Secretory avocado idioblast oil cells: evidence of their defensive role against a non‐adapted insect herbivore

Abstract

AbstractWe tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblast oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hübner) larvae are generalists that do not feed on avocados. By contrast, Sabulodes aegrotata (Guenée) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on avocados. All bioassays were performed at a naturally occurring concentration of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume more control than treated diet. In contrast, idioblast oil cells had no significant antifeedant effects on the adapted S. aegrotata and P. includens larvae. Subsequent experiments designed to assess resistance mechanisms separated pre‐ingestive (behavioral) and post‐ingestive (physiological) effects of the avocado idioblast oil cells, and the extracted idioblast oil, on the two adapted herbivores. Post‐ingestive adaptation was the mechanism that allows feeding. Because the impact of the avocado idioblast oil cells was greatest on the performance of non‐adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortality and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, reduced developmental rates, and antifeedant activity in the non‐adapted insect indicate that defense against non‐adapted herbivores may be an important function of idioblast cells in avocados.