Objective This paper aims to explore the mechanism of the difference in the food spectrum of larvae of Dioryctria sylvestrella and D. abietella, and to provide scientific basis for the difference of feeding behavior between the two species of Dioryctria larvae.
Method We observed the ultrastructures of the mouthparts and sensilla of the two species of Dioryctria larvae by scanning electron microscopy (SEM) observation.
Result The SEM results showed that there were differences in the structure of the mouthparts of these two species larvae. The epipharynx of D. sylvestrella larvae has ossification in two places, while D. abietella larvae has three places. There were more incisors on the upper mandibular of D. sylvestrella larvae than that on D. abietella larvae. The spinner mouth of D. sylvestrella larvae was larger than that of D. abietella larvae. Both of the two species of Dioryctria larvae had five kinds of sensilla and one kind of keratinous teeth on the mouthparts. They were sensilla trichomes, sensilla chaetica, sensilla basiconca, sensilla styloconica, sensillum digitiformium, and spiculesteeth, respestively. However, there were differences in the size and distribution of the same sensilla between the two species. The sensilla basiconca at the junction of the first and second segments counted from the end of the maxillary palp of D. sylvestrella larvae was type Ⅱ, while the sensilla at the same place of D. abietella larvae was type Ⅲ. There were also differences in the length and shape of two species of Dioryctria larvae in the same type of sensilla.
Conclusion The difference in the structure of the mouthparts of the two Dioryctria larvae is one of the reasons for the different feeding behaviors. In addition, the difference in length and shape between the same sensilla has a certain effect on the feeding behavior of the two species of Dioryctria larvae. The five kinds of sensilla distributing on the mouthparts of the two species of Dioryctria larvae play an important role in the perception of the hardness of the host plant tissue, the volatiles on the surface of the host, and the airflow, and help the larvae to locate the host. This study clarifies the structure of mouthparts, types, quantity and distribution of the sensilla of the larvae of these two species of Dioryctria, and discusses the functions of sensilla. It is expected to lay a foundation for further exploring the mechanism of food spectrum difference between the larvae of two species of Dioryctria.