Au-dela du cinquieme instar, les larves etaient trop developpees pour etre sensibles aux facteurs declanchant la diapause presents dans ces tiges. La pigmentation larvaire comme critere pour la diapause d'estivation est discutee.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1570-7458.1978.tb02832.x

The condition of the host plant during aestivation‐diapause of the stalk borers chilo partellus and chilo orichalcociliella (lepidoptera, pyralidae) in kenya

Stalk 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 were comparable 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 a reduction 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.

/pdf/ecological-and-physiological-aspects-of-aestivation-diapause-3lrxjtjloi.pdf

Ecological and physiological aspects of aestivation-diapause in the larvae of two Pyralid stalk borers of maize in Kenya

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Ecological and physiological aspects of aestivation-diapause in the larvae of two Pyralid stalk borers of maize in Kenya