Luko Hilje

Cultural practices for managing Bemisia tabaci and associated viral diseases

Abstract Whiteflies ( Bemisia spp.) and the viruses they vector cause extensive losses to many horticultural and agronomic crops throughout the tropics and subtropics. These losses have spurred a worldwide search for cost-effective management strategies. Cultural practices can play a significant role in integrated pest management (IPM) systems targeting whiteflies, because of their preventative nature. Yet, cultural practices have received disproportionately little attention from researchers, possibly due to the difficulty of testing by conventional methods. Practices such as crop-free periods, altering planting dates, crop rotation, and weed and crop residue disposal, perform well only if used on a regional scale and therefore are difficult to test or demonstrate experimentally. Growers may also be reluctant to adopt cultural practices such as living barriers, high planting densities, floating row covers, mulches, and trap crops, that require significant changes in conventional cropping practices. Nonetheless, we have seen adoption in recent years of some cultural practices to manage whiteflies, such as crop planning that includes host-free periods, and various forms of screened exclusion. This review focuses on research efforts, field utilization, and the potential of cultural practices to manage the whiteflies and associated viral diseases.

Development of Hypsipyla grandella (Zeller) (Lepidoptera: Pyralidae) in response to constant temperatures

Developmental response of the mahogany shootborer, Hypsipyla grandella (Zeller), to temperature was determined in laboratory trials in Turrialba, Costa Rica. Two hundred fresh eggs (less than 24h old) were placed individually inside glass flasks, and exposed to seven constant temperatures (10, 12.5, 15, 20, 25, 30 and 35°C), in separate experiments. These were carried out in environmental chambers, at 80-90% relative humidity and 8L:16D photoperiod. After hatch, larvae were fed tender foliage of Spanish cedar (Cedrela odorata) and were inspected every 24h to determine larval and pupal duration, and adult emergence, at each temperature. Relationships between these variables and temperature were analyzed by means of ANOVA, regression and correlation procedures. Temperature greatly influenced development of H. grandella immature stages and adult emergence; development time varied between 30 days (30oC) and 104 days (15°C). Larval mortality was high at those temperatures, reaching values of 90% (15°C) and 45% (30oC). Also, mortality of the first larval instar was relatively high (51-75%) at all temperatures in the range 15-30°C, except at 25oC (14%). Pupal weight was lower at extreme temperatures (15oC and 30oC). Estimation of a common lower thermal threshold for all immature stages (8.5oC) allowed calculation of a general thermal constant (1320 degree-days), which could be used to predict population peaks in the field.

Population trends and damage patterns of Hypsipyla grandella (Lepidoptera: Pyralidae) in a mahogany stand, in Turrialba, Costa Rica

Abstract  1 Population trends of Hypsipyla grandella and their relationship with abiotic (humidity, precipitation and temperature) and biotic factors (availability of mahogany shoots and natural mortality agents), as well as damage patterns, were studied for 16 months in Turrialba, Costa Rica.

Effect of farm diversity on harvesting of coffee leaves by the leaf-cutting ant Atta cephalotes

1 In Mesoamerica, shade trees are often included within coffee (Coffea arabica) agroforestry systems. Shade trees potentially protect the main crop by increasing vegetational diversity and reducing insect herbivory through one or more mechanisms.

Hopper (Homoptera: Auchenorrhyncha) diversity in shaded coffee systems of Turrialba, Costa Rica

Diversity of hopper species (suborder Auchenorryncha) in coffee (Coffea arabica) plantations with no shade (C) was compared with the diversity in plantations with shade of either poró (Erythrina poeppigiana) (CP) or poró plus laurel (Cordia alliodora) (CPL) in Turrialba, Costa Rica. Species-abundance and rarefaction curves were plotted for each system, and indices of diversity (Shannon-Wiener), dominance (Simpson), species evenness, and similarity (Jaccard) were calculated. The majority of hopper species and individuals belonged to the Cicadellidae family. A particular species dominated in each system: Graphocephala sp. (C), Fusigonalia lativittata (CP) and Hebralebra nicaraguensis (CPL). The richness and diversity of hopper species were highest in the CP system, followed by the CPL and C systems. Species similarity was closest between the CP and CPL systems, but varied considerably according to plant component and geographic location of each plot. Even though hoppers have not been reported as coffee pests in Mesoamerica, some of them cause serious problems elsewhere.

