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

We tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblas t oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hubner) larvae are generalists that do not f eed on avocados. By contrast, Sabulodes aegrotata (Guenee) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on a vocados. All bioassays were performed at a naturally occurring concentratio n of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume m ore control than treated diet. In contrast, idioblast oil cells had no sign ificant antifeedant effects on the adapted S. aegrotata and P. includens la rvae. Subsequent experiments designed to assess resistance mechanisms separ ated 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 a llows feeding. Because the impact of the avocado idioblast oil cells was gr eatest on the performance of non-adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortali ty and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, redu ced developmental rates, and antifeedant activity in the non-adapted insect indicate that defense against non-adapted herbivores may be an important f unction of idioblast cells in avocados.