The effect of insecticide application by dropleg sprayers on pollen beetle parasitism in oilseed rape

Abstract

Dropleg sprayers apply pesticides below the flower horizon of oilseed rape plants and thus reduce unwanted side effects on pollinating insects. Whether this technique benefits parasitoids of seed and pollen feeding insect pests has not been studied earlier. To answer this question, we first assessed the vertical distribution of pests and parasitoids using a portable aspirator. In addition, parasitism rates of pollen beetle, Brassicogethes aeneus Fabricius (Coleoptera: Nitidulidae), by the larval parasitoid Tersilochus heterocerus Thomson (Hymenoptera: Ichneumonidae) were compared in conventional and dropleg sprayed fields over four years (2016–2019), using the neonicotinoids thiacloprid and acetamiprid. Our results show that seed and pollen feeders were mainly found in the flowering canopy, while the predominant location of parasitoids was species-specific. Among pollen beetle parasitoids, Phradis interstitialis Thomson (Hymenoptera: Ichneumonidae) was more abundant below flowering canopy (63% of total catch), whereas T. heterocerus was mainly caught in the flowering canopy (84% of total catch). In the spraying experiments, average parasitism rates of pollen beetles by T. heterocerus ranged between 55 and 82% in the untreated controls. In the dropleg spray treatments, parasitism rates did not differ significantly from control levels, with the exception of thiacloprid application in 2019. In contrast, conventional spray applications resulted in a reduction of parasitism rates by up to 37% compared to the control for at least one of the insecticides in three out of four years. The impact of conventional application differed between years, which may be explained by the temporal coincidence between spray application and the immigration of parasitoids into the crop. We conclude that dropleg spraying exerts lower non-target effects on the main biological control agent of pollen beetle.

The potential for reducing non-target effects on parasitoids of Ceutorhynchus obstrictus Marsham (Coleoptera: Curculionidae) through spatially targeted insecticide applications

“…The weather around Braunschweig was warmer and drier than average in the period from May 2018 to July 2019, which might have favoured M. morys during the two years period. As this species was mainly found in the upper crop layers in a previous study (Hausmann et al 2021), we assumed, in advance, that M. morys could benefit from spatial targeted insecticide sprays using dropleg technique. Results show that the emergence of M. morys in the Dropleg Biscaya treatment was not statistically different from any other treatment, while after conventional application of Biscaya, significantly less parasitoids emerged compared to the control and the Dropleg Mospilan treatment in 2019.…”

“…However, Haye et al (2018) found evidence that climatic conditions define the ecological niches of T. perfectus and M. morys. But in the same field, studies on the vertical distribution of both species in the flowering crop have also revealed first evidence for spatial niche partitioning, as M. morys is mostly abundant in the horizon of open flowers, whereas T. perfectus is found in lower vegetation layers (Hausmann et al 2021). For an integrated pest management, it is necessary to preserve natural antagonists from harmful side effects of insecticides, as far as possible, and spatial targeting of insecticides is one way to achieve this (Ulber et al 2010a).…”

“…pollen beetle parasitoids (Ulber et al 2010a). Biscaya, on the other hand, can have harmful effects on tersilochine parasitoids San Martin 2014, 2017;Hausmann et al 2021). The applications were carried out at the full flowering stage of the OSR crop (BBCH 65-67), independent of infestation rates, with a driving speed of 7 km h −1 and a pressure of 2.8 bar using water volumes of 300 l ha −1 .…”

“…Using the dropleg application technique, residues of (also systemic) insecticides in nectar and pollen were reduced (Wallner 2014;Brandes 2021). Spatial targeting of insecticides, using droplegs in flowering OSR, can increase parasitism rates of pollen beetle larvae compared to conventional application (Hausmann et al 2021). The objective of our study was, to investigate the effects of insecticide applications, application techniques on pod infestation by CSW larvae and on its parasitism rates, and on parasitoid species composition and the interaction between application technique and parasitoid species.…”

Efficacy of Different Fungicide Spraying Techniques on the Infestation with Kabatiella zeae and Formation of Fusarium Mycotoxins in Forage Maize

“…In contrast, the so-called dropleg technology offers an opportunity to reach lower plant organs of the maize plant with the fungicide, which are difficult to achieve with the overhead spraying technique. This spraying technique generally allows sub-canopy treatments with various pesticides, e.g., in row crops in horticulture [10,11] as well as field crops such as oilseed rape [11][12][13]. Compared to the overhead spraying technique, the nozzle carriers are not directly attached to the spray boom, but to the lower end of the dropleg, which are prolonged elastic nozzle holders that hang freely floating underneath the boom down into the crop canopy during spraying [12,13].…”