Abstract
Abstract Different profiles of secondary compounds are released by plants after herbivore attack. Many of these compounds are used by predators and parasitoids to locate herbivores that are damaging leaves. Such an induced indirect defense was tested with the Caryocar brasiliense-ant system in the dry season, when C. brasiliense has old leaves, and in the rainy season, when C. brasiliense has new leaves. A total of 20 plants were analyzed per season. Two opposite leaves of the same branch were selected for each plant. Approximately 40% of the area of one leaf was removed (treatment leaf) while the other leaf remained intact (control). The number of ants that visited each leaf was counted simultaneously for a period of 15 minutes and the mean difference in ant number was tested by paired t-test. The mean number of ants differed significantly between treatment and control only in the rainy season (t = 3.004, df = 19, p = 0.007). This finding suggests the presence of induced defense in this system only when the leaves are young. The study supports the Optimal Defense Theory since young leaves of C. brasiliense with artificial damage attracted significantly more ants than leaves without damage and represents the first evidence of an induced defense mechanism in the C. brasiliense-ant system.
Highlights
Plant defense strategies against herbivores can be categorized as either direct or indirect
The mean number of ants on treatment leaves was greater than that of control leaves in both seasons, the difference was only significant in the rainy season, suggesting induced defense in this system only when leaves are young
Herbivory of young leaves tends to have a greater effect on plant fitness than does herbivory on mature leaves (Jurik & Chabot 1986)
Summary
Plant defense strategies against herbivores can be categorized as either direct or indirect. Direct defenses negatively affect herbivore metabolism via toxic compounds and digestibility reducers (Speight et al 1999, Marquis 2012) whereas indirect defenses occur when the plant attracts other insects that can reduce herbivore damage (Bixenmann et al 2013). In the latter strategy, the plant provides chemical cues or rewards (food and/or shelter) to attract predators and parasitoids of the herbivores (Boege & Marquis 2005). Such new VOC profiles may inhibit oviposition or reduce herbivore performance (induced direct defense), or act as a cue for predators and parasitoids to locate herbivores that are causing damage (induced indirect defense) (Turlings et al 1995, Paré & Tumlinson 1999, Carroll et al 2006, Heil 2014). Kost & Heil (2006) demonstrated that VOCs released by damage plants could be recognized by undamaged neighboring co-specific plants, acting as a trigger for increased secretion of extrafloral nectar
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