Insect growth is influenced by two major environmental factors: temperature and nutrient. These environmental factors are internally mediated by insulin/insulin-like growth factor signal (IIS) to coordinate tissue or organ growth. Maruca vitrata, a subtropical lepidopteran insect, migrates to different climate regions and feeds on various crops. The objective of this study was to determine molecular tools to predict growth rate of M. vitrata using IIS components. Four genes [insulin receptor (InR), Forkhead Box O (FOXO), Target of Rapamycin (TOR), and serine-threonine protein kinase (Akt)] were used to correlate their expression levels with larval growth rates under different environmental conditions. The functional association of IIS and larval growth was confirmed because RNA interference of these genes significantly decreased larval growth rate and pupal weight. Different rearing temperatures altered expression levels of these four IIS genes and changed their growth rate. Different nutrient conditions also significantly changed larval growth and altered expression levels of IIS components. Different local populations of M. vitrata exhibited significantly different larval growth rates under the same nutrient and temperature conditions along with different expression levels of IIS components. Under a constant temperature (25°C), larval growth rates showed significant correlations with IIS gene expression levels. Subsequent regression formulas of expression levels of four IIS components against larval growth rate were applied to predict growth patterns of M. vitrata larvae reared on different natural hosts and natural local populations reared on the same diet. All four formulas well predicted larval growth rates with some deviations. These results indicate that the IIS expression analysis explains the growth variation at the same temperature due to nutrient and genetic background.

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This study investigated the effects of different temperatures (15, 20, 25, 27, 30, 35, and 40 °C) on the development rate of Spodoptera exigua (Hübner) eggs, larvae, pupae, and total immatures on plant hosts (soybean, maize, potato, and green pea). The eggs of S. exigua developed successfully at all the tested temperatures, except at 40 °C. The total developmental time (egg-adult) decreased with an increasing temperature from 15 to 35 °C on plant hosts. Stage-specific parameters such as the lower threshold temperature (TH) were determined using linear and nonlinear models (Sharpe-Schoolfield-Ikemoto [SSI]). The lower developmental threshold (LDT) and thermal constant (K) were determined using a linear model. The LDT and K for the total immature stage had respective values of 11.9 °C and 397.27° -day (DD) on soybean, 11.6 °C and 458.34° -day (DD) on maize, 11.2 °C and 446.23° -day (DD) on potato, 10.7 °C and 439.75° -day (DD) on green pea, and 12.2 °C and 355.82° -day (DD) on the artificial diet. The emergence frequency of adult S. exigua over the full range of constant temperatures was simulated using nonlinear developmental rate functions and the Weibull function. This study predicted the spring emergence date in the first to second weeks of June, with approximately five generations for plant hosts. The interaction of temperature and plant host also influenced the development and longevity of the adults. Overall, the findings of this study may be useful for predicting the number of generations, occurrence, population dynamics in crop fields, and management of S. exigua.

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Potato virus Y (PVY) (Potyviridae: potyvirus) is a serious emerging virus affecting seed potato worldwide. It affects the seed potato by transmitting non‐persistently via aphids. Sometimes this virus induces symptomless infection and is hard to detect in potato. So, it requires a specific and quick diagnosis for efficient examination. Recently, a reverse‐transcription polymerase chain reaction (RT‐PCR)‐based PVY detection method has been developed from plant as well as insect vectors. However, it is a complex and time consuming method. Here, we developed a simple PVY detection method that uses boiling for releasing the viral RNA from aphid stylets, and amplification by PVY‐specific primers located in the viral coat protein gene. The method is suitable for various strains. This simplified method could save time compared to earlier detection methods because of the simplified RNA extraction step. Following this procedure, we tested this one‐step RT‐PCR‐based PVY detection method by using three PVY vectoring aphid species (Myzus persicae, Aphis gossypii and Macrosiphum euphorbiae) as well as other sucking insects such as thrips, Frankliniella occidentalis. The reliability of a newly developed primer set was suitable for RT‐PCR and procedures were successfully demonstrated for virus detection. This PVY detection method is rapid, easy to use and suitable for large‐scale testing in laboratories of seed potato, and could potentially be applied to virus‐free seed potato production.

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