Production of induced volatiles by Datura wrightii in response to damage by insects: effect of herbivore species and time.

Natural enemies of herbivorous insects utilize numerous chemical cues to locate and identify their prey. Among these, volatile plant compounds produced after attack by herbivores may play a significant role (hereafter herbivore-induced plant volatiles or HIPVs). One unresolved question is whether the composition of the volatile cue blends induced by different herbivore species differ consistently enough to indicate not only that the plants are damaged by herbivores but also the identity of the herbivore species causing the damage. We studied HIPV production in the undomesticated plant species Datura wrightii in the laboratory when damaged by either of two leaf-chewing herbivore species, Lema daturaphila or Manduca sexta, or when damaged by L. daturaphila and the piercing-sucking bug, Tupiocoris notatus, or both L. daturaphila and T. notatus, for 24 hr. HIPV production was monitored 1 d before induction, the day of induction, and for 7 d after induction. In all experiments, both the quantities and composition of the HIPV blends varied with the time since induction as different components reached peak production at different times after induction. HIPV blends did not differ consistently with the herbivore species causing the damage. For plants damaged by both L. daturaphila and T. notatus, greater amounts of HIPVs were produced than by plants damaged by either species alone, but the amounts did not differ from that predicted as the sum from damage inflicted by each herbivore species independently. The HIPVs of D. wrightii are a general rather than specific indicator of damage by herbivores. Because generalist predators are the most abundant natural enemies in this system, general cues of herbivore damage may be all that are required to facilitate the discovery by predators of plants damaged by any of several suitable prey species.

Production of herbivore-induced plant volatiles is constrained seasonally in the field but predation on herbivores is not.

Natural enemies of herbivorous insects utilize numerous cues to locate and identify their prey. One class of such cues is volatile organic compounds (VOCs) often produced by plants after attack by herbivores (hereafter herbivore-induced plant volatiles or HIPVs). Under simplified laboratory conditions, natural enemies often make clear choices between different HIPV blends, but such clear choices may be more difficult in more complex field environments. We studied the impact of VOC production by the undomesticated species, Datura wrightii on predation of eggs and larvae of Lema daturaphila by the omnivore, Geocoris pallens in the field. HIPV production in D. wrightii is developmentally and seasonally constrained to the early stages of plant growth even though L. daturaphila and G. pallens inhabit plants throughout the plant's growing season. We, therefore, asked if predation of L. daturaphila by G. pallens might be similarly constrained seasonally. Higher levels of VOC production were associated with higher levels of predation throughout the growing season, and the greater quantities of VOC production in May caused greater increases in predation than did VOC production later in the season (June-September). However, predation in the absence of VOC production ranged from 60-70% in June-September compared to only 14% in May, probably because plants were already colonized by predators later in the season. High levels of VOCs in response to herbivore damage by D. wrightii therefore may aid in the discovery of herbivore-damaged plants early in the season but the seasonal decline in VOC production does not limit predation of L. daturaphila by G. pallens later in the season.