Stanislav Trdan

How reliable are thermal constants for insect development when estimated from laboratory experiments

The concept of thermal time, where a linear relationship between developmental rate (d) and environmental temperature (T) is assumed for plants and poikilothermic animals (∂ = a + b · T ; a is the intercept, b the slope, and ∂ the model estimate of d) (e.g., Bonhomme, 2000), has been widely used as an approximation of physiological time (Trudgill et al., 2005). This approach has been especially common in studies of insect development (e.g., Hon e k & Kocourek, 1990; Hone k, 1996), as temperature plays a major role in the course of their life. Although a linear approximation of the generally non-linear relationship between developmental rate and temperature (e.g., Briere & Pracros, 1998) is valid only within a limited range of temperatures, the simplicity of the thermal time approach makes it very valuable in practice. Thermal constants [lower developmental threshold (LDT) and sum of effective temperatures (SET)] can be estimated from the assumed linear relationship, usually determined from laboratory observations of insect development at different constant temperatures. The LDT, which is defined as the temperature at which the development of insect ceases (d = 0) (e.g., Jaro8 ik et al., 2004; Trudgill et al., 2005), can be estimated as LDT = − a/b. The SET, which is defined as the temperature difference above LTD when total development (d = 1) is reached in a single day (e.g., Jaro8 ik et al., 2004; Trudgill et al., 2005), can be estimated as SET = 1/b (Figure 1). Because the values d = 0 and d = 1, which determine LDT and SET, are extremes in developmental rates and therefore outside the range of values used for the development of the linear model, the estimation of thermal constants is a typical example of extrapolation. This stimulated us to investigate how uncertain the estimates of thermal constants actually are; we base our investigation on laboratory experiments, due to the extrapolation inherent to the computational procedure.

Control of the Colorado potato beetle (Leptinotarsa decemlineata [Say]) on potato under field conditions: a comparison of the efficacy of foliar application of two strains of Steinernema feltiae (Filipjev) and spraying with thiametoxam.

The efficacy of entomopathogenic nematodes in controlling Leptinotarsa decemlineata on potato was tested in a field experiment. A Slovenian strain of Steinernema feltiae (B30) was used with the commercial product Entonem and the insecticide thiametoxam. The application of the nematodes and thiametoxam was repeated twice. In the second application half of the first dose (250.000 infective juveniles m–2) was used. Observing the population dynamics of the pest, it has become apparent that entomopathogenic nematodes significantly decreased the number of larvae, while no effect on their eggs and adults was confirmed. In controlling larvae in both years thiametoxam showed the best results in general, meanwhile treatments with EPNs showed no differences. In both years the highest yield was obtained when thiametoxam was used (2008: 24.38 t ha–1; 2009: 21.60 t ha–1), while the lowest yield was in the control treatment (2008: 7.65 t ha–1; 2009: 6.94 t ha–1). Different treatments of EPNs showed differences, and yields ranged from 16.44 (S. feltiae B30 low concentration) to 17.81 t ha–1 (Entonem high concentration) in 2008, and between 11.06 (S. feltiae B30 low concentration) and 14.88 t ha–1 (Entonem high concentration) in 2009. Both strains of entomopathogenic nematodes showed sufficient activity when controlling larvae, but unfortunately the high price of these biological agents at the moment makes it difficult for their broader implementation in the systems of potato production.ZusammenfassungDie Wirksamkeit entomopathogener Nematoden bezüglich der Bekämpfung des Kartoffelkäfers (Leptinotarsa decemlineata) wurde im Feld an Kartoffeln untersucht. Ein slowenischer Stamm von Steinernema feltiae (B30) und das kommerzielle Produkt Entonem wurden mit dem Insektizid Thiametoxam in zweifacher Wiederholung verglichen. Bei der zweiten Anwendung wurde mit 250.000 Nematodenlarven m–2 die halbe Dosis der ersten ausgebracht. Populationsdynamische Beobachtungen am Schadtier ergaben, dass die entomopathogenen Nematoden zwar die Anzahl der Käferlarven signifikant verminderten, die Anzahl der Eier und Adulten aber nicht beeinflussten. Gegenüber den Käferlarven zeigte Thiametoxam in beiden Jahren generell die stärkste Wirkung, während sich die Wirkung der Varianten mit entomopathogenen Nematoden nicht voneinander unterschied. Die höchsten Kartoffelerträge wurden in beiden Versuchsjahren mit Thiametoxam erzielt (2008: 24,38 t ha–1; 2009: 21,60 t ha–1), während die geringsten Mengen in der Kontrolle geerntet wurden (2008: 7,65 t ha–1; 2009: 6,94 t ha–1). Die verschiedenen Behandlungen mit Nematoden wirkten unterschiedlich; die Erträge variierten zwischen 16,44 (S. feltiae B30 in geringer Dosierung) und 17,81 t ha–1 (Entonem in hoher Dosierung) im Jahr 2008, sowie zwischen 11,06 (S. feltiae B30 in geringer Dosierung) und 14,88 t ha–1 (Entonem in hoher Dosierung) im Jahr 2009. Beide untersuchten Nematodenstämme zeigten gegenüber den Larven des Kartoffelkäfers ausreichende Wirksamkeit, aber ihr derzeitig hoher Preis beeinträchtigt noch ihre verbreitetere Anwendung im Kartoffelbau.

