Resistance mixtures reduce insect herbivory in strawberry (Fragaria vesca) plantings: leaf damage and yield data

The transition towards more sustainable plant protection with reduced
pesticide use is difficult, because there is no ‘silver bullet’ available
among non-chemical tools. Integrating several plant protection approaches
may thus be needed for efficient pest management. Recently, increasing the
genetic diversity of plantations via cultivar mixing has been proposed as
a possible method to reduce pest damage. However, previous studies have
not addressed either the relative efficiency of exploiting cultivar mixing
and intrinsic plant herbivore resistance or the potential utility of
combining these approaches to increase cropping security. Here, using a
full factorial experiment with 60 woodland strawberry plots, we tested for
the relative and combined effect of cultivar mixing and intrinsic plant
resistance on herbivore damage and yield. The experiment comprised two
levels of diversity (“high” with ten varieties and “low” with two
varieties), and three levels of resistance (“resistant” comprising only
varieties intrinsically resistant against strawberry leaf beetle
Galerucella tenella; “susceptible” with susceptible varieties only; and
“resistance mixtures” with 50:50 mixtures of resistant and susceptible
varieties). The experiment was carried out over two growing seasons. Use
of resistant varieties either alone or intermixed with susceptible
varieties in “resistance mixtures” reduced insect herbivory.
Interestingly, resistant varieties not only reduced the mean damage in
“resistance mixtures” by themselves being less damaged, but also protected
intermixed susceptible varieties via associational resistance. The effect
of higher genetic diversity was less evident, reducing herbivory only at
the highest level of herbivore damage. In general, herbivory was lowest in
plots with high diversity that included at least some resistant varieties,
and highest in low diversity plots consisting only of susceptible
varieties. Despite this, no significant difference in yield (fruit
biomass) was found, indicating that strawberry may be relatively tolerant.
Our results demonstrate that combined use of high genetic diversity and
resistant varieties can help reduce pest damage and provides a useful tool
for sustainable food production. “Resistance mixtures” may be particularly
useful for sensitive food crops where susceptible varieties are
high-yielding that could not be completely replaced by resistant ones.