Use of grafting to prevent Hypsipyla grandella (Zeller) (Lepidoptera: Pyralidae) damage to new world Meliaceae species.

The susceptible species Cedrela odorata and Swietenia macrophylla to attack by Hypsipyla grandella (Zeller) larvae were grafted onto the resistant species Khaya senegalensis and Toona ciliata. Six-month-old grafted plants were then compared to their reciprocal grafts and to both intact (non-grafted) and autografted plants for damage due to H. grandella larvae and for their effects on larval performance. Two experiments were conducted: one in which the apical bud of the main plant shoot was inoculated with H. grandella eggs, and the other in which the bud was inoculated with third instars. Damage in each experiment was assessed by the number of frass piles, number and length of tunnels, number of damaged leaves, and damage to the apical bud. Larval performance was evaluated in terms of time to reach pupation and pupal weight and length. In both experiments, plant damage differed significantly among treatments (P < 0.03). Resistant rootstocks conferred resistance to susceptible scions. In both experiments, grafting by itself, regardless of the rootstock and scion combination, also reduced damage caused by H. grandella larvae. Scions of autografted susceptible species had similar resistance to susceptible scions grafted on resistant rootstocks. Few larvae reached pupation, and their pupal weight and length were similar.

A model for assessing the effect of distance on disease spread in crop fields

Standard logistic regression is applied to model spatio-temporal plant disease spread in crop fields. The binary response variable observed at a given point in time is the disease status Y, taking on the value Y=1 for plants newly infected and Y=0 for healthy plants. The predictor variable used and tested here is the distance of a newly infected or healthy plant to the closest plant that was previously diseased, hypothesizing that disease spread originates largely from foci within the crop field. The method is illustrated with three sets of artificial data, and with four real data sets from published studies and our own studies. In the artificial data sets a random disease spread, a distance-dependent, and a both distance- and orientation-dependent disease spread were simulated, and then clearly identified as such by the corresponding logistic regression models. For one of the real data sets (citrus variegated chlorosis) the results correspond directly to the published results where clustering of disease incidence was found, which is one form of distance dependence. For the other published data set (papaya ringspot) no statistically significant effect of distance was identified. The non-randomness of disease spread as identified in the original source is supposedly due to reasons other than the distance effect investigated here. In one of the tomato fields analyzed, a clear distance dependence of the tomato yellow mottle disease (ToYMoV) spread was found. In the other field, where disease progress was very fast, no effect of distance could be observed. The proposed method is a flexible analysis tool. It does not require the plants in a regular lattice; rather the plants location is defined by its metric coordinates. Like in any spatio-temporal analysis, the definition of the time interval between measurements plays an important role. Too short or too long intervals will make it impossible to identify a systematic pattern in disease spread. The method tests not only for non-randomness of disease spread in general, but it also helps to identify the effect of specific predictor variables. Thus, a specific hypothesis can be researched, helping to obtain insight into the mechanisms of disease spread and helping to develop specific disease management approaches.

Coffee farm diversity and landscape features influence density of colonies of Atta cephalotes (Hymenoptera: Formicidae).

ABSTRACT The density of colonies of leaf-cutting ants, Atta cephalotes L. (Hymenoptera: Formicidae), was measured and compared among coffee (Coffea arabica L.) plantations in five management categories: monoculture conventional, diversified conventional, diversified organic, highly diversified conventional, and highly diversified organic. Twenty-four small farms (<4 ha) in Turrialba, Costa Rica, were included in this study. Within-farm and off-farm (landscape) variables were measured and tested for their relationship with A. cephalotes colony density. Total ant colony density (colonies per ha) and density of new colonies shortly after a nuptial flight were significantly greater in the coffee monoculture conventional system, compared with all other systems. Total ant colony density and density of new colonies were inversely related to percentage of shade within the farms. Within farms, colony density was greater near edges adjacent to riparian forest than those adjacent to nonforested land. Regardless of edge type, plots closer to the edge (0–30 m) had greater colony density than those furthest from the edge. At the landscape scale, density of new colonies was positively related to fallow land use coverage within a 2,000-m buffer radius and to forest coverage within a 500-m radius. Results indicate that coffee farm management practices and landscape level factors can affect A. cephalotes colony densities. Understanding such practices and factors could assist in the development of better management methods of these injurious insects in coffee farms. Increased diversification in coffee farms, possibly due to the greater shade associated with it, may reduce colonization by the ants, which are considered forest gap specialists.