The unique interaction between the summer annual desert plant Salsola inermis Forssk and weevils residing on its roots: mutualism or parasitism?

1. Herbivores and parasites are likely to impose less damage on their host when their growth rate is slow and their dependency on the host is high. Accordingly, it was hypothesised that evolution would favour neutral or even beneficial interactions between a below-ground herbivore and a plant during the harsh season in a desert ecosystem. 2. This study characterised the relationship between the summer annual plant Salsola inermis Forssk (Chenopodiaceae) and weevils developing in a mud chamber attached to its roots in the Negev Desert of Israel. Plant seedlings were exposed to adult weevils (Conorhynchus palumbus Olivier or Menecleonus virgatus Schoenherr; Coleoptera: Curculionidae) in a controlled outside setting, to induce oviposition and larval establishment. The following were quantified: plant growth, above-ground biomass, fruit biomass, and fruit size, as well as relative C and N contents, and isotopic signatures (δ¹³C and δ¹⁵N) in plant tissues. 3. Exposure to weevils did not reduce plant survival but significantly and negatively affected plant growth and seed production. However, these effects were mainly due to above-ground herbivory by adults rather than root herbivory by larvae, and might have been overestimated. Interestingly, %N and δ¹⁵N were significantly higher, and the C:N ratio was significantly lower, in plants with larval establishment, suggesting that weevils affect the plant nitrogen budget. 4. The overall results do not support the notion of mutualistic interactions; yet, slow consumption, a low infestation level, and, possibly, N supplementation to the plant may enable the plant to tolerate herbivory under natural conditions.