Agriculture Reference
In-Depth Information
Macro-invertebrates: root and exudate grazers and litter feeders
Rhizophagy is not a common feeding habit in soil macro-invertebrates and only a limited
number of studies have been conducted on the abundance and impact of rhizophagous
organisms on root production in the absence of severe attack on roots; such studies are
particularly rare in natural ecosystems. In most cases, estimates are based on assumed
feeding regimes. In the humid savannas of Lamto (Côte d'Ivoire), rhizophagous inver-
tebrates only comprise 0.3 % of the total biomass of communities (Lavelle, 1983a).
Higher estimates have been made for a range of diverse ecosystems: 3.8 % of biomass in
forest patches in tropical Mexico and 40.3 % in adjacent pastures where larval Coleoptera
may become serious pests (Lavelle
et al
., 1981); 2.55 g dry weight (23.6 % of biomass)
in a Swedish grassland soil (Lohm and Persson, 1977). Scott
et al.
(1979) estimated
the total consumption of underground parts by soil organisms at 7-26 % of root biomass.
In agro-ecosystems where organic
resources are scarce, relative consumption is
generally enhanced.
Rhizophagous groups include a large number of larval Coleoptera and Diptera,
Myriapoda, Homoptera, some Acari, and a number of other micro-arthropods. Major
groups of soil invertebrates like termites, ants or earthworms do not seem to feed on
living roots to a significant extent. Baylis
et al.
(1986), however provide some isotopic
evidence for root feeding in temperate earthworms. Spain
et al.
(1990) using natural
abundance of in biomass of the earthworm
Pontoscolex corethrurus
in a sugarcane
plantation concluded that these worms feed partly on fresh sugarcane root material
which may comprise exudates and dead or living root biomass. However, as indicated
in Chapter I, root litter is not a good quality resource for geophagous earthworms.
The endogeic tropical earthworm
Millsonia anomala
was fed soil mixed with 1 % organic
matter of different origins. When fed fresh and two-week-decomposed root litter of
savanna Gramineae,
M. anomala
had a lower growth rate than in the control 0-10 cm soil
(Lavelle
et al.,
1989). In this case, root material had high concentrations of secondary
compounds that were detrimental to growth.
In an experiment where clover (
Trifolium repens
) had been labelled with
in situ
for six hours on six different occasions, a number of soil invertebrates became radioactive,
thus demonstrating feeding on living roots. An unexpected 22.9 % of earthworms
appeared to feed on live roots. Other root feeders included larval Coleoptera (23.1 %),
'heat-extracted Annelids'
i.e.,
Enchytraeidae plus earthworms (13.8 %), Collembola (0.9 %)
and nematodes (0.3 %), whereas Acari, adult Coleoptera and Araneae did not show
any labelling (Baylis
et al.,
1986). Observations by Piearce (1978) and Ferrière (1980) on
the gut contents of lumbricid earthworms occasionally report the occurrence of root
debris in guts of some temperate Lumbricidae. In tropical soils, specialised root feeding
species have not yet been found. However, root litter is the major source of young
organic matter in a number of ecosystems and sapro-rhizophages comprise a large
proportion of the endogeic invertebrates considered to be geophagous or humivorous.
A wide taxonomic spectrum of arthropods may achieve crop pest status through their
severe effects on crop yields. They include Myriapoda (Chilopoda, Diplopoda and
Symphyla), Acari, Collembola, Hemiptera, Orthoptera (Gryllotalpidae), Coleoptera,
Diptera, Lepidoptera and some Isoptera (termites). Calculated yield losses from these
pests may range from less than 10 % up to 40-50 % (Schwartz and Classen, 1981).