Quantifying the damage of red deer ( Cervus elaphus ) grazing on grassland production in southeastern Slovenia

The influence of red deer (Cervus elaphus) grazing on grassland production for forage conservation at the forest border during the vegetation period was studied on three locations (Mala gora, Cvišlerji, and Mačkovec) in the Kočevje region (SE Slovenia). The experiment lasted from the 25th of March until the 8th of October 2002. Portable cages were used to exclude red deer from grazing the herbage. At four sampling dates in the season, herbage air dry matter (DM) yield was measured at three different observations (cage-protected plot, cage-protected plot only two to three weeks before sampling date, otherwise freely grazed (removed), and unprotected plot). The results from the experiment showed that red deer grazed on grassland through the entire season and that the regeneration capability of sward was the highest in summer, middle in spring and smallest in autumn. On unprotected plots, an average reduction of 50% of herbage DM yield was found with the most damaged sites also up to 80% reduction.

Activity of four entomopathogenic nematode species against young adults of Sitophilus granarius (Coleoptera: Curculionidae) and Oryzaephilus surinamensis (Coleoptera: Silvanidae) under laboratory conditions

Four entomopathogenic nematode species (Steinernema fel-tiae, Steinernema carpocapsae, Heterorhabditis bacteriophora, and Heterorhabditis megidis) were tested in a laboratory bioassay with the aim of studying their efficacy in control of the adults of two stored grain pests, Sitophilus granarius and Oryzaephilus surinamensis. Activity of the biological agents studied was determined at three different concentrations (500, 1000, and 2000 infective juveniles [IJs] per adult) and temperatures (15, 20, and 25°C). The granary weevil mortality rate was higher than the mortality rate of the saw-toothed grain beetle. Heterorhabditis megidis proved to be the least efficient in control of both pests, while no significant differences were recorded between any of the other three nematode species. The experiment demonstrated that the entomopathogenic nematodes were most efficient in the control of S. granarius at 20°C (LC50 after 7-day exposure 803–1195 IJs/adult) and 25°C (LC50 505–1175 IJs/adult). A satisfactory level in control of the pest O. surinamensis was reached at 20°C (LC50 921–1335 IJs/adult). The concentration of the suspension used in our experiment was shown to be a less important factor affecting the biological activity of nematodes against the adults of both stored grain pests. Though the use of entomopathogenic nematodes for control of the tested pests is not possible at the present time, it may be possible to combine this approach with some other (biotechnical) methods in the future.ZusammenfassungVier entmopathogene Nematodenarten (Steinernema feltiae, Steinernema carpocapsae, Heterorhabditis bacteriophora und Heterorhabditis megidis) wurden in einem Labor-Biotest hinsichtlich ihrer Wirksamkeit bei der Bekämpfung zweier Vorratsschädlinge des Getreides, Sitophilus granarius und Oryzaephilus surinamensis, untersucht. Ihre Wirksamkeit wurde in drei verschiedenen Konzentrationen (500, 1000 und 2000 Infektionslarven [IL] pro Adultem) und Temperaturen (15, 20 und 25°C) ermittelt. Die Mortalität des Kornkäfers S. granarius übertraf dabei die des Getreideplattkäfers O. surinamensis. Heterorhabditis megidis war die ineffektivste Nematodenart gegenüber beiden Vorratsschädlingen, während zwischen den drei anderen Nematodenarten keine signifikanten Unterschiede auftraten. Das Experiment zeigt, dass die entomopa-thogenen Nematoden den Kornkäfer am effektivsten bei Temperaturen von 20°C (LC50 nach 7 Tagen 803–1195 IL/Adultem) und 25°C (LC50 505–1175 IL/Adultem) kontrollieren. Ein befriedigender Bekämpfungserfolg gegenüber dem Getreideplattkäfer wurde bei 20°C (LC50 921–1335 IL/Adul-tem) erreicht. Die Konzentration der verwendeten Nemato-densuspension beeinflusste den Bekämpfungserfolg gegenüber beiden Schädlingsarten dagegen weniger. Obwohl die Verwendung entomopathogener Nematoden zur Kontrolle der beiden untersuchten Vorratsschädlinge des Getreides zur Zeit noch nicht praktikabel ist, könte dieser Ansatz zukünftig in Kombination mit anderen (biotechnischen) Verfahren zum Erfolg führen.

