Services and disservices provided by wood ants (Formica polyctena) in an apple plantation

Summary

Ants can due to their high numbers and by being organized as superorganisms provide a number of services for plants including commercial crops. First, ants may prey on or deter arthropod and in some cases even vertebrate pests (Way & Khoo 1992; Goheen & Palmer 2010). Secondly, they may fertilize their host plants by depositing fecal spots on plant tissue. These fecal spots may contain important nutrients that can be taken up directly by plant leaves (Vidkjær et al. 2016; Pinkalski et al. 2018). In addition, ants may enrich the soil around plant roots with increased nutrient levels due to their foraging activities (Folgarait 1998; Frouz et al. 2008; Wagner & Fleur Nicklen 2010). Lastly, ants may also provide plant services by reducing plant pathogen incidence. For example by eating fungal spores (Letourneau 1998), excrete antibiotics (Peng & Christian 2005; Gonzalez-Teuber, Kaltenpoth & Boland 2014) or by deterring diseases vectors (Letourneau 1998; Roux et al. 2011). Provision of these three services by ants may lead to highly effective biocontrol programs. In some cases ant based biocontrol is more efficient than conventional control based on synthetic pesticides, and even at lower costs (Offenberg 2015).
Not all ant activities are positive for plants. Many ant species attend honeydew-producing homopterans that feed on plant sap and that can act as disease vectors (Delabie 2001). As ants improve hygiene and protect homopterans partners against natural enemies, they may increase in numbers when ant-attended. In this way ants may become indirectly harmful to plants (Wäckers et al. 2017) even though cases exists where ant-attended homopterans show a positive net effect on plant fitness as the presence of homopterans lead to increased ant activity ultimately leading to protection against more detrimental herbivores (Styrsky & Eubanks 2007). The importance of this disservice to agriculture is illustrated by a number of research activities aimed at developing techniques to disrupt ant-homopteran symbioses in commercial crops. For example by preventing ants access to plant canopies by placing sticky barriers around tree trunks or by offering sugar to ants via artificial feeders (Nagy, Cross & Markó 2013; Nagy, Cross & Markó 2015; Wäckers et al. 2017). The latter will provide ants with a carbohydrate source that is an alternative to the honeydew produced by the attended homopterans. This alternative may result in ants neglecting their partners or even lead to ants starting to prey on them (Offenberg 2001).
In the present study we evaluated the services and disservices provided by wood ants (Formica polyctena) that was transplanted into an apple plantation to control winter moth larvae.