Defoliating Insect Immune Defense Interacts With Induced Plant Defense During a Population Outbreak

Abstract

During population outbreaks, top-down and bottom-up factors are unable to control defoliator numbers. To our knowledge, details of biotic interactions leading to increased population density have not been studied during real population outbreaks. We experimentally assessed the strength of plant defenses and of insect immunocompetence, assumed to contribute to active insect resistance against parasitoids and pathogens, in the geometrid Epirrita autumnata during a steep increase in population density. We demonstrated rapid (same-season) induced resistance in the foliage of its host, mountain birch. The response was systemic, spreading throughout the tree, and retarded larval growth rate by approximately 10%. On the other hand, no direct delayed carry-over effects were found in the next season in larval growth rate, mortality, or pupal mass. Larval damage to a tree during the previous year, however, significantly (by approximately 13%) accelerated the advance of the immune response (measured as melanization of an implant inserted into the pupal hemocoel). The encapsulation rate correlated positively with larval mortality in trees in which larvae had been introduced the previous year, but not in control trees. Both of these observations suggest that induced plant defense was associated with an increased insect immunocompetence during the population increase.

Foliar Phenolics are Differently Associated with Epirrita autumnata Growth and Immunocompetence

“…Setup In the summer of 2002, 100 mature mountain birches were randomly selected from a natural population near the Kevo Subarctic Research Station (Northern Finland, 69°45′N, 27°01′E), as described in Kapari et al (2006). The birches were mature with at least three ramets (stems).…”

“…We inserted an implant (a nylon monofilament, 2 mm long, 0.20 mm in diameter, rubbed with sandpaper) into the hemocoel of 1-wk-old pupae. Pupae were incubated +14°C for 1 hr to allow encapsulation and then frozen in liquid nitrogen (the method is described in detail in Kapari et al 2006). Later, the implant was removed and photographed from three different angles under a light microscope with a digital video recorder.…”

“…Later, the implant was removed and photographed from three different angles under a light microscope with a digital video recorder. As a measure of encapsulation rate, we used the average gray values of three video pictures (the observed gray values were subtracted from the gray value of a clean implant (see details in Kapari et al 2006). The ability to encapsulate abiotic material is related to the ability to encapsulate a parasite (Paskewitz and Riehle 1994;Gorman et al 1996).…”

“…Survival was measured as the number of larvae in one tree surviving until pupation. Survival percentages ranged from 0 to 93%, with a mean of 42% (SD=20) (Kapari et al 2006).…”

“…These analyses search for factors with delayed, negative density dependence, implicitly suggesting that the increase phase does not demand any explanation at the chemical level. Kapari et al (2006), however, have demonstrated that during the early increase phase of the Epirrita autumnata (Bkt.) Lep., Geometridae (autumnal moth) cycle, the immune defense of this natural and important defoliator of the B. pubescens varies among individual host trees.…”