More intraguild prey than pest species in arachnid diets may compromise biological control in apple orchards

Understanding the full diet of natural enemies is necessary for evaluating
their role as biocontrol agents, because many enemy species do not only
feed on pests but also on other natural enemies. Such intraguild predation
can compromise pest control if the consumed enemies are actually better
for pest control than their predators. In this study, we used gut
metabarcoding to quantify diets of all common arachnid species in Swedish
and Spanish apple orchards. For this purpose, we designed new primers that
reduce amplification of arachnid predators while retaining high
amplification of all prey group. Results suggest that most arachnids
consume a large range of putative pest species on apple but also a high
proportion of other natural enemies, where the latter constitute almost a
third of all prey sequences. Intraguild predation also varied between
regions, with a larger content of heteropteran bugs in arachnid guts from
Spanish orchards. There was also a tendency for cursorial spiders to have
more intraguild prey in the gut than web spiders. Two groups that may be
overlooked as important biocontrol agents in apple orchards seem to be
theridiid web spiders and opilionids, where the latter had several
small-bodied pest species in the gut. These results thus provide important
guidance for what arachnid groups that should be targets of management
actions, even though additional information is needed to quantify all
direct and indirect interactions occurring in the complex arthropod food
webs in fruit orchards.