Herbivory in Myrtillocactus geometrizans (Cactaceae): Do Parasitoids Provide Indirect Defense or a Direct Advantage?

Abstract

Plants respond to herbivory in diverse, complex ways, ranging from avoidance or tolerance to indirect defense mechanisms such as attracting natural enemies of herbivores, i.e., parasitoids or predators, to strengthen their defense. Defense provided by parasitoids to cultivated plants is well documented and is used in biological control programs. However, its effectiveness on wild plants under natural conditions has been little studied. Such is the case of the cactus Myrtilllocactus geometrizans (known in Mexico as garambullo), which is consumed by the soft-scale insect Toumeyella martinezae (herbivore) which, in turn, is host to the parasitoid wasp Mexidalgus toumeyellus, and mutualist with the ant Liometopum apiculatum, that tenders and protects it. This study explores the role of the parasitoid as an indirect defense, by examining its effect on both the herbivore and the plant, and how this interaction is affected by the presence of the mutualistic ant. We found that scales adversely affect the cactus’ growth, flower, and fruit production, as well as its progeny’s performance, as seedlings from scale-infested garambullo plants were shorter, and it also favors the presence of fungus (sooty mold). The parasitoid responded positively to herbivore abundance, but the presence of ants reduced the intensity of parasitism. Our results show that parasitoids can function as an indirect defense, but their effectiveness is reduced by the presence of the herbivore’s mutualistic ant.

Damage in a saguaro (Carnegiea gigantea) population in the El Pinacate and Gran Desierto de Altar Biosphere Reserve, Sonora, Mexico

“…De Bonis et al (2017), Evans et al (2003, 2005), and Turner and Funicelli (2000) considered that this disease is an important factor that affects longevity of the individuals and increases mortality rates, causing a decrease in saguaro populations. Additionally, the absence of mycelial fungi in this tissue eliminates the alternative of a disease caused by fungi or rot, related to damage by insects as has been observed in cacti of the genera Cephalocereus , Isolatocereus , Pachycereus , Pilosocereus , Stenocereus , (Bravo‐Avilez et al, 2019), Trichocereus (Barbarich et al, 2021), and Myrtillocactus (Callejas‐Chavero et al, 2022).…”

“…The attack by curculionids is worrisome since the number of plants attacked as well as the number of species attacked seems to be increasing (Bravo‐Avilez et al, 2014; López et al, 2016). In addition, delving into this issue is important since recent studies have documented complex interactions between cacti and their herbivores where organisms from other trophic levels could be involved (Callejas‐Chavero et al, 2022). This example shows us the complexity of the relationships between herbivores and cacti that, despite their physical and chemical defenses, are attacked by organisms with special adaptations.…”