The role of food nutrition: effects on foraging behavior, dealing with plant secondary metabolites and parasites in Helicoverpa armigera larvae

Abstract

Nutrition has a fundamental influence on all living organisms. The amount and balance of different nutrients can impact the behaviour and physiology of animals. When offered nutritionally complementary diets, animals tend to regulate their intake of nutrients to a point that is optimal for their requirements. This mixing behaviour was investigated in early instar larvae of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) using a putative optimal artificial diet (OP) and high protein (HP) and high carbohydrate (HC) artificial diets based on protein (p) and carbohydrate (c) ratios. Larvae were allowed to choose between the same kind of diet cubes (effectively no-choice), or diet cubes with different protein-to-carbohydrate (p: c) ratios (choice test). In no-choice tests, I found that first instar larvae remained longest on OP diet and spent least time on HC diet, whilst third instar larvae remained longest on HC diet and spent least time on OP diet. First instar larvae moved most when provided with HC diet, while third instar larvae moved most when provided with OP diet. However, both stages moved least when allowed to choose between diet cubes with different p: c ratios. The relative growth rate decreased when larvae moved often, but this influence was not evident when larvae fed on HC diet. Larvae that fed only on HC diet had the highest relative growth rate, followed by larvae with access to all diets simultaneously, indicating a behaviour to mix nutrient intake. Insect herbivores encounter secondary metabolites when feeding on host plants. How insects deal with plant secondary metabolites in a complex nutrient environment is unclear. The influence of a classic plant secondary metaboliteallyl glucosinolate (sinigrin), and its hydrolysed product allyl isothiocyanate (AITC) on the development of H. armigera when fed on diets with different p: c ratios was investigated, and metabolized products in the frass were analysed. As expected, AITC had a greater effect than sinigrin on H. armigera survival, development, and pupal weight. However, AITC at low doses appeared to have a positive effect on the parameters above. Both sinigrin and AITC can induce detoxification activity in the gut, and the reaction was related to diet protein concentration. High protein diet can provide adequate free amino acid, especially cysteine, that is part of the detoxification process. Nutrient content of the diet plays an important role influencing how plant secondary metabolites are handled. Infection by parasites is another stressor that insects may encounter. A new parasite that is similar to Ophryocystis elektroscirrha (McLaughlin & Myers) (Neogregarinorida: ii Ophryocystidae) (OE) was found on H. armigera. OE is a relatively benign neogregarine protozoan parasite that infects monarch (Danaus plexippus (Linnaeus) (Lepidoptera: Nymphalidae)), queen ((Danaus gilippus (Cramer) (Lepidoptera: Nymphalidae)) and lesser wanderer (Danaus petilia (Stoll) (Lepidoptera: Nymphalidae)) butterflies. Spores of this new OE-like parasite were insp...