According to the current catalogs, the biggest problem is the taxonomy of parasitoids. The two key factors deciding host range are host taxonomy and shared ecology of hosts. Environmental constancy can also affect host range. Experiments showed that constant environment can promote host specialization. Hosts feeding on early successional plants have fewer specialists than those feeding on late successional plants, because these hosts are less predicatable to parasitoids.
Molecular markers are obviously advantageous for host range study.
The biggest problem in traditional investigations is the rearing of hosts, and incapability of direct collection of field data. Molecular marker can solve these problems, and may be used in studies on parasitoid population, spatial distribution, parasitism and food webs in nearly real-time ways. There also exist interactions among parasitoids, which finally shapes parasitoid load. Host range and parasitoid load are the most important issues in parasitoid community ecology. There is a great difference between the number of parasitoid species attacking different hosts.