Top-down and bottom-up regulation of herbivores: Spodoptera frugiperda turns tables on endophyte-mediated plant defence and virulence of an entomopathogenic nematode

Abstract.  1. The fungus Neotyphodium lolii forms a symbiotic relationship with its grass host Lolium perenne (perennial ryegrass). The fungus benefits from access to plant nutrients and photosynthate, whereas the plant benefits from acquired chemical defence against herbivory.
2. This study examined the potential for endophyte-mediated plant defences to influence interactions between fall armyworm Spodoptera frugiperda , and the entomopathogenic nematode Steinernema carpocapsae and clarified biological mechanisms underlying the observations made.
3. In laboratory and greenhouse experiments, S. frugiperda larvae were fed endophytic or non-endophytic L. perenne then exposed to S. carpocapsae or injected with the nematodes' symbiotic bacteria Xenorhabdus nematophila .
4. In all instances, S. frugiperda larvae fed endophyte-infected grass suffered significantly lower mortality than those fed non-endophytic plants. Although larvae fed endophyte-infected grass often had significantly lower biomass than those fed uninfected grass, these differences did not account for altered susceptibility to S. carpocapsae .
5. Endophyte-mediated reductions in herbivore susceptibility to the nematode pathogen represent a herbivore adaptation that effectively turns the tables on both plant and natural enemy by reducing the virulence of the nematodes' symbiotic bacteria while expanding the temporal window of herbivory.     

Évaluation de la toxicité deBacillus thuringiensis surSpodoptera frugiperda

Résumé Cinquante deux souches deB. thuringiensis appartenant à 13 sérovars ont été testées sur des chenilles néonates deSpodoptera frugiperda en contaminant la surface du milieu semi-synthétique d'élevage. Deux souches du sérovarkenyae et une autre du sérovartolworthi provoquent le plus de mortalité, suivies par les souches des sérovarsaizawai etkurstaki. Les souches les moins actives appartiennent aux sérotypesalesti, dendrolimus, sotto etcolmeri. L'action des souches sur le développement larvaire a aussi été abordée. Les souches des sérovarskenyae, aizawai etkurstaki ont ralenti le développement des chenilles, tandis que les souches des sérovarsalesti, sotto etcolmeri n'ont eu aucun effet.      

Effect of CryIC toxin from Bacillus thuringiensis on larval feeding behavior of Spodoptera exigua

The lack of data on the effect of Bacillus thuringiensis (B.t.) toxins on larval feeding behavior of the pest Spodoptera exigua (Hübner) (Noctuidae: Amphypyrini) prompted us to investigate the effect of three delivery systems of CryIC, a commercial formulation, inclusion bodies, and the activated CryIC toxin. The commercial formulation was the least and CryIC toxin the most lethal form to neonates of susceptible colonies. All but two of the treatments in choice tests with neonates and third instars showed significant avoidance of B.t. treated diet, with greater proportion of larvae from susceptible (UCR-S and AUBURN-S) and resistant (AUBURN-R) colonies on untreated diet than on diet treated with any of the CryIC forms and concentrations tested. Furthermore, third instars consumed significantly more control than treated diet for all CryIC forms, colonies and concentrations. The avoidance of CryIC toxin by neonates and third instars strongly suggests that CryIC, which also is present in the commercial formulation and in the inclusion bodies, is responsible for eliciting avoidance behavior by S. exigua larvae. Behavioral observations of third instars in a no-choice test on either treated or control diet indicated that questing behavior in susceptible larvae appears to be positively related with presence of CryIC toxin in the diet. Furthermore, resistant third instars were on the whole more active than susceptible thirds on both treated and control diet. Resistant thirds raised on CryIC treated diet (AUBURN-RC) spent more time eating treated diet than resistant larvae raised on control diet (AUBURN-R), suggesting that diet conditioning plays an important role on feeding behavior of S. exigua. The implications of these results are discussed.     

