Helicoverpa armigera (Hubner): can wheat stubble protect cotton plants against attack?

When investigating strategies for Helicoverpa armigera (Hubner) control, it is important to understand oviposition behaviour. Cotton (Gossypium hirsutum) was sown into standing wheat (Triticum astivum L.) stubble in a closed arena to investigate the effect of stubble on H. armigera moth behaviour and oviposition. Infrared cameras were used to track moths and determine whether stubble acted as a physical barrier or provided camouflage to cotton plants, thereby reducing oviposition. Searching activity was observed to peak shortly before dawn (03:00 and 04:00 h) and remained high until just after dawn (4 h window). Moths spent more time resting on cotton plants than spiralling above them, and the least time flying across the arena. While female moths spent more time searching for cotton plants growing in wheat stubble, the difference in oviposition was not significant. As similar numbers of eggs were laid on cotton plants with stubble (3.5/plant SE +/- 0.87) and without stubble (2.5/plant SE +/- 0.91), wheat stubble does not appear to provide camouflage to cotton plants. There was no significant difference in the location of eggs deposited on cotton plants with and without stubble, although more eggs were laid on the tops of cotton leaves in wheat stubble. As the spatial and temporal distribution of eggs laid on the cotton plant is a crucial component of population stability, eggs laid on the upper side of leaves on cotton plants may be more prone to fatalities caused by environmental factors such as wind and rain. Therefore, although stubble did not influence the number of eggs laid, it did affect their distribution on the plant, which may result in increased mortality of eggs on cotton plants sown into standing wheat stubble.

Structure and ultrastructure of the silk glands and spinneret of Helicoverpa armigera (Hubner) (Lepidoptera : Noctuidae)

This study provides comprehensive documentation of silk production in the pest moth Helicoverpa armigera from gland secretion to extrusion of silk thread. The structure of the silk glands, accessory structures and extrusion apparatus are reported. The general schema of the paired silk glands follows that found for Lepidoptera. Morphology of the duct, silk press, muscle attachments and spigot are presented as a three-dimensional reconstruction and the cuticular crescent-shaped profile of the silk press is demonstrated in both open and closed forms with attendant muscle blocks, allowing advances in our knowledge of how the silk press functions to regulate the extrusion of silk. Growth of the spigot across instars is documented showing a distinctive developmental pattern for this extrusion device. Its shape and structure are related to use and load-bearing activity. (c) 2005 Elsevier Ltd. All rights reserved.

Oviposition and reproductive performance of a generalist parasitoid (Trichogramma pretiosum) exposed to host species that differ in their physical characteristics

The response of generalist egg parasitoids to alternative natural hosts that are present simultaneously is not well known. We investigated the behavior of Trichogramma pretiosum Riley (Hymenoptera: Trichogrammatidae) in relation to two field hosts Helicoverpa armigera Hubner and Spodoptera litura Fabricius, in choice and no choice tests. We quantified the effects of natal host species and post-emergence adult age on the oviposition preference of the parasitoids. H. armigera eggs were consistently preferred over S. litura eggs, regardless of the natal host and adult age. When only S. litura eggs were available as hosts, they were parasitized at statistically similar rates to H. armigera eggs (average of 17 +/- 2.7 vs. 13 +/- 3.0, H. armigera to S. litura). The adult lifespan and lifetime fecundity of T. pretiosum were variable but were affected by natal host species and/or host species to which they were exposed. Mean lifespan and fecundity of parasitoids that had developed in H. armigera eggs and were exposed to H. armigera eggs for oviposition were 13.9 +/- 1.8 days and 98.7 +/- 11.0 adult offspring. By contrast, those that developed in S. litura eggs and were exposed to S. litura eggs for oviposition lived for 7 +/- 0.9 days and produced 53.8 +/- 8.0 adult offspring. The ovigeny index (OI) was significantly lower in the parasitoids exposed to H. armigera eggs than in those exposed to S. litura eggs, regardless of the natal host, indicating that H. armigera eggs sustain the adult parasitoids better than S. litura eggs. These results are used to predict parasitoid behavior in the field when both hosts are available. (c) 2006 Elsevier Inc. All rights reserved.