Arthropods Associated with Amaranthus hybridus in Southwestern Nigeria and Aggregation Patterns of Gasteroclisus rhomboidalis, Hypolixus nubilosus (Coleoptera: Curculionidae) and Brown Marmorated Stink Bug, Halyomorpha halys (Hemiptera: Pentatomidae) in Relation to Host’s Morphology

Aims: To generate data on arthropod communities associated with grain amaranth, Amaranthus hybridus (Caryophyllales: Amaranthaceae) within a single organic cropping system in Southwestern Nigeria and to study arthropod-host interactions which can be potentially exploited in cultural management.

Place and Duration of the Study: The study was conducted from September to November, 2016 and repeated in December, 2016 to February 2017 during the dry season in Southwestern Nigeria.

Methodology: The farm area was divided into twenty plots containing four beds each. The total area of the field was approximately 180 m2. Survey of arthropods on amaranth was commenced at six days after sowing and continued until maturity. Scheduled daily visits to the field was done at 6:00-9:00 am, 12:00 noon-3:00 pm and 6:00 pm-8:00 pm to scout for insects. At 40-45 days, when the flowering heads have developed,the height of 24 plants per plot were measured and ranked randomly into three height ranges (Tall, >100 cm; Intermediate, 50-100 cm; Short, <3 cm), using a marked meter stick. Another 24 plants per plot were classified into three stem girth ranges (Large, > 6 cm; Intermediate, 3-6 cm; Thin, <3 cm), using a calliper and the plants were tagged. Density of Halyomorpha halys on the inflorescence of the plants in the different height groups were sampled. Activities of borers, Hypolixus nubilosus and Gasteroclisus rhomboidalis mixed infestation on plants in the different stem girth ranges were assessed, using stem damage characteristics.

Results: Approximately, 41.7% of the total arthropods occurred during the vegetative stage while the remaining 58.3% occurred during flowering and fruiting. Beneficial insects, (25% of the total arthropod community) were predatory and they occurred during fruiting and flowering stages. The beneficial insects, Dolichurus spp., Scelifron spirifex and Lasius niger belong to the Order Hymenoptera. Other beneficial insects were Dysnactus trigorus (Diptera), Cora marina (Odonata) and Mantis religiosa (mantodea). H. halys was responsible for damage to the flowering heads, while the stems were attacked by H. nubilosus and G. rhomboidalis.

Conclusion: The borers showed behavioural preference in relation to stem girth of amaranth plants while H. halys preferred tall plants. The observed host preference behaviours can be useful in cultural management.