Abstract
The transition toward more sustainable plant protection with reduced pesticide use is difficult, because there is no “silver bullet” available among nonchemical 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 10 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 provide 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.
Highlights
Modern crop production is heavily dependent on synthetic pesticides, which are known to interfere with important ecosystem services such as pollination and biological control (Ma et al, 2000; Stanley et al, 2015) and may impair human health (McCauley et al, 2006)
Plant resistance significantly reduced leaf damage throughout the experiment (Table 2; Figures 2A,B; Supplementary Tables S1 and S2, Supplementary Figures S1A,B): that is, plants growing in susceptible-only plots had more herbivore damage compared to plants growing in resistant or resistance mixture plots
Susceptible varieties had fewer herbivore damaged leaves when growing in resistance mixtures with resistant plants compared to when they were growing among other susceptible varieties (Tables 3A,B; Figures 4A,B) but there was no such effect for resistant plants (Tables 3A,B)
Summary
Modern crop production is heavily dependent on synthetic pesticides, which are known to interfere with important ecosystem services such as pollination and biological control (Ma et al, 2000; Stanley et al, 2015) and may impair human health (McCauley et al, 2006). Most examples of associational resistance against herbivores have dealt with interspecific plant neighbors, but theoretically similar mechanisms could protect susceptible plant genotypes growing in the vicinity of resistant genotypes of the same plant species Such effects have been demonstrated for phytopathogens; for example, susceptible rice cultivars intermixed with resistant cultivars experienced reduced damage by fungal pathogens, leading to increased yields of the susceptible cultivars (Zhu et al, 2000). High intrinsic resistance of some cultivars interferes with herbivore performance (War et al, 2012) and increases the functional diversity of the plantation when intermixed with susceptible cultivars (Bustos-Segura et al, 2017) These and similar results suggest that the efficiency of cultivar mixing in reducing herbivory could be amplified if the traits responsible for resistance to herbivores are identified and optimized. We expected that associational resistance in resistance mixture plots would reduce herbivory on susceptible plants, that is, that the presence of resistant varieties would reduce damage on and increase yield of their susceptible neighbors compared to when susceptible varieties are grown alone (homogenously susceptible), and that this effect would increase with the number of varieties used
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