9.1.1. Root-feeding insects
Out-of-sight herbivores feeding on the roots of plants have been neglected in studies of insect—plant interactions, although it is recognized that 50–90% of plant biomass may be below ground. Root-feeding activities have been difficult to quantify in space and time, even for charismatic taxa like the periodic cicadas (Magicicada spp.). The damaging effects caused by root chewers and miners such as larvae of hepialid and ghost moths, and beetles including wireworms (Elateridae), false wireworms (Tenebrionidae), weevils (Curculionidae), scarabaeids, flea-beetles, and galerucine chrysomelids may become evident only if the above-ground plants collapse. However, lethality is one end of a spectrum of responses, with some plants responding with increased above-ground growth to root grazing, others neutral (perhaps through resistance), and others sustaining subcritical damage. Sap-sucking insects on the plant roots such as some aphids (Box 11.2) and scale insects (Box 9.1) cause loss of plant vigor, or death, especially if insect-damaged necrotized tissue is invaded secondarily by fungi and bacteria. Although when the nymphs of periodic cicadas occur in orchards they can cause serious damage, the nature of the relationship with the roots upon which they feed remains poorly known (see also section 6.10.5).
Soil-feeding insects probably do not selectively avoid the roots of plants. Thus, where there are high densities of fly larvae that eat soil in pastures, such as Tipulidae (leatherjackets), Sciaridae (black fungus gnats), and Bibionidae (March flies), roots are damaged by their activities. There are frequent reports of such activities causing economic damage in managed pastures, golf courses, and turf-production farms.
The use of insects as biological control agents for control of alien/invasive plants has emphasized phytophages of above-ground parts such as seeds and leaves (see section 11.2.6) but has neglected root- damaging taxa. Even with increased recognition of their importance, 10 times as many above-ground control agents are released compared to root feeders. By the year 2000, over 50% of released root-feeding biological control agents contributed to the suppression of target invasive plants; in comparison about 33% of the above-ground biological control agents contributed some suppression of their host plant. Coleoptera, particularly Curculionidae and Chrysomelidae, appear to be most successful in control, whereas Lepidoptera and Diptera are less so.