Assessment of the pest status, parasitism levels and efficacy of chemical control of Agrotis segetum (Lepidoptera: Noctuidae)

Abstract

The common cutworm, Agrotis segetum (Denis & Schiffermüller) (Lepidoptera: Noctuidae) is a pest of maize seedlings. During the past four planting seasons in South Africa, cutworms has resurfaced as a threat to maize production. No information existed on the economic impact of this pest in South Africa and since the chemical control of A. segetum has largely been described as inefficient, the need existed to investigate factors that may improve its management and affect insecticide efficacy. The overall objective is to improve our understanding of the influence of biotic and abiotic factors in controlling the population of A. segetum. The first objective of this study was therefore to conduct a survey among grain farmers to assess the economic impact of this pest, and to determine the possible association between cultivation practices, weed management strategies, chemical control approaches used and the severity of pest infestations. The survey revealed that the economic loss due to cutworm infestation of maize, soybean and sunflower amounted to R44.875 million on the 150 363 ha represented in this survey. Information provided by survey respondents indicated tendencies of higher pest infestation in no-till production systems, and in crop fields where weed control occurred less than three weeks prior to planting. Results further showed that the most affected crop was maize, and it is reported for the first time that this pest also has a significant economic impact on soybean production. Survey results showed that 97.0% of farmers applied insecticides for cutworm control and highlighted that applications were largely not done in accordance with label recommendations. Although pesticide label instructions indicate that insecticides must be applied at high water volumes (300 L/ha) and when the soil is damp, general tendencies recorded from questionnaire responses was that lower than the prescribed water volumes were used and that poor insecticide efficacy was experienced when products were applied under dry soil conditions. The second objective of this study was to establish the natural occurrence and diversity of biological control agents of A. segetum within the country's maize production regions. Therefore, cutworm larvae were collected from 14 localities and reared in a laboratory until the emergence of a biocontrol agent, or until they died or moths emerged. Overall cutworm mortality due to parasitism was high (43.6%). Three parasitoid species were identified, i.e., Macrocentrus collaris (Hymenoptera: Braconidae), Gonia bimaculata (Diptera: Tachinidae) and Periscepsia carbonaria (Diptera: Tachinidae). Cutworm mortality due to entomopathogenic viruses, nematodes and fungi were 26.1, 15.7 and 3.2% respectively. Since pesticide label instructions indicate that insecticides must be applied onto soil when the soil is damp, the third objective of this study was to assess the influence of different soil moisture levels on larval behaviour and to relate possible differences in larval behaviour to differences in the efficacy of insecticides. Third- (L3) and 5th-instar (L5) larvae were exposed to field-like soil moisture conditions (dry, damp or wet) in a laboratory and greenhouse setting. Results showed that the behaviour of L3 and L5 larvae differed under different soil moisture conditions. While L3 larvae showed limited movement into the soil, most of the L5 larvae moved to a depth of < 22 mm into the soil, irrespective of soil moisture level. This confirmed the photopositive behaviour of L3 larvae, and tunnelling behaviour of L5 larvae, and showed that soil moisture conditions influence larval behaviour, which may have an indirect effect on insecticide efficacy.