Athayde Tonhasca

The effects of crop diversification on herbivorous insects: a meta‐analysis approach

Abstract. 1 We used a meta‐analysis to evaluate the hypothesis that diversified crops are subject to lower density of herbivorous insects. This method consists of integrating findings of independent studies by calculating the magnitude of treatment effects (effect size). 2 A literature review covering a period of over 10 years yielded twenty‐one studies that contained sufficient information for the meta‐analysis. The data were analysed according to three criteria: yearly replications of the same study were considered independent data so that research repeated over time would have greater weight; a second meta‐analysis was conducted with results of only 1 year because of possible lack of independence between results of the same study; the third approach was to remove studies involving Brassica spp. from the analysis because they are redundant from the standpoint of testing a general hypothesis of crop diversification. 3 The three analyses resulted in effect sizes of 0.50, 0.35 and 0.27, respectively, and all were statistically significant. These results correspond to 69%, 63% and 60% differences between average insect densities for treatment (diversified crops) and control groups. 4 We considered the last criterion for data analysis as the most appropriate because of lowest bias from non‐independence between samples. Therefore we concluded that crop diversity caused moderate reduction of herbivorous insect populations. 5 The implications and limitations of meta‐analyses are discussed.

First record of Phorid Parasitoids (Diptera: Phoridae) of the leaf-cutting ant Atta bisphaerica Forel (Hymenoptera: Formicidae)

Phoridae flies parasitize several ant species, including many Atta leaf-cutting ants. In this note, the attacks of three coexisting phorid species (Myrmosicarius grandicornis Borgmeier, Apocephalus attophilus Borgmeier and Neodorhniphora bragancai Brown) against Atta bisphaerica Forel workers in a pasture located in Vicosa County, Minas Gerais State, Brazil, are reported. The first two species of phorids are known to parasitize other leaf-cutting ants. N. bragancai, however, has been found only around A. bisphaerica nests. Each of these phorid species selects ants engaged in different tasks and oviposits in specific sites of the ant body.

Aggregation patterns of Bemisia tabaci in response to insecticide applications

We measured the effect of insecticide applications on the distribution patterns of the sweet potato whitefly, Bemisia tabaci (Gennadius), in fields of cantaloupe, Cucumis melo L., in Yuma, Arizona. Whitefly infestations were measured by counts of adults, eggs, first to mid/fourth instars, and late‐fourth instars (‘red‐eyed’) nymphs. Adults were sampled from the entire leaf, and immature stages were counted in a 1‐cm2 area of a leaf. The indices b (Power Law), β (Patchiness regression) and Id (Morisita) indicated that all life stages were aggregated, but results for the three indices were not similar for determining the relative aggregation levels between treatments. In general, indices β and Id indicated higher aggregation in insecticide‐treated fields, whereas b had mixed results. The Morisita index was sensitive to a few unusually high means among a series of low densities in the treatment plots, what could be attributed to refuges due to failure in the insecticide applications. Despite the usefulness of the Power Law and the Patchiness regression for describing the relationship between spatial or temporal variability and mean densities, we suggest that Id is more appropriate for expression of spatial distribution because it is based on a precise definition of aggregation.

Binomial sampling plans for estimating Bemisia tabaci populations in cantaloupes

Early season infestations of the sweet potato whitefly,Bemisia tabaci (Gennadius), on cantaloupes,Cucumis melo L., were determined by counts of the number of adults per leaf in fields near Yuma, Arizona. We used these data to develop binomial sampling plans based on the relationship between mean densities of whiteflies per leaf,m, and proportion of leaves infested with more thanI whiteflies,P I, according to the empirical model lnm=a′+b′ ln[−ln(1−P I)]. The models were developed for the presence-absence approach (I=0) and for a cutoff value of three whiteflies per leaf (I=3). Four independent data sets were used to evaluate the models. Both methods yielded reliable predictions at low infestation levels, but some of the higherm values were overestimated. As the tentative economic threshold forB. tabaci is three adults per leaf, which corresponds to lowP I values, results of the binomial sampling were satisfactory for pest management purposes.