Experimental Suppression of Red Imported Fire Ants (Solenopsis invicta) Has Little Impact on the Survival of Eggs to Third Instar of Spring-Generation Monarch Butterflies (Danaus plexippus) Due to Buffering Effects of Host-Plant Arthropods

Abstract

The eastern migratory population of the monarch butterfly (Danaus plexippus) has shown evidence of declines in recent years. During early spring, when the population is at its smallest, red imported fire ants (RIFA) (Solenopsis invicta) have been implicated as having devastating effects on monarch egg and larval survival, but there are no conclusive experimental data to support this contention. The purpose of this study was to determine the effect of RIFA on the survival of spring monarch eggs to third instar larvae. Three treatments were analyzed: control plots, RIFA-suppressed plots, and RIFA-enhanced plots. Other host-plant arthropods were also documented. In control plots, monarch survival was unrelated to RIFA abundance on or around the plants. For both years combined, RIFA suppression had little impact on monarch survival. In one of the two years, higher survival occurred in the suppressed treatment, but confidence in this difference was low. In control plots, monarch survival increased with increasing numbers of other arthropods (not including RIFA) on the host plant. Predator pressure did not vary relative to arthropod abundance, and RIFA only occupied plants in large numbers when large numbers of other arthropods were also present. The presence of RIFA did not affect predator pressure. RIFA artificially drawn onto host plants created artificially high predator pressure, and monarch survival was low. Long-term use of bait to control RIFA may not be cost-effective provided surrounding biodiversity is high. Efforts to promote spring monarchs should focus on promoting biodiversity in addition to planting milkweed.

Butterfly abundance is higher in areas treated for fire ants

“…Calvert (2004) later reported that monarch larval survival is higher within small exclosures (~20 m 2 ) treated for fire ants than outside. In contrast, other researchers in Texas reported that a higher percentage of monarchs survived to third instars closer to red imported fire ant mounds than farther away from fire ant mounds (Hudman 2018) and that fire ants had no significant effect on fifth instar survival (Scott 2019). Monarch survival from egg to fifth instar is very low (7–10%; Nail et al 2015) even in areas without fire ants, so it is not clear if fire ants decreased monarch survival, or if fire ants removed or replaced other predators (Calvert 2004, Hudman 2018).…”

“…In contrast, other researchers in Texas reported that a higher percentage of monarchs survived to third instars closer to red imported fire ant mounds than farther away from fire ant mounds (Hudman 2018) and that fire ants had no significant effect on fifth instar survival (Scott 2019). Monarch survival from egg to fifth instar is very low (7–10%; Nail et al 2015) even in areas without fire ants, so it is not clear if fire ants decreased monarch survival, or if fire ants removed or replaced other predators (Calvert 2004, Hudman 2018). Alternatively, Nixon (2019) reported that monarch survival was higher on plants with higher densities of alternative prey available.…”