Peter Esbjerg

Influence of Temperature and Soil Moisture on Some Population Growth Parameters of CYRTOMENUS BERGI (Hemiptera: Cydnidae)

Abstract Abundance of Cyrtomenus bergi Froeschner has been reported regularly under moist and damp conditions. The influence of temperature and soil moisture on development time and mortality of first, third, and fifth instars, longevity and fecundity of C. bergi adult females, as well as hatching time and rate of eggs were determined under laboratory conditions at different temperature and soil moisture levels. Population growth is optimal around 26°C (constant temperature) and a soil moisture regime ranging from moist (field capacity) to wet soil (between field capacity and water saturation). Wet soil (44% gravimetric soil water) promotes high mean fecundity in young adult females, reducing generation time and favoring population growth compared to that seen in moist soil (33.5% gravimetric soil water, field capacity). The lower temperature threshold for development was 14.7°C. Neither egg hatching nor molting from fifth instars to adults occurred above 31°C. The lower soil moisture threshold for immature development was between dusty (19% gravimetric soil water) and very dry soil (22% gravimetric soil water) and between very dry and dry (25.5% gravimetric soil water, wilting point) for adult female survival and oviposition. Third instars were most tolerant to extreme temperatures. These abiotic limitations to population growth together with other findings concerning host plant regime and movement in soil may explain patterns of local and regional abundance.

Oviposition response of the Turnip moth to soil moisture

Abstract The survival of cutworms (Agrotis segetum Schiff., Lepidoptera: Noctuidae) in northern Europe is highly dependent on soil moisture conditions, and the abundance and damage caused by this crop pest mirrors the weather conditions that determine the soil moisture. Experiments have demonstrated that soil moisture causes mortality of very young larvae, whereas medium-sized larvae suffer increased mortality if the soil is very moist. We therefore hypothesise an oviposition preference of the Turnip moth for dry soil, as such behaviour may favour the survival of offspring. The present study describes experiments on A. segetum oviposition, given a choice of dry or moist soil. Furthermore we tested whether soil moisture affected oviposition on dead plant stalks that were mimicked by a steel wire object. The main result is a clear oviposition preference for dry over moist soil, meaning that increased observational efforts may be needed to predict cutworm presence in row crops on sandy soils. Another result is that objects such as plant material will receive a certain proportion of the eggs, but the proportion decreases if the soil is dry.

Effects of weed harrowing frequency on beneficial arthropods, plants and crop yield

Weed harrowing is an alternative to herbicides but it may have negative effects on epigaeic arthropods. We assessed the effects of frequent (four) versus two harrowings during the growing season on the density and diversity of generalist arthropods and the weed flora. Collection by flooding was used to estimate arthropod densities, after collection of weeds and crop for biomass estimation. The predatory guild of spiders Tachyporus spp. carabids was reduced by frequent harrowing. Spiders and Tachyporus spp. were the most sensitive, with up to 38% higher density in plots harrowed only twice compared with four times. Furthermore, a marginally significant decrease in arthropod diversity was found after four harrowings. The negative effect of frequent harrowing on arthropods was probably caused by a combination of direct lethal effects and habitat disruption. Additional harrowings reduced weeds with weed seed biomass, on average, being 77% higher and species diversity being 48% higher in plots harrowed only twice compared with four times. There was a positive relationship between weed biomass and predatory arthropod guild density, supporting the hypothesis that weeds promote generalist arthropod predators. Weed harrowing should be carried out early and limited to a minimum to protect flora and fauna. Frequent harrowing had no positive effect on yield, suggesting that more weeds can be tolerated to support biodiversity and biocontrol.

Mortality of first instar larvae of the European pine shoot moth (Rhyacionia buoliana Schiff.) on waterlacking needles of lodgepole pine (Pinus contorta Loud.).

In 8 experiments with 250 larvae of Rhyacionia buoliana placed on watered and non-watered shoots of Pinus contorta, a significantly higher percentage of successful larval borings was found on non-watered shoots. The growth rate of larvae seemed to be higher on the drier shoots. The results underline the increased risk of heavy infestations following drought years, give a reason for the rather frequent infestations found on poor soils, and may be used in Denmark as an important parameter for prognoses.