Impact of Climate Change on Developmental Dynamics of Thrips tabaci (Thysanoptera: Thripidae): Can It Be Quantified?

Abstract We attempt to quantify the impact of future climate change on the developmental dynamics of onion thrips in Slovenia. Monthly averaged results of simulations of future climate from four different general circulation models (GCMs) were projected to local scale by empirical downscaling. The GCM simulations were based on two emission scenarios (IPCC SRES A2 and B2). Local estimates of monthly averaged air temperatures for five locations in Slovenia were adjusted for an additional four emission scenarios (SRES A1T, A1F1, A1B, and B1) using a pattern scaling technique. They were further transferred to a daily scale using a first-order autoregressive model. A simple degree-day model, based on data reported in the literature, was used to relate the development of onion thrips to temperature. Potential changes in the period with favorable developmental conditions for onion thrips (i.e., temperatures above the lower developmental threshold) and in the number of generations per season were estimated with regard to the expected future climate change in Slovenia. The changes are influenced by the magnitude of temperature increase, its asymmetry within the year, and present climate conditions. Using this approach, one can obtain quantitative estimates of the impact of climate change on the developmental dynamics of an insect pest, but one must be fully cognizant of all the assumptions made in the procedure, which introduce uncertainties in the final results. Further research is needed to evaluate the plausibility of such simplified projections.

GLUCOSINOLATES IN PLANT PROTECTION STRATEGIES: A REVIEW

This review discusses the importance of glucosinolates in plant protection. The Brassicaceae, which are culti- vated worldwide, use glucosinolates and their decomposition products to defend themselves against attacks by harmful organisms. The glucosinolate content varies among individual plant species, plant organs and developmental stages. The glucosinolate content in plants is also affected by biotic and abiotic factors, while the type or quantity of glucosinolate de- termines the susceptibility of the plants to insect pests. These facts can pose a problem when implementing this knowledge in cultivation of the Brassicaceae, especially in regions with moderate climates where Brassicaceae crops are exposed to attacks by a large number of harmful organisms. Under these circumstances, it is essential to research new, or to improve the existing environmentally acceptable methods of protecting Brassicaceae plants against economically important pests.

Impact of entomopathogenic nematodes on adults of Phyllotreta spp. (Coleoptera: Chrysomelidae) under laboratory conditions

Abstract In 2005, preparations of four species of the entomopathogenic nematodes Steinernema feltiae, S. carpocapsae, Heterorhabditis bacteriophora, and H. megidis, were tested under laboratory conditions for their ability to kill adult flea beetles, Phyllotreta spp. (Coleoptera: Chrysomelidae). The nematode preparations were tested at doses of 200, 1000, and 2000 IJs per adult and at temperatures 15°C, 20°C, and 25°C. The numbers of beetles killed were recorded two, four, six, and eight days after treatment. The nematodes were more effective at 20°C and 25°C than at 15°C. At 20°C, the nematodes had killed between 44% (H. megidis at the lowest dose tested) and 77% (S. feltiae at the lowest dose tested) of the beetles eight days after treatment. At the two highest doses tested, the S. feltiae, S. carpocapsae, and H. bacteriophora preparations each killed at least 74% of the beetles at 25°C. Steinernema feltiae was the most effective nematode (LC50=483–1467 IJs/adult) and, as an alternative to chemical insecticides, appears to have the highest potential for controlling overwintered flea beetles (May) under field conditions. The nematodes S. feltiae, S. carpocapsae and H. bacteriophora would all be suitable for controlling adult flea beetles during warm summer months, when flea beetles occur in high numbers in Slovenia.

