Biological control of Diloboderus abderus (Coleoptera: Scarabaeidae) larvae using Steinernema rarum CUL (Nematoda: Steinernematidae) and Heterorhabditis bacteriophora SMC (Nematoda: Heterorhabditidae)

Abstract

The advance of no-till management systems in agriculture favored the increase of Diloboderus abderus (Sturm) populations in most of the cultivated areas in South America. Damage to wheat crops (Triticum aestivum Linnaeus) is caused by the larvae, commonly known as white grubs, which consume seeds and roots, weakening or killing the plants. Entomopathogenic nematodes (EPNs) are insect parasites usually used as biological control agents of larvae of Scarabaeidae species. However, a great variability in pathogenicity and virulence against different white grubs has been observed among EPN species and isolates of a single species. The aim of this work was to evaluate the pathogenicity of Steinernema rarum (de Doucet) CUL isolate, Heterorhabditis bacteriophora (Poinar) SMC isolate and their symbiotic bacteria against D. abderus larvae. Wheat plants were grown in plastic pots under greenhouse conditions and one larva (L1, L2 or L3) of D. abderus was placed in each pot and then inoculated with 1000 EPN IJs. H. bacteriophora caused 95 and 45% mortality to L1 and L2, respectively. No L3 mortality caused by the studied isolates was observed. Field experiments were conducted in wheat plots in 2013 and 2014. Treatments consisted of surface application of S. rarum or H. bacteriophora (2.5 × 109 IJs/ha) and chlorpyrifos; control treatments contained only distilled water. Evaluations were made at day 0, 30 and 60 after treatments by randomly selecting eight sites from each plot and determining the number of D. abderus L2. Larval populations were reduced in plots treated with H. bacteriophora and chlorpyrifos. Larval populations were reduced in plots treated with H. bacteriophora and chlorpyrifos. The plots treated with S. rarum did not show differences in the insect population with respect to the control. EPN symbiotic bacteria were injected into the hemocoel of each larval stage to determine virulence against D. abderus. D. abderus larval mortality caused by symbiotic bacteria was significantly influenced by bacterial concentration and larval stage. H. bacteriophora showed a greater reproductive capacity than S. rarum. We conclude that H. bacteriophora SMC have potential for use as biological control agents of D. abderus first and second-instar larvae.