Remarkable susceptibility of the diamondback moth (Plutella xylostella) to ingestion of Pir toxins from Photorhabdus luminescens

Genes encoding Pir toxins were cloned and sequenced from Photorhabdus luminescens (Enterobacteriaceae) strain Hm. Cultures of Escherichia coli expressing the Pir A and B proteins were highly toxic when fed to larvae of Plutella xylostella L. (Lepidoptera: Plutellidae), as had been reported previously. Histological examination of P. xylostella larvae fed with recombinant E. coli revealed gross abnormalities of the midgut epithelium, with profound swelling and shedding of the apical membranes. However, the recombinant E. coli had no effect on the growth or mortality of larval Heliothis virescens F. (Lepidoptera: Noctuidae), Manduca sexta L. (Lepidoptera: Sphingidae), Lymantria dispar L. (Lepidoptera: Lymantriidae), or Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae). Based on these results, P. xylostella is at least 300‐fold more susceptible to Pir toxins than other insect species tested, suggesting that they may not be broadly useful as insecticidal proteins. Because Pir B has sequence similarities with N‐terminal portions of Cry proteins from Bacillus thuringiensis, we also tested the recombinant E. coli against a strain of P. xylostella that is resistant to the Cry 1A toxin, but found no difference in mortality between resistant and susceptible strains.     

Delaying evolution of insect resistance to transgenic crops by decreasing dominance and heritability

The refuge strategy is used widely for delaying evolution of insect resistance to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Farmers grow refuges of host plants that do not produce Bt toxins to promote survival of susceptible pests. Many modelling studies predict that refuges will delay resistance longest if alleles conferring resistance are rare, most resistant adults mate with susceptible adults, and Bt plants have sufficiently high toxin concentration to kill heterozygous progeny from such matings. In contrast, based on their model of the cotton pest Heliothis virescens, Vacher et al. (Journal of Evolutionary Biology, 16, 2003, 378) concluded that low rather than high toxin doses would delay resistance most effectively. We demonstrate here that their conclusion arises from invalid assumptions about larval concentration-mortality responses and dominance of resistance. Incorporation of bioassay data from H. virescens and another key cotton pest (Pectinophora gossypiella) into a population genetic model shows that toxin concentrations high enough to kill all or nearly all heterozygotes should delay resistance longer than lower concentrations.     

Insect resistance of transgenic tobacco expressing an insect chitinase gene

Chitinase expression in the insect gut normally occurs only during moulting, where the chitin of the peritrophic membrane is presumably degraded. Thus, insects feeding on plants that constitutively express an insect chitinase gene might be adversely affected, owing to an inappropriately timed exposure to chitinase. This hypothesis was tested by introducing a cDNA encoding a tobacco hornworm (Manduca sexta) chitinase (EC 3.2.1.14) into tobacco via Agrobacterium tumefaciens-mediated transformation. A truncated but enzymatically active chitinase was present in plants expressing the gene. Segregating progeny of high-expressing plants were compared for their ability to support growth of tobacco budworm (Heliothis virescens) larvae and for feeding damage. Both parameters were significantly reduced when budworms fed on transgenic tobacco plants expressing high levels of the chitinase gene. In contrast, hornworm larvae showed no significant growth reduction when fed on the chitinase-expressing transgenics. However, both budworm and hornworm larvae, when fed on chitinase-expressing transgenic plants coated with sublethal concentrations of a Bacillus thuringiensis toxin, were significantly stunted relative to larvae fed on toxin-treated non-transgenic controls. Foliar damage was also reduced. Plants expressing an insect chitinase gene may have agronomic potential for insect control     

The macroglomerular complex of the antennal lobe in the tobacco budworm moth Heliothis virescens : specified subdivision in four compartments according to information about biologically significant compounds

The functional organisation of the male specific macroglomerular complex in Heliothis virescens has been studied by tip recordings of sensilla trichodea type 1 combined with cobalt-lysine stainings and by intracellular recordings of antennal lobe projection neurons combined with neurobiotin stainings. The antennal lobe, the macroglomerular complex and the stained axons/dendrites were reconstructed by camera-lucida. Some were further computer reconstructed in three dimensions. The results showed that: 1) The macroglomerular complex consisted of four anatomically separated compartments; 2) A large compartment (the cumulus) at the entrance of the antennal nerve received input from receptor neurons responding to the major pheromone component; 3) Another large compartment, located dorso-medially of the cumulus (the dorso-medial compartment) received input from receptor neurons tuned to the second pheromone component; 4) Two ventrally located compartments received input from two receptor neuron types, co-localized in the same sensillum. Each neuron type responded strongest to one of two interspecifically acting signals, shown to interrupt the pheromone attraction. 5) The function of the dorso-medial compartment was further verified by selective arborizations in this compartment by a projection neuron showing strong response to antennal stimulation with the second pheromone component. At low concentration, the neuron responded synergistically to stimulation with the binary pheromone mixture. Accepted: 29 July 1998     

