Low Infestation Levels Research Articles

Within-Vine Distribution and Damage of Sweetpotato Weevil, Cylas formicarius elegantulus (Coleoptera: Curculionidae), on Four Cultivars of Sweet Potato in Southern Florida

The sweetpotato weevil (SPW), Cylas formicarius elegantulus (Summers) is the most economically important insect pest of sweet potato, Ipomoea batatas (L.) Lam., worldwide. Low level infestations reduce quality and marketable yield (Proshold 1983). Terpenoids that are produced by sweet potato in response to SPW feeding may make even slightly damaged roots unfit for human consumption (Uritani et al. 1975, Akazawa et al. 1960). Yield losses of up to 60-80% have been attributed to SPW (Hua 1970, Subramaniam et al. 1977, Mullen 1984). Economically effective control measures for SPW in regions with large resident populations are lacking. There are no insecticides that will adequately control weevils in an infested field (Stall et al. 1984) because SPW immature stages are spent within vines and/or roots. For this reason, other SPW management approaches, such as host plant resistance, have been explored (Waddill & Conover 1978, Mullen et al. 1980a,b, 1981, 1982, 1985). With one exception (Waddill & Conover 1978), resistant cultivars have not been field tested in regions with high SPW population pressure. The present study was conducted to evaluate SPW resistance in four sweet potato cultivars in southern Florida, a region with a dense SPW population. Also, we determined the -impact of a mechanical barrier (polyethylene mulch) on field plant protection from SPW. Lastly, we compiled preliminary data on the within-vine distribution of SPW in sweet potato. The experiment was conducted in 1986 at the Tropical Research and Education Center in Homestead, Florida. Four sweet potato cultivars, I. batatas cv. Resisto, Regal, Sumor, and NC1135, were planted in a split-plot design with three replications. Plastic polyethylene mulch (white/black, 38 microns thick covering a 0.9-m bed width) was the whole-plot factor and cultivar was the split-plot factor. Cultivars were handplanted on 22 April in raised beds with 1.8 m centers. Slips (0.3-m long) of each cultivar were planted 0.3 m apart in one row per bed. Replicate plots consisted of 40 slips

Recurrent selection for potato leafhopper resistance in potato,Solanum tuberosum L. Gp. Tuberosum

ASolanum tuberosum L. Gp. Tuberosum population was recurrently selected for resistance to potato leafhopper (PLH) (Empoasca fabae, Harris). Primary selection was for low levels of nymphal infestation under natural infestation in field tests. The selection procedures allowed completion of a selection generation per year. The original population and seven selection-generation populations were tested for PLH resistance for three years at two locations. Averaged over six tests, seven generations of selection reduced the level of nymphal infestation by 71% and severity of plant damage (hopperburn) by 45%. The rate of reduction slowed in later generations.

Development of two bruchid populations ( Bruchidius atrolineatus (Pic) and Callosobruchus maculatus (Fab.)) infesting stored cowpea ( Vigna unguiculata Walp) pods in Niger

The development of populations of two species of bruchids, B. atrolineatus and C. maculatus, in traditional stores containing cowpea pods was analysed in the Niamey area of Niger. At the beginning of the storage period, the first generation of B. atrolineatus adults is sexually active, mates and oviposits on the pods. Later, as the population increases dramatically, there is a correspondingly higher percentage of adults exhibiting reproductive diapause. Experiments showed that the diapausing insects leave stores during the dry season. This factor subsequently induces a regression of population growth inside the stores at the end of the dry season. There was no development of the C. maculatus population in the present experiment, because of the low level of infestation in the field before harvest and the fact that climatic conditions were probably unfavourable for this species in the Niamey area. In stores, the rate of B. atrolineatus population increase is very high in a limited number of generations resulting in significant losses of cowpea seeds.

