Control efficacy of azadirachtin on the fall armyworm, Spodoptera frugiperda (J. E. Smith) by soil drenching.

The fall armyworm (FAW), Spodoptera frugiperda, is an important agricultural pest species native to the Western Hemisphere and has recently invaded to Africa and Asia. Owing to the development of pesticide resistance and environmental contamination, ecofriendly pesticides are desirable for FAW control. Azadirachtin is a plant-derived natural pesticide with low toxicity to humans and the natural environment. Azadirachtin is primarily applied by foliar spraying; however, this approach lowers the efficacy of controlling target insects owing to photodegradation and might give a harmful effect on nontarget beneficial insects. Thus, we investigated whether applying azadirachtin to soil improves FAW control and its toxicity to corn plants. Soil drainage of azadirachtin exhibited no phytotoxic effects on corn plants but significantly reduced the larval body weight and delayed the developmental period of each larval instar of FAW. Applying 10, 15, and 20 ppm azadirachtin to soil inhibited larval growth by 68%, 76%, and 91%, respectively. Furthermore, the survival rate of FAW gradually decreased when larvae were fed azadirachtin-treated corn leaves. Collectively, this is the first study suggesting the systemic efficacy of azadirachtin by soil drenching against FAW.

Effects of Lymantria dispar multiple nucleopolyhedrovirus and Bacillus thuringiensis var. kurstaki on different larval instars of Lymantria dispar asiatica.

Outbreaks of Lymantria dispar asiatica (the Asian spongy moth; Lepidoptera: Erebidae) occur sporadically, causing widespread damage to forest and fruit trees. Owing to the development of pesticide resistance and environmental contamination, biopesticides, including L. dispar multiple nucleopolyhedrovirus (LdMNPV) and Bacillus thuringiensis var. kurstaki (Btk), can significantly contribute to controlling overall larval stage of this species. Although both pathogens are highly effective at the larval stage, their effects on different instar stages have not been investigated. In this study, we analyzed the mortality and lethality in different L. dispar asiatica instars exposed to single or combined pathogen treatments. Treatments with low or medium LdMNPV concentrations induced lower mortality and had higher LT50 values at the 4th and 5th instars compared with other instars, whereas high LdMNPV treatments induced high mortality in all instars, with higher LT50 values at later instars. Treatment with Btk induced a rapid 100% mortality in all instars, with higher LT50 values for the later instars. The combination of LdMNPV and Btk delayed the killing time compared with the effects of single treatments, with the effect being more pronounced in the 1st and 5th instar stage than at other stages at low Btk concentrations. Our findings indicate that the pathogenic effects of LdMNPV and Btk on L. dispar asiatica differ according to larval stage, thereby providing novel insights into enhancing the biological control efficacy of these agents against L. dispar asiatica in the field.

Genetic diversity of cryptic species of Bemisia tabaci in Asia.

Bemisia tabaci is a species complex consisting of various genetically different cryptic species worldwide. To understand the genetic characteristics and geographic distribution of cryptic species of B. tabaci in Asia, we conducted an extensive collection of B. tabaci samples in ten Asian countries (Bangladesh, Indonesia, Japan, Korea, Myanmar, Nepal, Philippines, Singapore, Taiwan, and Vietnam) from 2013 to 2020 and determined 56 different partial sequences of mitochondrial cytochrome oxidase subunit I (COI) DNA. In addition, information on 129 COI sequences of B. tabaci identified from 16 Asian countries was downloaded from the GenBank database. Among the total 185 COI sequences of B. tabaci, the sequence variation reached to 19.68%. In addition, there were 31 cryptic species updated from 16 countries in Asia, that is, Asia I, Asia I India, Asia II (1-13), Asia III, Asia IV, Asia V, China 1-6, MEAM (1, 2, K), MED, Australia/Indonesia, Japan (1 and 2). Further, MED cryptic species consisted of 2 clades, Q1 and Q2. This study provides updated information to understand the genetic variation and geographic diversity of B. tabaci in Asia.

Impact of Rice and Potato Host Plants Is Higher on the Reproduction than Growth of Corn Strain Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae).

The fall armyworm (FAW), Spodoptera frugiperda, is an invasive pest species that has recently increased its range in most African and Asian countries, causing significant losses to crop yields, especially corn. To develop effective management strategies, it is particularly important to study the biology of FAW in various crops. Here, we utilized the age-stage, two-sex life table to examine the development, survival, and reproduction rate of the corn strain FAW on three different host plants: corn, rice, and potato. The corn strain FAW successfully completed its life cycle in rice and potato, as well as corn plants. However, the growth, developmental time, survival, and reproduction rate differed among the three host plants. The preadult survival rates in corn, rice, and potato were 92%, 81%, and 77%, respectively. Similarly, mean generation time was significantly shorter in corn (35 days), followed by rice (41 days) and potato (42 days), indicating more generations in corn. Interestingly, the net reproduction rate varied greatly among the three host plants. In corn-fed FAW, the net reproduction rate was 472 offspring per individual, whereas, in rice and potato crops, the rates were only 213 and 86 offspring per individual, respectively. Our results suggest that alternative host plants, such as potato and rice, have more effect on reproduction than the growth of corn strain FAW. These results may be useful in predicting the population dynamics of FAW and understanding the potential damage to crops, thus contributing to an appropriate management strategy in the newly FAW-invaded agricultural ecosystems.