Traditional and modern methods for the identification of thrips (Thysanoptera) species

Many thrips are pests of commercial crops due to the damage they cause by feeding on developing flowers or vegetables. Thrips may also serve as vectors for plant diseases, such as tospoviruses. Their small size and predisposition towards enclosed places makes them difficult to detect by phytosanitary inspection. In this review, several methods available for identifying thrips, including their advantages and disadvantages, are discussed. A combination of different methods gives the most reliable identification. Relatively new morphometric, molecular and biochemical methods for identifying thrips species represent valuable alternatives for situations in which correct identification with classical morphological methods is very difficult, time consuming or virtually impossible. However, traditional morphological methods should not be neglected, especially because adequate identification using morphological keys is usually an indispensable first step in the development and validation of these new modern methods. In addition, modern systems may still require specimen identification to the genus level via morphological keys, or such keys may be recommended to confirm the results of modern identification methods.

Field efficacy of deltamethrin in reducing damage caused by Thrips tabaci Lindeman (Thysanoptera: Thripidae) on early white cabbage

Our research assessed how the number of sprayings with deltamethrin influenced the efficiency against onion thrips, Thrips tabaci, on early white cabbage. The study was performed in 2002 and 2005 using the early white cabbage cultivar Parel, which is very susceptible to the attack of onion thrips. The cabbages were planted in a block-design experiment, with percentage of damage on the exterior leaves recorded according to the positions of the leaves (i.e. outer to inner). Although the attack of thrips was much greater in 2002 than in 2005, nevertheless, control plants in both years showed much more damage than plants with at least one insecticide spraying. In 2002, there was little difference between one and two sprayings, but three sprayings showed a significantly improved result. However, three sprayings of this insecticide is generally not permitted in farming practice. In 2005, no differences in efficacy among one, two and three sprayings were recorded. We also assessed a high threshold (up to 10% damaged leaf surface) and low threshold (up to 1% damaged leaf surface) for the removal of damaged leaves. In 2002, both thresholds were exceeded for control plants, but in 2005, the attack of thrips was mild enough that neither threshold was exceeded. In this case, one spraying was sufficient to reduce leaf damage below the higher (more tolerant) threshold, but three sprayings were needed to reduce leaf damage below the lower (more stringent) threshold. Therefore, in Slovenia or other parts of the world with similar climatic conditions, we recommend one spraying as broadly effective and recommend the more tolerant threshold for leaf removal.

Field efficacy of three insecticides against cabbage stink bugs (Heteroptera: Pentatomidae) on two cultivars of white cabbage

Abstract In 2002 – 2003, three insecticidal substances were tested for their ability to reduce the damage inflicted on plants of white cabbage by cabbage stink bugs, Eurydema ventrale and E. oleracea (Heteroptera: Pentatomidae). The two cultivars of cabbage tested were: the early cultivar, ‘Tucana F1’, which requires about 60 days from transplanting to harvest, and the mid-early cultivar ‘Hinova F1’, which requires about 120 days. In a field experiment, the growth and damage on cabbage plants sprayed with malathion (two sprays), potassium soap [potassium salts of fatty acids] (five sprays), and refined rape oil (five sprays) were compared to the growth and damage on unsprayed plants. Potassium soap and refined rape oil are considered to be environmentally friendly insecticides. In 2002, plants of the cultivar ‘Tucana’, that were treated with malathion had the highest plant weights [PW], head weights [HW], HW/PW ratio, and the lowest number of frame leaves. In the drier year of 2003, there were no significant differences in damage between the three treatments for either cultivar, except during the last assessment done on 13 August. In 2003, plants of ‘Tucana’, treated with potassium soap, had the highest PW and HW. The significantly lowest PW and HW values were recorded from the untreated plants. For the cultivar ‘Hinova’, the PW and HW were heaviest from the plants treated with malathion. In both years, the HW/PW ratio of Hinova (mid-early cultivar) did not differ between treatments, while the plants of ‘Tucana’ (early cultivar) treated with malathion had the highest ratio. Of the three substances tested, malathion was the most effective at reducing the damage on white cabbage plants caused by cabbage stink bugs. However, in an attempt to reduce the use of synthetic insecticides, instead of malathion, we recommend the use of refined rape oil (49 EUR/ha), as it is as effective as potassium soap (81 EUR/ha), but much cheaper.