Effect of host resistance on the feeding and reproductive performance of Haemaphysalis punctata and Ixodes ricinus ticks

A study was made to assess the influence of host response on Haemaphysalis punctata and Ixodes ricinus biology by comparing the effects of feeding ticks on previously uninfested hosts with those fed on previously sensitised hosts. Adult H punctata were fed on sheep; larvae and nymphs of H punctata and I ricinus were fed on rabbits. The engorgement weights of all instars studied that fed on sensitised hosts fell very gradually with successive infestations. These weight reductions were greater at high than at low levels of infestation. In previously sensitised hosts, the engorgement period of all instars was prolonged: the percentage of instars which completed engorgement and engorgement weights were reduced. The host's clinical and immune response markedly affected the fecundity of adult female ticks and the viability and hatchability of the laid eggs. Eighty per cent of female H punctata successfully laid eggs following the first infestation and the hatchability of eggs was 85 per cent. The numbers successfully laying eggs decreased to 60 per cent during the fifth and sixth infestations and was less than 20 per cent following the seventh infestation; the hatchability decreased similarly. There was no marked effect on the development of immature instars when fed on sensitised rabbits.

Within-plant Distribution of, and Partial Compensation for, Stink Bug (Heteroptera: Pentatomidae) Damage to Soybean Seeds

Yield, pod filling, weights of 100 seeds, viability, and vigor of seeds from upper and lower halves of soybean, Glycine max (L.) Merrill, plants were examined in response to four levels of natural infestation (population means of 3.8, 3.0, 1.5, and 0.3 bugs per meter of row) by a stink bug complex that included Nezara viridula (L.), Acrosternum hilare (Say), and Euschistus spp. Reductions in yield occurred as stink bug infestation levels increased; these reductions primarily were confined to upper halves of plants. Feeding damage, as evidenced by reductions in pod filling, seed viability, and seed vigor, was restricted to upper halves of plants until infestations reached the highest level (i.e., 3.8 bugs per meter of row). These results demonstrated that stink bugs fed preferentially on seeds in upper halves of plants until high infestation levels forced bugs to feed in lower halves. Results also suggest that feeding by stink bugs at low infestation levels (i.e., 1.5 bugs per meter of row) effected increases in weights of 100 seeds. Damage-free seeds compensated for damaged seeds by exhibiting increases in weights of 100 seeds of as much as 43.8%.

Insecticides for Suppression of Cotton Fleahoppers in the Texas Blacklands, 1986

Abstract Cotton was planted 4 Apr on 38 inch row spacings and was cultivated using standard management practices for the area. Four insecticide treatments and an untreated check were compared in a randomized block design with 4 replications. Plots were 12 rows wide and 95 ft long, but only the middle 8 rows were sprayed. Applications were made with a high-clearance, self-propelled ground sprayer using TX-4 hollow cone nozzles and calibrated to deliver 5 GPA at 40 psi and 4 mph. Insecticide treatments were applied 29 May and 6 Jun. Fleahopper density was sampled on 28 May and 4 Jun, prior to each application of insecticides. A final sample was delayed until 16 Jun due to rain. No additional insecticide applications were made because of the maturity of the crop and the low level of fleahopper infestation. Sampling consisted of examining 30 randomly selected plant terminals/plot for fleahopper adults and nymphs. Cotton fruiting rates were monitored over a 25-day period by counting white blooms on plants within a designated 20-foot section of row in each plot. White bloom counts were made Monday-Friday. Yield data were collected in a stratified harvest from two separate pickings of cotton from the same 20 ft row section used for counting white blooms. The harvested cotton was bur-extracted, ginned and weighed. Data were anlayzed by conventional analysis of variance and multiple comparison procedures.

Avaliação da resistência dê cafeeiros às raças dê Meloidogyne incognita

No Centro Experimental de Campinas do Instituto Agronômico, de abril de 1984 a agosto de 1985, realizaram-se dois experimentos, em vasos, para avaliar a resistência de mudas de sete linhagens de Coffea arabica ('Mundo Novo': CP388-17, CP379-19, CP501 e MP376-4; 'Catuaí Amarelo' H2077-2-5-62 e 'Catuaí Vermelho' H2077-2-5-81, e 'Caturra Amarelo') e dois de C. canephora (Robusta: 'Guarini' col. 10 e 'Kouillon' col. 67-14) às quatro raças de Meloidogyne incognita. No primeiro experimento, cada muda foi infestada com 8.000 ovos, passados cinco meses do transplante, e a avaliação, efetuada seis meses depois. No segundo experimento, as mudas foram infestadas cerca de um ano do transplante com 7.000 ovos cada uma e a avaliação realizada decorridos dez meses. Todas as plantas foram infestadas pelas raças 1 e 2; entretanto, a reprodução do nematóide foi menor nas plantas mais velhas. As raças 3 e 4 apresentaram baixas infestações e algumas reações de imunidade, principalmente a 4, que tem pouca importância prática pela sua pequena ocorrência. Os cultivares de Catuaí revelaram médias de notas de ootecas menores que as das linhagens de 'Mundo Novo', indicando menor suscetibilidade. Contudo, é importante ressaltar que a menor infestação não significa que as raças 3 e 4 sejam menos danosas ao cafeeiro quando o parasitam em campo.

