| Semanous bifasciatus is a destructive trunk borer, which can cause Platycladus orientalis and Sabina chinensis to die. The beetle larvae feed hiddenly, which made the control difficult. However, Pyemotes zhonghuajia (Acari:Pyemotidae) was found to parasitize the larva of the beetle in recent years, and has a better effect on low instar larvae. However, there is no much information on its parasitic mechanism. So the pyemotes selection, host positioning and the death mechanism should be discussed.In this paper, we studied the selectivity among different hosts and different instars. Firstly, the feeding character of the pyemotes was observed. Secondly, the interation between the pyemotes and the larvae was studied. Thirdly, the total protein content in hemolymph of P. zhonghuajia and Semanotus bifasciatus after infected by P. zhonghuajia were also measured. The research system of the parasitic mechanism was preliminary constructed. The results are stated below.1. The adult pyemotes were crawling on the surface of the larvae when they found the host, and then tried to sting the larvae, the larvae became paralyzed which was much easier for the pyemotes feeding. However, the mites would not fasten itself on the surface; they continued searching the spot more appropriate to be fixed. The epidermis of the larvae turned brown and there were so many spots in the surface. The pyemotes grew bigger after feeding, and the mites began to fasten on the surface when the turgor abdomen body was 2 or 3 times as large as the adult mites, which was unlike the previous reports. The pyemotes always fasten themselves at the segmacoria. The pyemotes fed on the whole body of host finally.2. The feeding behavior of the pyemotes and the selection of pyemotes to different hosts and different instars were observed using Six-Arm-Frame. The results showed that the pyemote were apt to the first larvae at the beginning.12 hours later, there were more pyemotes feeding on the surface of the second and third larvae. There was a significance different selection between the second larvae of Semanotus bifasciatus and the same instar of Adosomus sp. for the pyemotes. The significance was different at 8h,12h and 48h compared with the beginning. The host with low hardness epidermis and relatively high nutrition was easily be parasitized by the pyemotes.3. After being infested by Pyemotes zhonghuajia, the epidermis of the third larval instars and pupa of Semanous bifasciatus was observed under the scanning electron microscopy (SEM). The results showed that 5 kinds of sensillum were found in the third instar larvae including trichodea sensilla, chaeticum sensilla (â… ,â…¡,â…¢andâ…£) and 4 kinds of sensillum were found in the pupa including basiconca sensilla (â… ,â…¡,â…¢andâ…£). The distribution of these sensilla on the cuticle played an important role on the selection of the mites. Up to a certain degree, the parasitic mite was selective to the infection spot at the beginning. The pyemotes usually chose the segmacoria from the second to the sixth of larvae to parasitize, but not the same of the papae. The epidermis after being stabbed by stylets of the pyemotes formed an angled "D", which matches with the head construction of the pyemotes.4. Changes on the total protein content in hemolymph of Pyemotes zhonghuajia and Semanotus bifasciatus after infected by P. zhonghuajia was tested. The results showed that the total protein in the hemolymph of pyemotes turned out to be upward, while S. bifasciatus declined straight. There was a max range in the total protein of P. zhonghuajia between 36h and 48h, up to 61.78%. The content of S. bifasciatus changed significantly compared with control from 36h to 48h. The pyemotes began to search for the host and have a positive effect from 24h to 36. The critical stage that the pyemotes caused the larvae dead was from 36 to 48h.
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