Exigua Populations Research Articles

Selection and characterization of spinosad resistance in Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae)

The cross-resistance and biochemical mechanism of the beet armyworm, Spodoptera exigua (Hübner), to spinosad was studied in the laboratory. S. exigua population were collected from Shanghai suburb. After five generations of selection, the resistance of S. exigua to spinosad increased 345.4 times compared with the susceptible strain. There was no cross-resistance between spinosad and fenvalerate, phoxim, methomyl, abamectin, and cyfluthrin. When the inhibitors, PBO, TPP, DEF, and DEM were used as synergist in the susceptible strain and resistant strain, the synergistic ratio was 0.7-, 0.5-, 1.0-, and 0.6- fold for the susceptible strain, and 9.8-, 1.5-, 2.6-, and 1.5-fold for the resistant strain, respectively. The results revealed that PBO had significant synergistic effect on the resistant strain. The activity in vitro of microsomal- O-demethylase and glutathione S-transferase in the resistant strain was 5.2- and 1.0-fold of the susceptible strain, respectively. The results implied that microsomal- O-demethylase might be important in conferring spinosad resistance in the S. exigua population.

SPATIAL AND TEMPORAL OCCURRENCE OF BEET ARMYWORM (LEPIDOPTERA: NOCTUIDAE) MOTHS IN MISSISSIPPI

Throughout 1994-2000, adult beet armyworm, Spodoptera exigua (Hübner) populations were monitored in the delta and hill regions of Mississippi using pheromone traps. Significant differences in the mean number of moths trapped were found among different geographical areas of the state. A trend was observed where the greatest number of moths was found in the Mississippi Delta, located in the western region of the state. The lowest number of moths was found in the hills located in the eastern region of the state. An annual profile of beet armyworm populations in the western section of the Mississippi Delta also revealed that wide-scale immigration of this pest typically begins at 200 Julian days (mid-July). This date could be used as a benchmark to determine when and if population levels are high enough to have the potential to cause economic damage to crops in the Mississippi Delta.

Validation of a Comprehensive Process-Based Model for the Biological Control of Beet Armyworm, Spodoptera exigua, with Baculoviruses in Greenhouses

Abstract This paper describes the validation and sensitivity analysis of a process-based simulation model (BACSIM) for the control of beet armyworm, Spodoptera exigua, with baculoviruses. Model predictions are compared to results of independent greenhouse experiments in which second, third, or fourth instar larvae of S. exigua in chrysanthemum plots are treated with different concentrations of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) and S. exigua MNPV (SeMNPV), two viruses with distinct differences in infectivity and mean time to kill. BACSIM provides robust predictions for the control of S. exigua populations in greenhouse chrysanthemum with both AcMNPV and SeMNPV. Mortality levels caused by AcMNPV and SeMNPV were generally predicted within a 25% margin of error compared to the observed values. None of the deviations was higher than 40%. All values of simulated foliage consumption, caused by S. exigua populations treated with AcMNPV or SeMNPV applications, fell within 95% confidence intervals of measurements. Simulated time to kill was, in general, lower than the measurements. This discrepancy may be caused by the behavior of S. exigua larvae which feed on the underside of chrysanthemum leaves where they are protected from polyhedra. This suggests that the larval foraging behavior may play an important role in the efficacy of baculovirus applications and should be further studied experimentally. This validated model can be used for the pretrial evaluation of the efficacy of genetically modified baculoviruses as biological control agents and for the optimization of spraying regimes in chrysanthemum cultivation.

Breeding for resistance to Meloidogyne exigua in Coffea arabica by introgression of resistance genes of Coffea canephora

Breeding for resistance to root‐knot nematode Meloidogyne exigua in coffee may help in controlling this nematode, which causes substantial harvest losses throughout Latin America. Accessions of Coffea arabica, C. canephora and lines derived from the Timor Hybrid (wild C. arabica × C. canephora interspecific hybrid) were tested in a greenhouse for resistance to gall formation by counting individual galls on the roots of plants inoculated with second‐stage juveniles of a M. exigua population from Costa Rica. The level of introgression from C. canephora was also assessed on 28 resistant and susceptible genotypes in a molecular study with amplified fragment length polymorphism (AFLP) markers. The frequency of resistant plants was very low or even nonexistent in C. arabica and very high in C. canephora with > 78% immune plants and 100% resistant plants. Several lines derived from the interspecific hybrid revealed a high level of resistance similar to that observed in the parent C. canephora species. A molecular study of Timor Hybrid‐derived lines revealed high variability for the number of markers present in the study genotypes. A larger number of introgression markers was found in the group of resistant genotypes than in the susceptible genotypes. However, there were also lines with little introgression that had retained resistance. Based on genetic distances calculated from these markers, two groups of lines were revealed: those derived from Timor Hybrid CIFC1343, and those derived from Timor Hybrids CIFC832/1 and CIFC832/2, which may indicate the existence of different resistance genes in the two groups. The segregations observed in F1 and F2 progeny may be explained by at least a dominant gene. The lines derived from the Timor Hybrid are a worthwhile source of resistance to M. exigua that can be exploited to improve C. arabica with the help of molecular‐assisted selection.