Twospotted Spider Mite (Acari: Tetranychidae) Infestations on Soybeans: Effect on Composition and Growth of Susceptible and Resistant Cultivars

Effects of feeding of the twospotted spider mite, Tetranychus urticae Koch, on leaf and seed composition and dry matter accumulation in two soybean, Glycine max (L.) Merr., cultivars previously found to respond differently to mite infestation were investigated to gain a greater understanding of resistance mechanisms in soybeans to this wide host range plant pest. The two soybean cultivars, ‘Bonus’ and ‘Williams’, were infested with mites at the V3, V6, and R2 stages of development with 0, 10, and 200 mites per first fully expanded trifoliolate leaf. A relatively greater reduction in dry matter accumulation was seen in ‘Williams’ than in ‘Bonus’ at the same mite population level on the foliage. However, mite population increase was significantly lower on ‘Williams’ than on ‘Bonus’. Increasing mite infestation levels caused a slight increase in total soluble sugars and starch and a small decrease in total nitrogen and phosphorus levels of the leaves of both cultivars. Mite infestation had no effect on composition of seeds. Increasing mite damage caused a large reduction (55.26%) in leaf chlorophyll content and an even more dramatic loss (79.3%) in leaf carotenoid content with a greater loss in ‘Williams’ at the low mite infestation level. Results of these studies indicated that at least part of the increased resistance of ‘Williams’ to damage from T. urticae was due to a greater hypersensitive response of ‘Williams’ to mite attack. This response increased oxidizing conditions in the leaves, resulting in an extensive loss of carotenoids and chlorophyll. Increased oxidizing conditions in ‘Williams’ decreased mite population increases to a greater extent than it decreased net photosynthesis (dry matter accumulation).

Impact of Feeding by Tomato Fruitworm, Heliothis zea (Boddie) (Lepidoptera: Noctuidae), and Beet Armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on Processing Tomato Fruit Quality

Artificial infestations of Heliothis zea (Boddie) and Spodoptera exigua (Hubner) were made on three field plantings of processing tomatoes on a weekly basis in 1982 and 1983 to establish the damage relationship between infestations of each species during the season and damage at harvest. Comparisons of damage by both species were made to determine differences by week, level of infestation, and the week × level interaction. Damage was higher in the sampled plots during the season than in plots infested at the same time but sampled for damage at harvest. In general, the amount of damage recorded at harvest was greater as the date of infestation approached the harvest date. Differences in seasonal damage and damage at harvest were observed between weeks, between levels of infestation, and between the week × level interaction for all of the third-instar H. zea larval infestations, but only for the highest infestation levels of third-instar S. exigua . During 1982, when lower infestation levels were used, differences in damage due to timing of damage were not significant, although the general pattern held. During 1983, late instars were used, resulting in less mortality, higher levels of damage, and less variable damage.

Borer damage and estimation of loss caused by sugar-cane stem-borers in southern Ghana

Studies on damage by larvae of Eldana saccharina Walker, Chilo zacconius Bleszynski and Sesamia spp. to sugar-cane stands revealed a high degree of correlation between internode damage and stalk damage as well as between internode damage and sugar content of the canes. For every 1% increase in stalk damage there was a corresponding increase of 0.214% in the internode damage. The distribution of larvae inside the internodes was found to depend on the level of infestation. At low field infestation level (below 25 larvae per 100 canes) the distribution was random. As the level of infestation increases, the distribution becomes more aggregated. The distribution of damage in the sugar-cane field was found to depend on the overall stalk damage. The distribution tends to be more contagious when field level of stalk damage was less than 45%. The damage became evenly distributed when stalk damage rose above 60%. An estimation of average loss of sugar caused by stem-borers to the variety NCO 376 (at the field damage rate of 8.6, 10.6 and 3.1% damage for bottom, middle and top parts respectively of the cane) was U.S.S332.10 per ha of field.

