Meteorological and landscape influences on pollen beetle immigration into oilseed rape crops

Abstract

Heavy reliance on pesticide inputs to maintain crop yields has been an important aspect of agricultural intensification. Insecticide use has had detrimental impacts on pollinators and natural pest control agents, contributing to a decline in associated ecosystem services, and has also led to resistance development in pest populations. Throughout Europe, in oilseed rape (Brassica napus L.) crops, prophylactic use of insecticides against pollen beetles (Meligethes aeneus F. also known as Brassicogethes aeneus) has led to such issues, and there is an urgent need to develop more sustainable pest management practices for the crop. Although advice is available to oilseed rape growers regarding control thresholds, it may not be adhered to due to the expense of pollen beetle monitoring relative to the inexpensive cost of pyrethroid insecticides. Thus, the key to reducing prophylactic insecticide applications may lie with improved, less labour intensive methods of pollen beetle monitoring. For these to be realized, a better understanding is needed of the effects of agri-landscape features and meteorological conditions on pollen beetle immigration into the crop. In this study, based on data from four years of pollen beetle monitoring on a total of 41 field sites, we model the effects of meteorological (wind speed and direction, rainfall and accumulated temperature) and landscape (areas of woodland, residential gardens, the current and previous seasons' oilseed rape crops, and lengths of hedgerows and treelines) variables on directional sticky trap catches, at both the single trap and field scales. Meteorological variables, particularly accumulated temperature and wind speed were more important than landscape variables in predicting the abundance of pollen beetles immigrating into OSR fields. Sticky traps that were facing downwind caught more beetles than those that were facing across-wind or upwind; this is the first study to show at a landscape-scale, direct evidence for use of upwind anemotaxis by pollen beetles at the point of entry during immigration into the crop. At the field scale, the area of oilseed rape grown in the previous season was found to be positively related to trap catch, but no relationships with other landscape variables were found. Optimally-placed monitoring traps could complement existing decision support systems to reduce pollen beetle monitoring effort and encourage use of insecticides only when control thresholds are breached, thus enhancing the sustainability of oilseed rape production. Knowledge of the area of oilseed rape crops grown during the previous season in the surrounding landscape could contribute to risk assessment of potential pest pressure for individual OSR crops.

Keywords: Brassicogethes; Decision support systems; Insecticide resistance; Integrated pest management; Meligethes aeneus; Monitoring trap; Sustainable intensification.

Fig. 1

(a) Typical site layout showing directional sticky trap placement in oilseed rape crops relative to an assumed WSW prevailing wind direction (hollow arrows) and the boundaries of a sampled field (textured shading). Upwind (WSW-facing) and downwind (ENE-facing) traps (black stars with arrows to indicate facing direction) were placed 3 m into the crop on opposite sides of the field, along the plane of the WSW-to-ENE prevailing wind direction (short-dashed line). Fifteen of the sites had additional NNW- and SSE-facing cross-wind traps, placed on a NNW–SSE plane (long-dashed line), perpendicular to that of the prevailing wind. When the wind is from the prevailing WSW direction, pollen beetles flying upwind are expected to enter the crop from the ENE. (b) Landscape mapping zones and assignment of wind and trap directions to specific directional segments or ‘octants’. The circular area represents the 1000 m-radius zone within which landscape features were mapped in relation to a downwind trap (D; ENE-facing), and is divided into directional octants (labelled 0–7). Traps were assigned to the octant corresponding to the direction that they were facing (in the case of the downwind trap shown, octant 0). If the wind direction was from the ESE at 12:00 on a particular day (dashed straight arrow), ingress of upwind-flying beetles would be expected from a WNW (downwind) direction, and hence wind direction in this situation would be assigned to octant 3. The difference between wind direction and trap direction can then be calculated in terms of number of octants (in this case there was a 3-octant difference). However, if the wind direction was from the WSW, as in (a), then both the downwind trap and wind direction would be assigned to octant 0 (trap and wind direction aligned, a difference of 0 octants). Note that for modelling at the individual trap scale, only landscape features from the octant facing the trap and its two immediate neighbours (in this case octants 0, 1 and 7) were used.

Fig. 2

Predicted daily field catch of pollen beetles in traps placed in oilseed rape (OSR) crops (total of upwind and downwind traps) with 95% confidence limits for (a) wind speed (m/s) with 3 dhd accumulated temperature and 12 ha OSR in landscape in previous year, (b) accumulated temperature (dhd) with wind speed of 5 m/s and 12 ha OSR in landscape in previous year, (c) the area of OSR crops within a 1000 m radius with wind speed of 5 m/s and 3 dhd accumulated temperature.

Fig. 3

Predicted daily trap catch of pollen beetles with 95% confidence limits for (a) wind speed (m/s) with 3 dhd accumulated temperature, 4.5 ha oilseed rape (OSR) in landscape in previous year and trap aligned with wind direction, (b) the deviation between wind and trap direction in octants with wind speed of 5 m/s, 3 dhd accumulated temperature and 4.5 ha OSR in landscape in the previous year, (c) accumulated temperature (dhd) with windspeed of 5 m/s, 4.5 ha OSR in landscape in the previous year and trap aligned with wind direction, (d) the area of OSR crops within a 1000 m radius with windspeed of 5 m/s, 3 dhd accumulated temperature and trap aligned with wind direction.