Ectoparasitic Acugutturid Nematodes of Adult Lepidoptera

Noctuidonema guyaneme is an interesting ectoparasite of adult Lepidoptera that feeds on hosts from at least five families with its long stylet. Noctuidonema guyanense spends its entire life on the adult moth and is sustained as it is passed from moth to moth during host mating. Overlapping host generations are essential for parasite survival. This nematode occurs throughout tropical and subtropical America and is transported by at least one of its hosts, Spodoptera frugiperda, during migration to northern sites in the United States each spring. Noctuidonema guyanense debilitates its hosts. Research conducted to help determine the biological control importance of this nematode is reviewed. Two additional species, N. daptria and N. dibolia, are now known for Noctuidonema.     

Effects of genetic and environmental host plant variation on the susceptibility of two noctuids toBacillus thuringiensis

TwoApium graveolens var.rapaceum (L.) cultivars that differ in their suitability for the survival and growth ofSpodoptera exigua (Hübner) andTrichoplusia ni (Hübner) were used to examine the effect of genetic and seasonal environmental variation in host plant suitability on the efficacy ofBacillus thuringiensis subsp.kurstaki (Berliner). The effects of host plant genotype andB. thuringiensis were generally independent, so thatB. thuringiensis efficacy was greatest on the resistant host plant cultivar. Host plant suitability varied within growing season for both insect species but, while host plant suitability decreased with increasing plant age forT. ni, the response ofS. exigua to plant age was not as clear. Within season variation in host plant suitability affectedB. thuringiensis efficacy and the interaction betweenB. thuringiensis and host plant cultivar forS. exigua but not forT. ni. Soluble protein and Folin-Denis phenolic concentrations of host plant tissue were not correlated with changes in host plant suitability to either insect species.     

Effect of the pheromone biosynthesis activating neuropeptide on sex pheromone biosynthesis in Spodoptera littoralis isolated glands

The control of Spodoptera littoralis sex pheromone biosynthesis has been investigated with synthetic pheromone biosynthesis activating neuropeptide (PBAN) and different labeled tracers using an in vitro isolated gland system. Responsiveness of the glands to PBAN stimulation was impaired by careless tissue manipulation. The fact that PBAN is active in the isolated gland system suggests that this might be a target organ for this peptide in S. littoralis. As reported previously with Br-SOG extracts and intact females, label incorporation into the pheromone increased in glands treated with PBAN from all the precursors tested. However, the formation of labeled intermediates from d₅E11–14:Acid also occurred in glands incubated in the absence of the peptide, but the amounts of d₅Z9, E11–14:Acid were lower in PBAN treated glands than in controls. These results indicate that PBAN controls pheromone biosynthesis in S. littoralis by regulating the reduction of acyl moieties. © 1994 Wiley-Liss, Inc.     

Field Evaluation of a Commercial Formulation of the Spodoptera exigua (Lepidoptera: Noctuidae) Nuclear Polyhedrosis Virus for Control of Beet Armyworm on Vegetable Crops in Thailand

Small - scale field trials were conducted in the Kanchanaburi and Ratchaburi provinces of Thailand between December 1994 and March 1996 to gather efficacy data to support a registration dossier for a commercial formulation of a nuclear polyhedrois virus of beet armyworm , Spodoptera exigua, under the tradename Spod - X . A liquid concentrate formula tion of Spod - X was compared with two commercial standards , Bacillus thuringiensis subsp . aizawai (CenTuri 3 . 5 WDG) and the insect growth regulator tebufenozide (Mimic 20F , a benzoylhydrazine ecdysone agonist) , for control of beet armyworm on garden pea , Chinese kale , shallot and table grape . Spod - X applied at 4 - day intervals at 3 . 1 - 12 . 5 1011 occlusion bodies (OBs) ha 1 in a spray volume of 625 - 1000 l ha 1 provided excellent control of beet armyworm on garden pea and grape (75 - 100 % reductions) after the second application . On Chinese kale and shallot , rates of Spod - X in the rate 5 - 60 1011 OBs ha 1 sprayed every 4 days in a spray volume of 1000 l ha 1 provided significant protection from infestation of beet armyworm over the growing season; however , efficacy was poor to moderate . Tank - mixing Spod - X with the pyrethroid , deltamethrin (Decis3EC) , did not improveefficacy on shallot;however , combinations of virus and B. thuringiensis or tebufenozide achieved commercially acceptable levels of crop protection on Chinese kale compared with virus alone . Lower levels of control and crop pro tection observed with all materials on Chinese kale and shallot reflect , in part , the tendency of S. exigua larvae to feed within sheltered sites and directly damage marketable plant parts . In general , all rates of Spod - X performed as well as , or better than , the B. thuringiensis standard (56 . 3 - 105 g ha 1 a.i.) on all crops tested . In general , reductions of beet armyworm larval populations and plant damage provided by all labeled rates of the chemical standard , tebufenozide (125 - 250 g ha 1 a . i .), were marginally greater than all rates of Spod - X tested . On crops such as pea and grape , where beet armyworm predominately feed on exposed leaf surfaces , our data unequivocally support the use of Spod - X for cost - effective , season - long pest management . Strategies to improve the efficacy of virus on shallot and kale are discussed .     

