Effects of endophyte infection and methyl bromide on surface-dwelling and edaphic arthropods in tall fescue

Tall fescue is a common, cool-season pasture grass in the United States. When infected with the endophytic fungus, Acremonium coenophialum, it is associated with tall fescue toxicosis. Studies have been conducted to determine the effects of this grass/endophyte interaction on mammals as well as herbivorous arthropods. There has been little previous work done to determine the effects of the interaction on predators and decomposers. The objective of this study was to determine whether the presence of Acremonium coenophialum changed the community structure and population dynamics of mesoarthropods in tall fescue fields. The effects of previous methyl bromide treatment in half of the fields were also examined.

Collembola (springtails), Acari (mites), and Carabidae (ground beetles) were collected with pitfall traps for at least six weeks per season for one year. An alcohol mixture was used as a preservative in the field. Pitfall contents were collected twice weekly. Soil cores were taken once a month and arthropods were extracted either with a Crossley-Blair high-gradient soil extraction or by means of a heptane flotation technique.

The effects of endophyte-infected tall fescue on Collembola were species-specific. Sphaeridia pumilus, Sminthurus fitchi, Sminthurinus henshawi, and Isotoma viridis often had significantly higher populations in endophyte-infected (E+) tall fescue fields. Homidia soda and Pseudosinella violenta were more abundant in endophyte free (E-) fields. Lepidocyrtus cinereus had similar population densities in both E+ and E- fields. Association measures for Collembola indicated that E+- tall fescue selected a particular community composition. Carabids were collected in relatively low numbers in all fields and did not seem to be affected by the presence of the endophyte. Acari were more abundant in E- fields and therefore may have been adversely affected by the endophyte.

The effects of previous methyl bromide treatment were more apparent on truly edaphic species such as Parajapyx isabellae and Epilhommania sp. However, Isotoma viridis, S. henshawi, S. fitchi, Sminthurinus elegans, and Sphaeridia pumilus were most abundant in methyl bromide treated (MB+) fields. Lepidocyrtus cinereus and P. violenta were more common in fields not treated with methyl bromide (MB-). Homidia soda showed no preference for MB+ or MB- fields. Carabid populations were also lower in MB+ fields. Groups of Acari acted differently with respect to methyl bromide. Galumna sp. and other Oribatida populations were higher in MB+ fields, but Epilohmannia sp. was higher in MB- fields.

Additional studies on mesofauna in E+ and E- tall fescues would benefit from monitoring the alkaloid concentrations in the field. Together the information could bring about a better understanding of how the alkaloids produced by the grass/endophyte interaction affect organisms within the field. A different method of soil core extraction should be used to give a better estimate of the soil fauna present.