Combining Exclusion Techniques and Larval Death-Rate Analyses to Evaluate Mortality Factors of Spodoptera exigua (Lepidoptera: Noctuidae) in Cotton

By combining pesticide exclusion and cage exclusion techniques, the efficacy of natural enemies in reducing populations of Spodoptera exigua (Hubner), the beet armyworm, larvae was effectively demonstrated. Larval collections added information about parasitism and disease, and when combined with data from insecticide treatments, demonstrated that differences in S. exigua population densities usually were due to the action of predators. Death-rate analyses demonstrated that much mortality due to parasitism was contemporaneous with death from predation. When predator populations were not reduced by insecticides, most indispensable natural mortality was due to predation. When predators were eliminated, and S. exigua populations reached outbreak levels, most larvae died from disease in 1989 and from parasitism in 1990.

Effects of climatic factors on native arbuscular mycorrhizae and Meloidogyne exigua in a Brazilian rubber tree (Hevea brasiliensis) plantation

The root‐knot nematode Meloidogyne exigua and arbuscular mycorrhizal (AM) fungi may both occur in the roots of Brazilian rubber trees (Hevea brasiliensis). AM fungi may stimulate plant growth whereas nematodes usually reduce it. Variations of native AM fungi and M. exigua populations in soil and roots of rubber trees were studied for one year in a Brazilian plantation. The number of AM spores in the soil was generally greater in the rainy season than in the dry season, although AM colonization of roots was unaffected by season. During the dry season, numbers of juveniles and eggs of M. exigua in roots were lower than in the rainy season. A site without nematodes in the soil or roots showed the greatest numbers of AM spores in soil and highest AM colonization of roots. A negative correlation was observed between the percentage of AM colonization and the number of second‐stage juveniles in soil and second‐stage juveniles and eggs in roots. Microscope observations revealed (i) tissue specificity for each of the microorganisms in the roots, with a cortical location of mycorrhizae and a mainly vascular cylinder location of nematodes, and (ii) that Gigaspora was the most abundant AM genus in the plantation soil.

Epizootics caused by a nuclear polyhedrosis virus in populations ofspodoptera exiguain southern Spain

A nuclear polyhedrosis virus (NPV) has been identified as the etiological agent causing epizootics in natural populations of Spodoptera exigua in the south of Spain. The incidence of this NPV has been determined from samples of larvae collected from sunflower fields and vegetable greenhouses at two sites located 500 km apart. The NPV was the only disease agent identified at both sites. Through the period surveyed the percentage of NPV‐infected larvae increased from 4.3% to 23% and from 22.5% to 100% in S. exigua larvae collected from sunflower fields and vegetable greenhouses respectively. No mortality attributed to the NPV was found in Spodoptera littoralis, Heliothis armigera, Peridroma saucia and Plusia gamma larvae collected in the same survey from the same sites. This is the first report of epizootics caused by NPV in S. exigua populations in field and greenhouse crops in Europe.

Resistance to insects by cabbage lines developed in New York when grown in South Texas

Brassica oleracea (L.) germplasm, developed at the New York Agricultural Experiment Station (NYAES), Geneva, as advanced cabbage breeding lines with resistance to lepidopterous pests, was grown in south Texas. Resistance, as measured in terms of insects present on plants, of the breeding lines to arthropod pests was compared with that of cultivars commonly grown in south Texas by monitoring populations of insect pests on the various lines and cultivars. Breeding lines developed at NYAES retained their desired characteristics for resistance to naturally occurring populations of Trichoplusia ni (Hübner) and Plutella xylostella (L.) and also were found to be resistant to Brevicoryne brassicae (L.). A red cabbage cultivar, Red Rock, was found to be more susceptible to Pemphigus populitransversus Riley populations than all other lines or cultivars. Murgantia histrionica (Hahn) was present during one growing season and some NYAES lines that were infested with low numbers of lepidopterous larvae and aphids became heavily infested with this species. Spodoptera exigua (Hübner) populations did not occur in great enough abundance to evaluate differences in resistance among lines and cultivars in 1984 or 1985.

Study of Phoma exigua Populations in the field

Strip-baits, consisting of filter-paper disks sandwiched between plastic tapes, were exposed in potato crops in 1968 and 1969. The frequency of isolation of Phoma exigua var. foveata and P. exigua var. exigua from the disks was related to the height of exposure of strip-baits above the soil, to the date of their exposure and to their position in the crop. Isolation of both varieties was high from strip-baits exposed 15 cm above the soil surface; wax. foveata was isolated most frequently from strip-baits exposed near potato plants grown from gangrene-diseased seed tubers while var. exigua was isolated most frequently outside the potato crop at the north end of the field. In comparison with 1968, the frequency of isolation in 1969 ofvar. foveata was almost halved whilst that of var. exigua increased almost threefold. In both years, following the harvest of the potato crop, the frequency of isolation of var. foveata tended to decline whereas that of var. exigua remained high for several months.