Seed selection

Many flowers produce nectar to attract pollinators, but also have adaptations to limit access to a specific group of pollinators, e.g. by the length of the corolla tube. Research showed that predatory hoverflies e.g. can use only a very limited range of flowers to obtain nectar from (Van Rijn & Wäckers 2016). The flowers on which they survive can be characterized as open or very shallow. The maximum flower depth of 1.6 mm is much less than for other hoverflies and most bees. Most umbellifer flowers (Apiaceae) meet these characteristics as well as e.g. Buckwheat and Sweet Alyssum. Although plants of the daisy family (Asteraceae) are generally recommended as resources for natural enemies, this study showed that most species from this family are actually unsuitable, exceptions are e.g. Chamomile, Corn Marigold and Yarrow. The problem probably lies in the narrow florets that, in combination with the anthers and stamen, limit the access to the nectar. Other exceptions are those plants that produce extrafloral nectar, such as Cornflower. Similar limitations can be observed for other natural enemies such as lacewings and parasitoid wasps (Wäckers & Van Rijn 2012).

The abundance of flowers with accessible nectar (depth <2 mm) ultimately defines the number of predatory hoverflies visiting the field margins (Van Rijn & Wäckers 2016).

To find the optimal flower mix for your field margin you should ideally go through the following identification steps:

1. Identify the main pests on your farm and the adjacent crop field in particular.
2. Identify the main natural enemies of those pest that are present in region.
3. Identify the flowers that are supportive for these natural enemies, but not for the pests itself.

In step 1 it is important not only to consider the crop planned on the field at which you want to make flower strips, but also crops that may be planted there in later years, so that the strip can remain at the same site for a number of years.

For step 2 it is not always easy to find the information needed. For some pests it is still not clear which native natural enemies are effective, e.g. for thrips and Colorado beetles. For many aphids however, natural enemies are well-known and present in many regions: lady beetles, hoverflies, lacewings, and parasitoids from various families. For other pests, better control can be expected from ground-dwelling natural enemies (such as carabid and staphyllinid beetles) which can more effectively be enhanced by soil improving measures than by flower strips, e.g. for root flies or wireworms.

For step 3 it is important to realize that nectar (or some other sugar source) is an essential food source for parasitoids as well for many winged predatory insects. Moreover, flowers show large variation in the level of concealment of the nectar, resulting from an evolutionary interaction with pollinating insects that vary in the lengths of their mouthparts. Given that the adults of almost all the natural enemies have short mouthparts, only plants with open or shallow flowers are suitable as nectar source for them. The critical depth however may slightly vary between groups and species of predators.

Even parasitoid families and species differ in the level their mouthparts are extended (Jervis & Vilhelmsen, 2000; Gilbert & Jervis, 1998). For species that lack these extensions even the moderate concealment in some umbellifer species such as Coriander may prevent them from feeding on its nectar. In small parasitoid wasps not only the mouthparts but often the whole head have to be brought down between the flower parts that conceal the nectary. In that case not only the depth but also the width of the access to the nectaries can be limiting (Patt, Hamilton, & Lashomb, 1997).

In some countries websites or folders are available that -based on the answers in steps one or two- provide suggestions for flowers to be planted in field margins. In the end, all suggested plants should be checked on their ability to grow in the specific soil and micro-climate of your field.

Guide to spontaneous flora suitable for auxiliaries (French only).

A field margin can support not only beneficial insects but also pest species or vectors of plant virussen (Wackers, Romeis, & van Rijn, 2007). Most plant feeding species have a limited range of related plant species where it can feed on (called ‘host plants’). Avoiding those species can prevent these unwanted effect of field margins. Note however, that it is not necessary to avoid all potential host plant species. When insect species are serious crop pest, the crop is generally the best food plant for this species. In that case, adding more plants of equal or lower food value next to your crop will have no significant effect on the abundance of this pest. It is a different case, when the host plants in your margin have a complementary function for the pest species. This either occurs when it grows in a different period than the crop, or when it provides a different type of food (e.g., pollen). In the first case adding the host plant can stretch the period that the pest is attracted to the side that the population can grow, which generally result in higher numbers. When it provides a different food source for the pest, such as pollen and nectar, the combination of the two food plants may result in a higher reproduction rate of the pest. This is the case for butterflies (Lepidoptera, e.g. diamondback moth), several herbivorous flies (Diptera, such as root flies and fruit flies), and for herbivorous thrips (Wackers et al., 2007). As for the natural enemies, mainly the adults are feeding on floral resources, whereas the larvae mainly feed on green plant tissue. When these pest species are a problem in your crop, plants that can be used by the pest species to feed on should be avoided in the field margin strips.

• Avoiding that certain species become dominant

• Mix of functional traits, spread in flowering period

• Abundance related to seed number and seed weight