Reduction of Feeding by Schistocerca piceifrons piceifrons (Orthoptera: Acrididae), Following Infection by Metarhizium anisopliae Var. acridum

Abstract

The Central American locust, Schistocerca pi ceifrons ssp. piceifrons (Walker), often becomes a major pest of staple and cash crops in Mexico and Central America (Barrientos-Lozano et al. 2002). In 1993, laboratory and field studies were initi ated to develop a biological control strategy for S. piceifrons. The National Centre for Biological Control (Centro Nacional de Referencia de Con trol Biol?gico-CNRCB) has in its entomopathogen collection 35 isolates of Metarhizium anisopliae var. acridum (M. a. acridum) from S. piceifrons in Mexico. Our laboratory studies have shown that the isolates MaPL32 and MaPL40 are 2 of the most virulent against S. piceifrons (Barrientos Lozano et al. 2002). Field trials with M. a. acridum oil-formulated spores have demonstrated effective control of S. gregaria Forskal in Africa (Langewald et al. 1997), Locusta migratoria (L.) and Chortoicetes ter minifera (Walker) in Australia (Hunter et al. 1999, 2001), Rammatocerus schistocercoides (Rehn) in Brazil (Magalhaes et al. 2001) and S. piceifrons in Mexico (Hern?ndez-Vel?zquez et al. 2003). One of the perceived disadvantages, evident in these tri als, is the length of time (13 to 14 d) that the target insect takes to die after the application. However, in practical terms, successful control of locust is re flected by a reduction in food consumption; a locust that has ceased to feed due to infection is no longer a significant pest (Moore et al. 1992). Studies by Moore et al. (1992) and Seyoum et al. (1994) both reported significant reductions in feeding by S. gregaria following infection with M. a. acridum; similar results were also observed with the grass hopper Zonocerus variegatus (L.) (Thomas et al. 1997). All of these reported studies were carried out using a high dose of the pathogen, in some cases with few insects per treatment, without con trols, and a mixture of instars. The aim of the present work was to examine the effect of a conidia low dose (6 x 104) per S. piceifrons adult on both feeding rate and feces production. First instars of S. piceifrons locusts were col lected in Tizim?n, Yucat?n, Mexico. Locusts were kept in 1 x 1 x 1 m cages inside a controlled envi ronment (CE) room and fed on a diet of leaves of the grass Cynodon dactylon (L.) and rolled oats. The mean daily temperature and relative humid ity (RH) in the CE room was 27?C and 70 ? 5% re spectively, with a photoperiod of 12:12 h (L:D). The fungal isolate was a monospore isolate (EH-502/8) of M. a. acridum MaPL40 from S. pi ceifrons (Hern?ndez-Vel?zquez et al. 1997). Fun gal conidia were produced by a diphasic method. In the first phase, the fungus was cultured in a liquid medium of 2% sucrose and 2% yeast extract (Jenkins & Prior 1993). Fungal biomass was transferred to rice in a plastic bag (250 g/bag) with a cotton filter for further growth and sporu lation. After 14 d the bags were opened in the CE room which allowed the fungus to dry to a mois ture content of approximately 9%. Conidia were separated from the rice by sieving through a 300 um mesh. The resulting conidia powder was then re-sieved through a 90-um mesh to remove any remaining rice-dust particles. Conidial suspensions were formulated in citro line (a mineral oil derived from petroleum for ag ricultural use in Mexico) by mixing 1 g of conidial powder in 100 mL with a magnetic stirrer. It was then adjusted to 3 x 107 conidia/mL with use of a Neubauer haemocytometer. Germination rates were determined by randomly examining 100 conidia per plate with a compound microscope (400x). A conidium was considered to have germi nated if the germ tube was at least as long as the width of the conidium. Germination of conidia was 95-100% immediately after formulation and just before locust infection. Each treatment consisted of 20 adult locusts, 7 d post-fledging, each receiving 2 uL of the formu lation applied on the pronotum via a Hamilton SGE syringe. Control groups received 2 uL of ei ther citroline or sterile distilled water with Tween 80 (0.1%, v/v, Difco ) without conidia. Following treatment, locusts were placed in subgroups of 4 insects in 13 x 10.5 cm plastic containers, covered with muslin. Containers were maintained at 28?C and 70 ? 5% RH. The experiment was replicated 4 times. Mortality was recorded daily for 12 d; dead insects were placed individually in Petri dishes on moist filter paper at 27?C for 4 d to allow growth