The role of the secondary plant compound 2,5-dihydroxymethyl 3,4-dihydroxypyrrolidine as a feeding inhibitor for insects

The cyclic amino alditol 2,5-dihydroxymethyl 3,4-dihydroxypyrrolidine (DMDP) has recently been shown to be an inhibitor of various glucosidases. We have investigated its role as a plant protective chemical against a range of phytophagous insects by examining its effects on development, feeding behaviour and functioning of taste receptors. We have shown that it can inhibit feeding and can be toxic. The mechanisms by which these effects are achieved are discussed in the context of the molecular structure of the compound.

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Résumé Récemment a été mis en évidence le puissant pouvoir inhibiteur de différents glucosidases. Présenté par l'amino-alditol cyclique, 2,5 dihydroxymethyl 3,4 dihydroxypyrrolidine (DMDP), isolé de graines de la légumineuse Lonchocarpus sericeus. Nous avons examiné le rôle de ce composé secondaire dans la stratégie défensive de la plante contre une gamme d'insectes phytophages, à travers son action sur le développement, le comportement alimentaire et le fonctionnement des récepteurs gustatifs. Nous avons montré que DMDP peut inhiber, suivant la dose, la prise de nourriture et, qu'aprs ingestion (par Spodoptera spp), il peut agir comme une toxine. Par suite de sa structure moléculaire, DMDP peut être considéré, non seulement comme l'analogue d'un sucre, mais aussi comme un alcaloïde. Le bilan des résultats de nos expériences suggère que l'efficacité de DMDP comme phagodissuadant, si elle ne provient pas de sa toxicité, découle principalement de ses propriétés d'alcaloïde.

     

Cyanide as a feeding stimulant for the southern army worm, Spodoptera eridania

ABSTRACT.

• 1 All instars of Spodoptera eridania larvae grow as well or better when cyanide is present in their diet as when it is absent. Concentrations up to 0.05% stimulate feeding in first to fourth instar larvae. Concentrations from 0.1% to 1.0% stimulate feeding in fifth and sixth instar larvae.
• 2 Three-day-old sixth instar larvae pre-exposed to cyanide are completely resistant to its acutely toxic effects, but previously unexposed larvae suffer reversible symptoms of poisoning when feeding on a diet containing 1.0% KCN.
• 3 A 1.0% dietary KCN exposure during the sixth instar reduces ecdysis to 17% adult emergence and completely inhibits oviposition.
• 4 Cyanide concentrations from 0.5% to 1.0% in the diet, although effecting increased growth rates, induce necrotic lesions in larval mid-gut epithelial cells.
• 5 Thiocyanate, one of the in vivo cyanide metabolites, at 0.5% in the diet reduces pupation to 23%, delays and reduces adult emergence to 20% and inhibits oviposition.
• 6 The preferred host plant of S.eridania is the lima bean, Phaseolus lunatus, probably due to its content of the cyanogenic glycoside linamarin. Dietary valine has no effect on the southern armyworm feeding and growth behaviour (Long & Brattsten, 1982) but dietary cyanide does. The lima bean is known to contain up to 31 ppm cyanide in some varieties.