Influence of crop management practices on bean foliage arthropods

Crop management practices can affect the population of phytophagous pest species and beneficial arthropods with consequences for integrated pest management. In this study, we determined the effect of no-tillage and crop residue management on the arthropod community associated with the canopy of common beans (Phaseolus vulgaris L.). Abundance and species composition of herbivorous, detritivorous, predaceous and parasitoid arthropods were recorded during the growing seasons of 2003 and 2004 in Coimbra County, Minas Gerais State, Brazil. Arthropod diversity and guild composition were similar among crop management systems, but their abundance was higher under no-tillage relative to conventional cultivation and where residues from the preceding crop were maintained in the field. Thirty-four arthropod species were recorded, and those most representative of the impact of the crop management practices were Hypogastrura springtails, Empoasca kraemeri and Circulifer leafhoppers, and Solenopsis ants. The infestation levels of major insect-pests, especially leafhoppers (Hemiptera: Cicadellidae), was on average seven-fold lower under no-tillage with retention of crop residues relative to the conventional system with removal of residues, whereas the abundance of predatory ants (Hymenoptera: Formicidae) and springtails (Collembola: Hypogastruridae) were, respectively, about seven- and 15-fold higher in that treatment. Importantly, a significant trophic interaction among crop residues, detritivores, predators and herbivores was observed. Plots managed with no-tillage and retention of crop residues had the highest bean yield, while those with conventional cultivation and removal of the crop residues yielded significantly less beans. This research shows that cropping systems that include zero tillage and crop residue retention can reduce infestation by foliar insect-pests and increase abundance of predators and detritivores, thus having direct consequences for insect pest management

Seasonal mortality factors of the coffee leafminer, Leucoptera coffeella

Seasonal population fluctuation of the coffee leafminer, Leucoptera coffeella (Guérin-Méneville & Perrottet) (Lepidoptera: Lyonetiidae), led to an investigation of its natural mortality factors during the rainy season when the population level is low and during the dry season when population peaks occur. Life-table data were colleted from insecticide-free plots within a 3 ha coffee plantation on the upper, medium and lower canopy. Leafminer mortality was similar among the canopy parts but varied in the two seasons studied. During the rainy season, the generational mortality averaged 94.3%, with 50.2, 33.7 and 10.4% occurring during egg, larval and pupal stages, respectively. During the dry season, total mortality was 89%, with 13.2, 61.0 and 14.8% occurring during egg, larval and pupal stages, respectively. Marginal mortality rates during the rainy season were highest for physiological disturbances, rainfall and egg inviability; but, in the dry season, they were highest for predaceous wasps, physiological disturbances and parasitoids. Egg and larval stages accounted for most of the mortality variation in the rainy season, while the combination of larval and pupal mortality better described the generational mortality in the dry season. Variation in mortality during the rainy season was primarily associated with egg inviability, rainfall and parasitoids. In contrast, predatory wasps and physiological disturbances were the main factors associated with mortality variation during the dry season. These results suggest that weather conditions, natural enemies and plant quality attributes are the main determinants of the population dynamics of L. coffeella

Partial purification and characterization of trypsin-like proteinases from insecticide-resistant and -susceptible strains of the maize weevil, Sitophilus zeamais

Serine proteinases from three strains of Sitophilus zeamais (Coleoptera: Curculionidae), one susceptible and two resistant to insecticides — one exhibiting fitness cost (resistant cost strain) and the other lacking it (resistant no-cost strain), were partially purified using an aprotinin–agarose affinity column providing purification factors ranging from 36.5 to 51.2%, with yields between 10 and 15% and activity between 529 and 875 µM/min/mg protein with the substrate N-α-benzoyl-l-Arg-p-nitroanilide (L-BApNA). SDS-PAGE of the purified fraction revealed a 56,000 Da molecular mass band in all strains and a 70,000 Da band more visible in the resistant no-cost strain. The purified proteinases from all strains were inhibited by phenylmethyl sulphonyl fluoride (PMSF), N-α-tosyl-l-lysine chloromethyl ketone (TLCK), aprotinin, benzamidine and soybean trypsin inhibitor (SBTI) characterizing them as trypsin-like serine proteinases. Trypsin-like proteinases from the resistant strains exhibited higher affinity for L-BApNA. The resistant no-cost strain exhibited Vmax-values 1.5- and 1.7-fold higher than the susceptible and resistance cost strains, respectively. A similar trend was also observed when using N-α-p-tosyl-L-Arg methyl ester (L-TAME) as substrate. These results provide support to the hypothesis that the enhanced serine proteinase activity may be playing a role in mitigating physiological costs associated with the maintenance of insecticide resistance mechanisms in some maize weevil strains

Altered cysteine proteinase activity in insecticide-resistant strains of the maize weevil: Purification and characterization

Insecticide resistance is usually associated with fitness costs, but such costs may be mitigated by increased energy and amino acid accumulation and mobilization as has been suggested in the maize weevil Sitophilus zeamais (Coleoptera: Curculionidae). To address this adaptation, cysteine proteinases (E.C. 3.4.22), one of the main proteinases in weevils, was purified from an insecticide-susceptible and two insecticide-resistant strains of the maize weevil (one with fitness costs, referred as resistant-cost, and the other without it, referred to as resistant no-cost) using thiol-sepharose affinity chromatography. Purification of the cysteine proteinases revealed a single 74,000 Da molecular mass band in the susceptible strain, two bands of 72,000 and 83,000 Da in the resistant cost strain, and two bands of 68,000 and 74,000 Da in the resistant no-cost strain. Purified cysteine proteinases of the three strains behaved differently regarding casein degradation and inhibition; the proteinases least sensitive to inhibition by the specific cysteine proteinase inhibitor E-64 were those from the resistant no-cost strain as indicated by their highest I50 value. The pH and temperature profile of cysteine proteinase activity differed among strains and although substrate affinity (i.e. KM) of the cysteine proteinases was similar, the Vmax value for cysteine-proteinases from the resistant cost strain was 3-fold and 5-fold higher than Vmax values for the resistant no-cost and susceptible strains respectively. Cysteine proteinase activity was highest for the resistant cost strain rather than the resistant no-cost. Therefore enhanced cysteine proteinase activity is unlikely to be playing significant role in mitigating the costs usually associated with insecticide resistance