Response of the cutworm Spodoptera litura to sesame leaves or crude extracts in diet

Abstract

The effects of extracts of sesame, Sesamum indicum L. (Liamiales: Pedaliaceae), and whole leaves of some selected cultivars of sesame were tested using a natural host Spodoptera litura (F.) (Lepidoptera: Noctuidae). Indices taken using the immature stages include; diet utilization, growth and development and induction of detoxification enzymes. The results indicate that S. litura generally selects its food amongst cultivars within 6 hours after food presentation. Growth and development of the insect is controlled also by plant acceptability and quality. Although all the cultivars tested significantly limit insect growth and development the variety 56S-radiatum did not allow a complete life cycle as pupation from first instar stage was 0%. Generally the crucial period for immature S. litura was the larval period, especially the first two instars where the weight of an insect fed on an experimental diet was three times lower than that of a control diet. The larval developmental period was greater than 40 days as compared to 17 days for insects fed a control diet. S. litura also had lowered efficiency in utilizing ingested food, from a low of 13% in a sesame cultivar to 45% in the control diet. The key detoxification enzyme was a glutathione s-transferase that was confirmed by a 6-fold increase between S. litura fed a plant cultivar vs. a control diet towards the substrate 1,2-dichloro-4-nitrobenzene. First and second instars of S. litura have a relatively reduced detoxification of enzymes in response to plant cultivar diets leading to low survival. A 3% v/w crude extract of the cultivars increased enzyme induction towards all the tested substrates.

Figure 1.

Schematic presentation of crude matter extraction from sesame leaves

Figure 2.

Survival beyond five days of the first 3 instars and pupation success from the first instar of Spodoptera litura fed with sesame leaves or a control diet. Hatched bar = First instar, open bar = Second Instar, solid bar = % Pupation. Second and third instars were initially reared on the control diet (n=30). % Pupation is the ratio of larvae that successfully developed from first instar to pupa with an initial number of 100 larvae.

Figure 3.

Food utilization per day of 3rd instar larvae of Spodoptera litura fed with sesame leaves for 5 days. Hatched bar = Weight of frass output, open bar = Weight of food intake, solid bar = % ECI. Bars are with means ± SD (n=30) ECI efficiency of conversion of ingested food by 3rd instars for 5 days: (weight gain/food consumed) × 100, (n=25).

Figure 4.

Change in detoxification enzyme induction between instars and also between whole leaf diets and plant extract diets towards various substrates. The bars represent the ratios of enzyme induction towards the respective substrates with the base line for no change at l on the y-axis: This is basically the row comparisons of Tables 2 and 3. Solid bars = 2^nd to 3^rd instar induction / 1st to 2nd instar induction, open bars = 3% v/w Extract induction / Whole leaf induction. Significant differences between induction ratios are denoted by *(P<0.05) and ** (P< 0.01) t-test.