Pyemotes zhonghuajia Yu, Zhang & He (Prostigmata: Pyemotidae), discovered in China, has been demonstrated as a high-efficient natural enemy in controlling many agricultural and forestry pests. This mite injects toxins into the host (eggs, larvae, pupae, and adults), resulting in its paralyzation and then gets nourishment for reproductive development. These toxins have been approved to be mammal-safe, which have the potential to be used as biocontrol pesticides. Toxin proteins have been identified from many insects, especially those from the orders Scorpions and Araneae, some of which are now widely used as efficient biocontrol pesticides. However, toxin proteins in mites are not yet understood. In this study, we assembled the genome of P. zhonghuajia using PacBio technology and then identified toxin-related genes that are likely to be responsible for the paralytic process of P. zhonghuajia. The genome assembly has a size of 71.943 Mb, including 20 contigs with a N50 length of 21.248 Mb and a BUSCO completeness ratio of 90.6% (n = 1367). These contigs were subsequently assigned to three chromosomes. There were 11,183 protein coding genes annotated, which were assessed with 91.2% BUSCO completeness (n = 1066). Neurotoxin and dermonecrotic toxin gene families were significantly expanded within the genus of Pyemotes and they also formed several gene clusters on the chromosomes. Most of the genes from these two families and all of the three agatoxin genes were shown with higher expression in the one-day-old mites compared to the seven-day-pregnant mites, supporting that the one-day-old mites cause paralyzation and even death of the host. The identification of these toxin proteins may provide insights into how to improve the parasitism efficiency of this mite, and the purification of these proteins may be used to develop new biological pesticides.

No abstract

Fall armyworm Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a destructive, migratory pest that seriously damages corn, sorghum, and other crops. The “insect killer” Pyemotes zhonghuajia (Trombidiformes: Pyemotidae) is an ectoparasitic mite of agricultural and forestry pests in China. To explore the biological control efficiency of P. zhonghuajia on S. frugiperda, we evaluated the parasitic selectivity of P. zhonghuajia on S. frugiperda of different life stages, and distribution of P. zhonghuajia on different body parts of S. frugiperda larvae and pupae, food intake of S. frugiperda larvae affected by P. zhonghuajia density, and field population dynamics of S. frugiperda in relation to the releasing population density of P. zhonghuajia. We found that P. zhonghuajia preferred 4th- to sixth-instar larvae and pupae of S. frugiperda for parasitism, and the ventral part of abdominal segments of fourth-instar larvae and the ventral part of pupal thorax for feeding. Freezing of S. frugiperda 4th instar larvae and pupae had little effect on the distribution of P. zhonghuajia on the surface of these two host stages. The parasitism of P. zhonghuajia regardless of mite population size (i.e., 5, 10, or 20 mites) could significantly decrease the cumulative food intake of fifth-instar larvae of S. frugiperda. Releasing two and three boxes (50,000 mites/box) of P. zhonghuajia significantly suppressed the field population of S. frugiperda on sorghum plants/25 m2 within 7 days. Our results show that P. zhonghuajia may be a potential biological control agent and can be used for biological control of S. frugiperda in the field.