Reliability of Degree-Day Models to Predict the Development Time of Plutella xylostella (L.) under Field Conditions.

The diamondback moth, Plutella xylostella (L.), is a cosmopolitan pest of brassicaceous crops occurring in regions with highly distinct climate conditions. Several studies have investigated the relationship between temperature and P. xylostella development rate, providing degree-day models for populations from different geographical regions. However, there are no data available to date to demonstrate the suitability of such models to make reliable projections on the development time for this species in field conditions. In the present study, 19 models available in the literature were tested regarding their ability to accurately predict the development time of two cohorts of P. xylostella under field conditions. Only 11 out of the 19 models tested accurately predicted the development time for the first cohort of P. xylostella, but only seven for the second cohort. Five models correctly predicted the development time for both cohorts evaluated. Our data demonstrate that the accuracy of the models available for P. xylostella varies widely and therefore should be used with caution for pest management purposes.

Assessing the Total Mortality Caused by Two Species of Trichogramma on Its Natural Host Plutella xylostella (L.) at Different Temperatures.

Trichogramma pretiosum Riley and Trichogramma atopovirilia Oatman & Platner are natural enemies of Plutella xylostella (L.) in Southern Brazil. Laboratory studies to evaluate parasitoids performance under different conditions, such as temperature regimes, are necessary to assess their potential as biocontrol agents of P. xylostella. In most studies involving Trichogramma, parasitism rate is the main parameter used to evaluate parasitoid performance, ignoring that parasitoids can cause egg mortality by feeding on the host content and/or to multiple drilling without laying eggs. This study was conducted to investigate three main issues: how temperature affects T. pretiosum and T.atopovirilia development on eggs of P. xylostella, whether or not these species respond differently to temperature, and how important is the mortality they cause besides parasitism on P. xylostella. Temperature effects (from 10 to 30°C) on development, survival, parasitism rate, mortality, and total mortality caused by T. pretiosum and T. atopovirilia on eggs of P. xylostella were evaluated. Temperature affected the development time, female longevity, parasitism rate, mortality not directly related to parasitoid larval development, and total mortality caused on the host. No significant differences were recorded for the estimated thermal requirements for T. pretiosum and T. atopovirilia. However, the higher mortality caused by T. pretiosum indicates that this parasitoid is the most suitable to be used against P. xylostella. Also, the results suggest that the use of parasitism rate as the only parameter to evaluate the performance of T. pretiosum and T. atopovirilia may underestimate the potential of these parasitoids in regulating pest populations.

Temperature-Dependent Parasitism, Survival, and Longevity of Five Species of Trichogramma Westwood (Hymenoptera: Trichogrammatidae) Associated with Anticarsia gemmatalis Hübner (Lepidoptera: Noctuidae).

Egg parasitoids of the genus Trichogramma Westwood play an important role in the control of the velvetbean caterpillar Anticarisa gemmatalis Hübner in soybean crops in Southern Brazil. The effectiveness of Trichogramma species as biocontrol agents is dependent of several factors, but their adaptation to the climatic conditions of the region where they will be released is one of the most important. In this context, this study evaluated the effects of five constant temperatures ranging from 14 to 30°C on parasitism, age-specific survival, progeny production and longevity of Trichogramma pretiosum Riley, Trichogramma atopovirilia Oatman & Platner, Trichogramma acacioi Brun, Moraes & Soares, Trichogramma lasallei Pinto, and Trichogramma rojasi Nagaraja & Nagarkatti. We demonstrate that temperature differently affected the species of Trichogramma investigated when using eggs of the velvetbean caterpillar. We also demonstrate that T. pretiosum and T. atopovirilia are better adapted to a wide range of temperatures than the other species evaluated, and are therefore better suited as biocontrol agents for applied biological control programs of A. gemmatalis.

Modelling reproduction of Plutella xylostella L. (Lepidoptera: Plutellidae): climate change may modify pest incidence levels.

Temperature is considered to be an important abiotic factor influencing insect reproduction. Despite the importance of Plutella xylostella L. (Lepidoptera: Plutellidae) as a pest of brassicaceous crops worldwide, the effects of temperature on its reproduction are not well understood. We evaluated the effect of constant temperatures ranging from 10 to 32.5°C on the reproduction of P. xylostella and developed an oviposition model for the species. The model combined temperature-dependent parameters of total fecundity, age-specific oviposition rate and age-specific survival. Additionally, we modelled population growth as a function of temperature. The estimated parameters allowed us to discuss the possible consequences of global warming on P. xylostella distribution. Temperature affected the length of pre-oviposition after adult emergence, oviposition period, longevity, total fecundity and egg viability. The model predicted that both daily egg production and length of oviposition period decreased at temperatures below 15°C and above 25°C. Population growth increased linearly with temperature in a range from 10°C to 25°C; however, the model predicted a reduction in population growth at temperatures above 28.6°C. Data suggested that temperature plays a critical role in P. xylostella reproduction, and subtle differences in average temperature could have an impact on its population growth. This is especially important in the context of global climate change, which in turn could alter the distribution and abundance of the pest in some regions of the world.

Performance of the wheat armyworm, Pseudaletia sequax Franclemont, on natural and artificial diets.

Armyworms in the genus Pseudaletia Franclemont are important pests of cereal crops worldwide. The development of pest management strategies is largely dependent on studies involving the relationship among the pest, its host plants and natural enemies, as well as its susceptibility to chemical and biological control strategies. Part of these studies requires rearing the species of interest under laboratory conditions, which can be greatly facilitated by the use of artificial diets. In the present study, the effects of three artificial diets on development, survival, reproduction and demographic parameters of P. sequax were compared to a natural host plant, Pennisetum clandestinum. A diet based on pinto beans, wheat germ and torula yeast provided the best results in comparison to those obtained with the host plant. The most important life history parameters used in the evaluation of the nutritional quality of the diets were fecundity, egg viability and pupae deformities. The intrinsic rate of increase and the net reproductive rate were higher when individuals were fed on the artificial diet if compared to the host plant, indicating the suitability of the tested diet for the rearing of P. sequax. This artificial diet has been used successfully for the rearing of P. sequax without any fitness effect for over 18 consecutive generations.

Development and survival of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) as a function of temperature: effect on the number of generations in tropical and subtropical regions.

The diamondback moth, Plutella xylostella (L.), is the most important pest of brassicaceous crops worldwide. Since temperature is the major abiotic factor influencing insect development and thermal requirements may vary among insect populations, it is important to know the effect of temperature on development and survival of a subtropical strain of P. xylostella. Development and survival of the diamondback moth was evaluated under seven constant temperatures ranging from 10°C to 35°C. Development was completed between 10°C and 32.5°C, but at 35°C all individuals died in the larval stage. Data were fitted to one linear and five nonlinear models. Considering as criteria the goodness of fit and the ability to estimate parameters of biological significance, the models Briere-1 and Briere-2 were the most adequate to describe the relationship between temperature and development of P. xylostella. The linear model demonstrated that P. xylostella required 312.5 degree-days above a lower threshold of 6.3°C to complete development. The degree-day model showed that the number of diamondback moth generations in the tropical region of Brazil is nearly twice the number in the subtropical region of the country. This result explains, at least in part, the higher population levels of this species in the tropical region of Brazil, and also demonstrates that P. xylostella is tolerant to a wide range of temperatures (6.1-32.5°C). Therefore, temperature cannot be considered a limiting factor for the occurrence of diamondback moth throughout the year in most regions of Brazil.