MINIMAL EVIDENCE FOR PLANT–INSECT ASSOCIATIONS IN THE SOUTH ASH PASTURE FLORA, MIDDLE PERMIAN OF NORTH-CENTRAL TEXAS

Levels of insect herbivory and floral diversity vary among Texan Permian floras, but the South Ash Pasture (SAP) plant assemblage is unprecedented in its minimal level of insect herbivory and low floral diversity. The SAP Flora is a mud-filled channel deposit and represents the youngest plant assemblage in the Blaine Formation (Guadalupian Epoch, 272-260 Ma). SAP consists of 15 recognized plant species, which has produced a rank abundance curve consisting of one very common strap-shaped, broadleaved conifer species, four less common conifers (including a parallel-veined large form) and the horsetail Sphenopyllum in intermediate abundance, and 12 rare conifer and gigantopterid species.

Of the 925 leaves evaluated for insect damage, only 3.6% displayed evidence of insect herbivory or oviposition. This herbivory was partitioned into five insect functional feeding groups on SAP leaves: margin feeding, hole feeding, piercing and sucking, skeletonization and galling. Two instances of oviposition also are present in SAP leaves. The heaviest concentration of herbivory occurred on the large and small gigantopterid species where 35.7% and 20% of the leaves, respectively, exhibited at least one type of insect damage. Other forms of herbivory occurred on the strap-shaped conifer and large, parallel veined conifer, the latter of which had ten times more damage than the former.

The low frequency of herbivory is almost certainly a result of the low plant diversity. A general tendency toward aridity during the middle Permian likely caused a shift in plant composition, which in turn reduced levels of insect herbivory. The lack of specialized insect herbivory also supports a turnover pattern in regional floras. The difference in damage frequency between the two species of broadleaved conifer may be due to differences in quantitative and/or qualitative secondary defense chemicals, typically enhanced in arid-adapted plants. Another possibility is co-association of the large, broadleaved conifer with a particular herbivore component community. The coarse-grained sedimentary rock of the SAP Flora also could have contributed to the low occurrence of insect herbivory.