The effect of intercropping beans on Eldana saccharina Walker (Lepidoptera: Pyralidae) arthropod predator populations in sugarcane.

Abstract

Commercial sugarcane in South Africa is a monoculture, and therefore lacks vegetation

diversity, which is instrumental in increasing associated faunal diversity. Diverse habitats tend

to support more stable populations of herbivorous and predacious animals. It is hypothesised

that lack of this diversity in sugarcane was partially responsible for the existence of Eldana

saccharina infestation levels that are higher than is commercially acceptable.

Amongst the available E. saccharina control strategies, is habitat management. This has been

developed with the view of increasing and enhancing predator foraging activity. Through

increasing arthropod predator abundance and activity, it is believed that E. saccharina control

may be enhanced. In this study, habitat diversity was increased through intercropping beans

within sugarcane. Arthropod populations were monitored throughout the sugarcane-growing

period, to determine what effect this intercropping had on known potential arthropod predator

populations of E. saccharina.

The study site was divided into two plots: the intercrop (beans planted within sugarcane rows:

sugarcane-bean intercrop) and sole sugarcane: control plot. At monthly intervals, epigeal

arthropods were sampled with pitfall traps, while foliage associated arthropods were sampled

with a suction trap. Predator activity at the base of the sugarcane stalk, where E. saccharina lays

its eggs was monitored with sticky traps. Sampling took place in the sugarcane-bean intercrop

and control plots as well as in the roadway bordering the study site. Epigeal predator habitat

preference was assessed by randomly placing pitfall traps in the sugarcane rows, bean rows,

interrows between sugarcane rows, interrows between sugarcane and bean rows and the

roadway. Corresponding with monthly trapping, an E. saccharina infestation and damage

survey was conducted. Environmental factors such as weather, light intensity, plant (beans and

sugarcane) phenology and weed density were measured, and their effect on E. saccharina

potential arthropod predators examined. At harvest, sugarcane stalks were sampled for sucrose

yield analysis.

Potential E. saccharina predators that were captured included species of the orders and/or

families Acarina, Blattidae, Formicidae (Pheidole megacephala and Dorylus helvolus) and

Araneida (Lycosidae, Oxyopidae, Thomisidae and Salticidae). P. megacephala and species of

Acarina were the only predators caught with all three trapping techniques, thus indicating that

they occurred both on the ground and foliage. D. helvolus and Acarina were the only predators

caught in significantly higher numbers in the intercrop, suggesting that increased habitat

management had positively affected their population sizes. D. helvolus were captured both on

the ground and length of sugarcane stalk, while species of Acarina were captured on the

ground, foliage and at the base of sugarcane stalk, indicating that they forage at the base of the

stalk, where E. saccharina activity is concentrated. Specific ground habitats preferred by D.

helvolus included the sugarcane rows and bean rows, while Acarina preferred the interrows

between sugarcane and bean rows.

Despite the generally low E. saccharina infestation levels during this study, significantly higher

levels of infestation occurred in the intercrop when compared to sole sugarcane. As expected

with high infestation, higher (although not statistically significant) damage occurred in the

intercrop. Surprisingly, sucrose yield and sugarcane stalk mass were slightly higher in the

intercrop.

The implications of the observations made during this study are discussed in the context E.

saccharina management.