Detection of honeybee tracheal mites ( Acarapis woodi) by simple staining techniques

A total of 12 staining techniques, including five anionic and five cationic stains, and two mordant stains, were tested as possible histopathological methods for the detection of the honeybee tracheal mite, Acarapis woodi. The cationic stains stained the parasitic mites intensively and the ensheathing tracheae weakly positive in thin thoracic sections. The improved clarity and contrast of the stained mites within the tracheae in the thin thoracic sections facilitated the detection process. Even at very low levels of infestation, stained mites were detected easily and accurately under a dissecting microscope at a magnification as low as 10 ×. Consequently, examination of dissected trachae under a phase compound microscope, currently used for diagnosis of the mites, is no longer necessary. The stained specimen can also be mounted permanently on microslides.

Competition of a Binary Weed System with Cotton (Gossypium hirsutum)

Cotton (Gossypium hirsutumL. ‘Stoneville 213’) was grown on Lucedale fine sandy loam with sicklepod (Cassia obtusifoliaL. ♯ CASOB) and a complex of redroot pigweed (Amaranthus retroflexusL. ♯ AMARE) and smooth pigweed (A. hybridusL. ♯ AMACH) in all possible combinations of 0, 1, 2, 4, 8, and 16 weeds of each species per 7.5 m of row. Seed cotton yields decreased as a quadratic function of increasing weed density. One pigweed and one sicklepod plant per 7.5 m of row reduced yields by 9 and 9.7% in 1979 and 1980, respectively. At low levels of infestation (≤4 weeds/7.5 m of row), the competitive effect of pigweed and sicklepod was additive; however, at the high densities, the competitive effect was not additive. Mechanical harvesting efficiency and cotton maturity were not decreased by any weed density. Sicklepod was more competitive than pigweed in both years.

Comparison and contrast of cotton and Desmodium tortuosum as hosts of Heliothis virescens

Desmodium tortuosum (Sw) DC (Papilionaceae) is a preferred wild host of Heliothis virescens (Fabricius) (Lepidoptera: Noctuidae) which is an important pest of cotton in Tolima, Colombia. From November 1976 until March 1979, cotton and D. tortuosum were compared and contrasted in the field as hosts of the pest. A very high infestation level of H. virescens in cotton in 1977 was preceded by an extremely low infestation level of the insect in D. tortuosum previous to the cotton-growing season. In 1978 and 1979 moderate infestation levels in cotton were preceded by moderately low infestation levels of H. virescens in D. tortuosum previous to the cotton growing season in both years.

Preharvest Infestation of Cowpeas by the Cowpea Weevil (Coleoptera: Bruchidae) and Population Trends during Storage in Florida

An examination of 62 one-bushel bags of cowpeas, representing 49 different fields, revealed an extremely low preharvest infestation level (mean 2.33 Callosobruchus maculatus [F.] adults) emerging per bushel per 2 weeks of storage. From similar initial infestation levels, populations either slowly dwindled away or after an 18-week delay increased exponentially.

Wind Dispersal of the Twospotted Spider Mite (Acari: Tetranychidae) in North Carolina Peanut Fields

The role of wind dispersal in the ecology of Tetranychus urticae Koch in North Carolina peanut fields was evaluated. Laboratory experiments demonstrated that even low-level infestations of T. urticae were dispersed by winds of only 8 km/h. However, the most mites were dispersed by 24-km/h winds when mites were massing. In the field the number of T. urticae caught was inversely proportional to trap height, and most mites are caught at about the height of peanut plants (0.6 m). More mites were caught blowing from southerly directions along with the predominant summer winds than from any other direction; however, mites were blown in all directions. Pesticides which caused T. urticae populations to increase in peanut fields resulted in significant increases in aerially dispersing mites. Mites were dispersed by wind in all treatments, on all dates, and with all populations measured.

Effect of various levels of infestation by the seed weevil (Ceutorhynchus assimilis Payk.) on the seed yield of oil-seed rape (Brassica napus L.)

