Herbivory and traits of Lupinus polyphyllus

Glyphosate is the most widely used non-selective herbicide in the world.
Glyphosate residues in soil can affect plant quality by modifying plant
physiology, hormonal pathways, and traits, with potential consequences for
plants’ interactions with herbivores. We explored these indirect effects
in the context of plant-herbivore interactions in a perennial,
nitrogen-fixing herb. We quantified leaf herbivory for glyphosate-exposed
and control plants grown in phosphorus-fertilised and non-fertilised
soils, and assessed the impacts of glyphosate treatment on traits related
to plant resistance against herbivores (leaf trichome density, leaf mass
per area) and performance (aboveground biomass, root:shoot ratio, nodule
number, nodule activity). Moreover, we conducted a laboratory feeding
experiment to compare the palatability of leaves from glyphosate-exposed
and control plants to a generalist mollusc herbivore. Herbivore damage and
intensity in situ increased during the growing season regardless of
glyphosate or phosphorus treatment. Glyphosate treatment reduced leaf
trichome density, but had no effect on the other plant traits considered.
Herbivore damage was negatively associated with leaf trichome density. The
feeding experiment revealed no difference in the feeding probability of
mollusc herbivores between glyphosate-exposed and control plants. However,
there was an interaction between glyphosate treatment and initial leaf
area for leaf consumption by herbivores: leaf consumption increased with
increasing leaf area in both groups, but at a lower rate for
glyphosate-exposed plants than for control plants. Our results show that
glyphosate residues in soil have the potential to indirectly affect
aboveground herbivores through changes in leaf quality, which may have
mixed consequences for folivore damage.