The Role of Insect Flower Herbivory in Native and Restored Prairies

Several studies have shown that insect herbivory on an inflorescence can limit not only the number of seeds but also can influence seedling recruitment in a population, ultimately having an impact on the size of a population. However, the absence of insect herbivores can result in a plant species becoming very abundant and on some occasions weedy. In an ongoing study, the impact of insect flower herbivory (i.e., presence or absence) on the reproductive output and regulation of population size is being addressed using the prairie species Eryngium yuccifolium Michx. (Apiaceae), rattlesnake master (Fig. 1).

Rattlesnake master is a very striking prairie species, having leaves similar to the yucca plant and prickly white flower heads (Fig. 1). This species can be found in the southeast portion of the Great Plains and is found in 80% of Illinois. In Illinois, particular interest is placed on this species because it serves as a food source for the state-threatened Eryngium stem-borer moth (Papiapena eryngii Bird [Noctuidae], Fig. 2). In addition, this species is considered to be a highly conservative species (i.e., found primarily in undisturbed tallgrass prairie). Although thought to be a conservative species, in some cases it can become extremely abundant in native prairies or even weedy in restored prairies. Several potential reasons for such abundance are high seed set, combined with abundance in the seed bank, and good germination. However, this does not explain why in other prairies, both native and restored, we find only a handful of rattlesnake master individuals.

During the summers of 1998 and 1999 while conducting a research project to determine different aspects of the reproductive biology of rattlesnake master, I discovered a lot of flower herbivory, up to 60%, in a native population. This differed from a restored population that had only 1% flower herbivory. As a result of this finding, several questions started developing: Do all populations (native and restored) of rattlesnake master have this flower herbivory? Do we find the same flower herbivores in all rattlesnake master populations? Is the lack of flower herbivory the reason for the extreme abundance of this species in some prairies? Can the size and degree of isolation of a rattlesnake master population determine the presence or absence of this flower herbivory? Do restorations of different ages have the same level of flower herbivory?

To answer these questions, I have been gathering data on 20 rattlesnake master populations in Champaign, Ford, Livingston, Vermilion, and Will counties. Ten of these populations are found in native prairies and 10 in restored prairies. Both native and restored populations range from a few individuals to thousands, and all populations are in isolated prairie fragments. In the case of restored prairies, I am using restorations that are 6 to over 25 years old. For each of these sites I am also collecting data on species diversity. At each site, I collect 3 flower heads from 20 individuals, when possible, and determine percent flower herbivory and percent fruit. In the process of determining percent flower herbivory and percent fruit set, I have discovered that many of the populations have some degree of flower herbivory (Fig. 3). This is detected by the presence of holes in the fruits or the entire ovary being gone. However, it is too early in the study to determine differences between native and restored prairies or patterns associated with size and age of restorations.

Why should we be spending time studying this plant-insect interaction? As with plant-pollinator interactions, plant-herbivore interactions can be used as an indirect way to measure success for prairie restorations. For example, if herbivores are contributing to the long-term population control of a species and they are found in our restorations, then it suggests that the restorations are resembling native prairies. So what was presumed to be negative interaction (herbivory) can be viewed as positive interaction and can be used as an indicator to determine the health of a prairie.

Brenda Molano-Flores, INHS CTAP Group

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