SUMMARYTo determine economic thresholds of pest infestation caged plots of oil-seed rape (Brassica napus L.) were infested with different populations of the seed weevil (Ceutorhynchus assimilis Payk.). Less than one seed weevil per plant resulted in pod infestation levels of seed weevil larvae that by themselves were insufficient to warrant insecticide application, but owing to secondary infestation of pod midge (Dasyneura brassicae Winn.), substantial seed loss occurred. Greater populations of seed weevil adults gave correspondingly larger levels of pod midge and diminishing yields.In commercial crops individual plants were able to compensate, and even to overcompensate for damage resulting from low levels of pest infestation, but increasing levels of infestation resulted in progressively fewer seeds per plant.The implications of the results on pest control and further research are discussed.

Immune responsiveness of the bovine host to repeated low-level infestations with Dermacentor andersoni.

The effects of low level infestations with adult Dermacentor andersoni (Stiles) on the immune responsiveness of Bos taurus cows and calves were examined. Exposure regimen consisted of one to four ten-day infestations, each with ten female and five male ticks. Tick feedings were separated by ten-day tick-free periods. Antibody responsiveness to infestation differed for cows and calves, calves uniformly displaying precipitating antibodies to D. andersoni salivary gland antigen (SGA) after all infestations. Most of the cows displayed precipitating antibodies before and after the first infestation, but by the end of the second infestation most cows had lost detectable precipitating antibodies to SGA. All tick-exposed cows and calves gave significant immediate and delayed reactions when skin-tested with SGA. Peripheral blood lymphocytes derived from cows and calves at the termination of a third and fourth infestation reacted in vitro in an antigen-specific manner to SGA, indicating the presence of an antigen-r...

Site and Host Factors Affecting the Sitka Spruce Weevil, Pissodes strobi1 , in Western Washington 2

Adult Pissodes strobi activity and brood production were compared at spruce plantations with high and low weevil infestation levels. During 2 years of observation, weevil activity started ca. 30 days earlier at the heavily infested plantation. Caging adults on terminals at both sites showed that the subsequent brood mortality was significantly higher at the plantation with low weevil damage. Poor brood survival was attributed mainly to resinosis.

Responses of ‘Kentucky-31’ Tall Fescue to Varying Forcipata loca DeLong & Caldwell Infestation Levels: Growth Chamber Study (Homoptera: Cicadellidae, Typhlocybinae) 1

The effects of various infestation levels of Forcipata loca Delong & Caldwell on the yield, tillers/plant, yield/tiller, leaf elongation rate, carbon dioxide exchange rate, specific leaf weight, leaf diffusive resistance, and water soluble carbohydrate concentration of ‘Kentucky-31’, tall fescue, Festuca arundinacea Schreb., were studied. The growth response of tall fescue to 9 and 27 leafhoppers/9.5 cm diam. plastic pot was increased tillering with corresponding decreases in yield and yield/tiller after each of 2 infestation periods. Leaf elongation rates were less at the low infestation level of 3 leafhoppers/pot. Photosynthesis and specific leaf weight were only affected at the high feeding level (27 leafhoppers/pot) and only after a second infestation period. There was no significant reduction in leaf diffusive resistance at either infestation level. Water soluble carbohydrate concentration in the leaves decreased with increasing infestation levels.

Influence of Cotton (Gossypium hirsutum) Densities on Competitiveness of Pigweed (Amaranthusspp.) and Sicklepod (Cassia obtusifolia)

Experiments were conducted from 1973 through 1975 on Lucedale sandy loam to determine the influence of in-row cotton (Gossypium hirsutumL. ‘Stoneville 213’) densities on the competitiveness of low-level infestations of sicklepod (Cassia obtusifoliaL.) and pigweed (Amaranthusspp.). Weeds were established at densities of 0, 4, 12, and 32 weeds per 15 m of row and allowed to compete the entire season with cotton grown at densities of 5, 10, or 20 plants/m of row corresponding to 47000, 94000 and 187000 cotton plants/ha. Conventional cultural practices were employed in these experiments. Cotton yields were inversely related to weed density; however, the density of cotton did not influence the competitive effect of sicklepod or pigweed. Pigweed or sicklepod dry weed weight was reduced when competing with 187000 cotton plants/ha.