Discussion
In contrast to the N limitation hypothesis, we showed that females
preferred (Fig. 2), survived longer (Fig. 3), and laid heavier eggs
(Fig. 4) on plants that had lower protein:carbohydrate ratios. Males
showed no difference in preference or survival to the fertilization
treatments. We ran the plant preference experiment five days after
fertilizing the millet. Despite there only being a marginal effect on
carbohydrate content at that point (Fig. 1A), female locusts selected
control plants over plants treated with high fertilizer. At that time,
there was no effect of fertilization on specific leaf area (SLA). After
12 days, SLA was lower in the control treatment than the high
fertilization treatment, indicating that control plants were tougher. It
is thus unlikely that leaf toughness played a significant role in plant
choice or locust performance in this study. After 18 days, the addition
of nitrogen to soil resulted in both higher protein and lower
carbohydrate levels in the millet leaves (Fig. 1), most likely because
carbon was mobilized for protein building (42).
Male body size may be more important than their diet in contributing to
egg size. Typically, egg size is positively correlated with the female’s
size and nutritional investment (43, 44); however, in this case total
egg mass was correlated only with male body size (Table 3). Presumably
this relationship is due to male contributing energetically to egg
production via the spermatophore that is transferred to the
females during mating. Nutrient transfer from the spermatophore to the
eggs has previously been demonstrated for grasshoppers although the
exact mechanism is poorly understood (45, 46). Females, relative to
males, were more attuned to dietary macronutrients, potentially due to
the significant impact on female survival (Figure 3) and/or the effect
of female nutrition on egg size (Figure 4). Larger eggs yield offspring
with higher performance traits across many species, suggesting egg size
is a good correlate of fitness (44). To our knowledge this is the first
time that increased plant protein:carbohydrates ratios have been shown
to have a direct negative impact on reproductive output in field
populations.
We hypothesize that migratory species are more likely to be carbohydrate
limited because migration is energetically costly. Flying metabolic
rates can be 20-100 times that of resting and insect thoracic muscles
have the highest known mass-specific oxygen consumption rates (47–49).
To accumulate fat reserves before and during migration animals usually
rely on nutrient and diet selection (36, 49, 50). Accordingly, in the
laboratory, several locust species have been reported to self-select
carbohydrates-biased intakes both as nymphs and adults (18, 51) and
locusts will increase carbohydrate consumption after long-distance
flights (52). By contrast, a study on millet in the United States found
that fertilization did not affect levels of herbivory for two
non-migratory grasshopper species (53). Female locusts may have been
more sensitive to carbohydrate limitation than males in this study
because females likely have higher energetic demands for flight (54).
Female grasshoppers, including locusts, are significantly larger than
males (55). For many species migration occurs pre-reproductively (56,
57) but this is not always the case for locusts and few other insect
species. For instance, female green darners (Odonata) migrate along the
eastern seaboard of the United States while gravid (58). Female O.
senegalensis (the current study species) can reproduce two to three
times during their lifetime, amongst migration events, and an ootheca’s
mass can exceed 150 mg, 20% or more of their body mass (59). These
factors may contribute to females having higher carbohydrate demands,
despite the established role of protein in optimization of reproductive
output (14, 60, 61).
Free-living locust populations may have difficulty securing these
preferred high carbohydrate, low protein diets. Despite the study taking
place in an arid environment, which are usually described as being N
poor (62, 63), most plants likely do not allow locusts to meet their
carbohydrate demands. We previously showed that the majority of
potential host plants across all major land use types in the region
(millet, groundnut, fallow, and grazing) were protein biased relative to
their preferred macronutrient balance of p1:c1.6 (64). Fields with the
lowest soil organic matter hosted plants with the lowest N content,
which attracted the most locusts (65). O. senegalensis is a major
millet pest; however, the millet leaves in this experiment, including
unfertilized millet, were protein biased relative to locusts’
carbohydrate biased demands (Figure 1; 62). For grasshoppers, including
locusts, carbohydrates are more difficult to extract from plants than
protein (66), potentially exacerbating the carbohydrate limitation
further.
There is increasing evidence that field populations of locusts and other
animals can be carbohydrate, not protein, limited (51, 67–69). Our
study illustrates a field scenario where carbohydrate limitation can
negatively affect fitness, suggesting far-reaching implications for
population and evolutionary dynamics. While plant carbon and nitrogen
are widely recorded for ecological studies, plant carbohydrate content
is rarely measured in relation to herbivore performance (70), despite
the demonstrated importance of macronutrient ratio on growth, survival,
lifespan, and reproduction across many taxa (14, 14, 60, 61, 71–77).
This study reinforces the realization that animals have different
nutrient demands based on life history and the importance of non-protein
energy for herbivore communities. This research provides new insights
for a complementary hypothesis to the N-limitation paradigm: the
carbohydrate limitation hypothesis.