Physiology of interspecific chemical communication in Heliothis moths

Abstract Electroantennograms were recorded from the antennae of adult male and female corn earworms, Heliothis zea (Boddie). A total of seventeen female moth sex pheromone components from several species were tested. Of these, two components elicited significantly greater responses than the other fifteen. These were (Z)-11-hexadecenal, a conspecific component, and (Z)-9-tetradecenal, a component found in the pheromone blend of a sympatric species H.virescens (F.) that inhibits attraction of H.zea males. The results from dose-response and selective adaptation studies indicate that there are separate populations of receptors for these two chemical signals on the antenna of male H.zea. The more sensitive population is selective for (Z)-11-hexadecenal, while the less sensitive one responds to (Z)-9-tetradecenal. These findings provide a physiological basis by which H.zea males can distinguish the interspecific repellent from the conspecific pheromone blend. It is likely that this discrimination contributes to reproductive isolation between these two species.     

DIETARY SILVER NANOPARTICLES REDUCE FITNESS IN A BENEFICIAL,BUT NOT PEST,INSECT SPECIES

Silver nanoparticles (AgNPs) have antimicrobial and insecticidal properties and they have been considered for their potential use as insecticides. While they do, indeed, kill some insects, two broader issues have not been considered in a critical way. First, reports of insect‐lethal AgNPs are often based on simplistic methods that yield nanoparticles of nonuniform shapes and sizes, leaving questions about the precise treatments test insects experienced. Second, we do not know how AgNPs influence beneficial insects. This work addresses these issues. We assessed the influence of AgNPs on life history parameters of two agricultural pest insect species, Heliothis virescens (tobacco budworm) and Trichoplusia ni (cabbage looper) and a beneficial predatory insect species, Podisus maculiventris (spined soldier bug), all of which act in agroecosystems. Rearing the two pest species on standard media amended with AgNPs led to negligible influence on developmental times, pupal weights, and adult emergence, however, they led to retarded development, reductions in adult weight and fecundity, and increased mortality in the predator. These negative effects on the beneficial species, if also true for other beneficial insect species, would have substantial negative implications for continued development of AgNPs for insect pest management programs.     

PHRAGMOPLASTS IN CYTOKINESIS OF CEPHALEUROS PARASITICUS (CHLOROPHYTA) VEGETATIVE GELLS 1

Cytokinesis in apical cells of actively growing cultures of Cephaleuros parasiticus Karsten sporangiate thalli was examined with transmission electron microscopy. A massive, interzonal cytokinetic microtubule spindle is anchored at its poles to the medial surfaces of the daughter nuclei at telophase. Later, the daughter nuclei are widely separated and no longer associated with the interzonal spindle; however, the spindle retains its shape and becomes a distinct phragmoplast with an array of vesicles, presumably derived from dictyosomes, aligned in the division plane. Fusion of the vesicles gives rise to a thin cell plate. Some bundles of microtubules in the spindle appear to mark the sites of plasmodesmata formation, but no endoptasmic reticulum is directly involved in plasmodesmata formation. No infurrowing or phycoplast array of microtubules is involved in the cytokinesis. The relationship, if any. between the metaphase-anaphase mitotic microtubule system and the interzonal cytokinetic spindle has not been determined. Cephaleuros parasiticus isone of only four green algae now known to contain a higher plant-like phragmoplast and cytokinetic process. The observations reported can be interpreted as very strong evidence for a phylogenetic affinity between the Trentepohliaceae and the Charophyceae, but consideration of ulvophycean features of the Trentepohliaceae such as motile cell ultrastructure and life histories precludes unequivocal assignment of the family to either the Charophyceae or Ulvophyceae.     

Mating status affects female choice when females are signalers

Sexual selection in animals has been mostly studied in species in which males are signalers and females are choosers. However, in many species, females are (also) signalers. In species with non‐signaling females, virgin females are hypothesized to be less choosy than mated females, as virgins must mate to realize fitness and the number of available males is generally limited. Yet, when females signal to attract males, mate limitation can be overcome. We tested how virgin and mated females differ in their calling behavior, mating latency, and in mate choice, using the tobacco budworm Chloridea (Heliothis) virescens as an example for a species in which females are not only choosers but also signalers. We found that virgin females signaled longer than mated females, but virgin and mated signaling females were equally ready to mate, in contrast to non‐signaling females. However, we found that virgin signaling females showed weaker mate preference than mated females, which can be explained by the fact that females increase their fitness with multiple matings. Mated females may thus further increase their fitness by more stringent mate selection. We conclude that signaling is a crucial aspect to consider when studying female mate choice because signaling may affect the number of available mates to choose from.