Biological control of pollen beetle in the garden

When spring temperatures exceed 15 degrees, the pollen beetle leaves its winter quarters and begins its destructive activities. The 1-2 mm small, shiny metallic pest has long been concentrated not only on oilseed rape, but also infests the flowers of important cultivated plants from the cruciferous family. On the hunt for pollen, the pest bites the closed buds, which then die. The harvest of cauliflower, broccoli or horseradish is threatened. Ornamental plants, such as blue cushions, take care of that. You can find out here how to biologically control the pollen beetle in the garden.

appearance and lifestyle

In order to get rid of the pollen beetle, the time factor plays an essential role. The earlier you recognize an infestation, the more effective the fight against it will be. Therefore, first familiarize yourself with the external appearance and way of life of the pest.

• Beetle is 1.5 to 2.5 mm small with an elongated oval body
• Color scheme in shimmering black-blue, violet or green
• Yellowish-white larvae, 1-4 mm long with a brown head
• Beginning of activities from 15 degrees air temperature
• The garden is infested from the edges of the beds

Pollen beetles bite into the still closed buds of cruciferous plants to get to the pollen. The damaged buds turn yellow, dry up and fall to the ground. In the further course, the female beetles drill into numerous buds in order to lay their eggs in them. Within a few days, the larvae hatch to feed on pollen and nectar. Fatally, the brood is capable of moving from flower to flower. At the end of the flowering period, the larvae drop to the ground, pupate and transform into the next generation of beetles within 14-18 days.

Monitoring and Damage Threshold

So that you do not miss the optimal date for biological control, yellow traps that have been set up provide information about the population development in the garden. Dedicated information is then provided by tapping and shaking tests on several plants. At midday, shake out endangered ornamental and crop plants over a jar and count the beetles. Preferably choose plants at the edge of the bed, as this is where the plague always begins. The following damage thresholds require immediate action against pollen beetle:

• At the beginning of bud formation: 1-2 beetles
• 14 days before the start of flowering: 4 beetles
• Immediately before flowering: 5-6 beetles

Depending on the development stage of the plants, some beetles can be tolerated. The decisive factor is not only the number of beetles, but also the constitution of the plants. In an unsuitable location with waterlogged soil and poor temperature and light conditions, total failure can threaten, while a strong plant easily tolerates 5-6 bugs.

Biological control methods

In recent years, agriculture has used chemical insecticides to combat the pollen beetle. Fatally, the pests developed a high degree of resistance to the preparations. Here the research was thrown back to its starting point. In contrast, hobby gardeners today benefit from the experience gained from the cultivation of organic rapeseed. The following methods have emerged, which can easily be transferred to the home garden:

collect

What the beetle collecting machines do on large fields, the environmentally conscious hobby gardener does by hand. In early spring, diligent collection of pollen beetles can reduce the risk of infestation by up to 30 percent reduce so that the tolerance limit for ornamental and useful plants is reached and no further action is required is.

Vegetable oil soap solution

By spraying the edges of beds with a mixture of vegetable oil, neutral soap and water, the immigration of pollen beetles can be reduced. It is important to note that you should only use curd or soft soap and not a standard cleaning product. If necessary, an emulsifier from the pharmacy should be mixed in so that the ingredients mix better.

rock flour

In the nature-loving management of the garden, fine rock flour serves as a natural fertilizer. They have also made a name for themselves in controlling plant diseases and pests. The plant additive also has a repellent effect on pollen beetle. While the fertilizing effect is a long time coming, fine rock dust immediately inhibits beetle and larvae feeding. In laboratory tests it was found that the effectiveness is based on the fact that the protective wax shell is dissolved. You can either apply the preparation dry using a powder syringe or dissolve it in water according to the dosage instructions in order to distribute it with the spray bottle.

• The first treatment with rock powder takes place at the same time as the buds form
• Carry out the second treatment just before flowering

Additional applications may be required depending on the weather, as both rain and strong winds reduce the effectiveness of the biological agent.

parasitic wasps

If the focus is on helpful insects against the larvae of the pollen beetle, parasitic wasps come into focus. So far, at least 9 species of these lacewings have turned out to be parasitic enemies. Intensive field tests by the Universities of Göttingen and Vienna brought the realization that in the near-natural designed Garden with a corresponding population of parasitic wasps, the mortality rate of the larvae is up to 54 percent lay. While special parasitic wasp species are bred in specialist companies for use in living spaces, this method is less suitable for beds in the open air. If you are striving to gain these beneficial insects as a protective aid in the fight against pests, appropriate places of retreat should be created. Ichneumon wasps feel at home here:

• Species-rich planted dry stone walls
• Mixed hedges
• Hollow tree trunks

If you frame the beds with colorful strips of flowers, this measure not only underlines the visual appearance, but also invites parasitic wasps to linger. From there they target the larvae of the pollen beetle, parasitize the brood and the next generation of parasitic wasps emerges.

neem

The neem tree (also called neem tree) is native to the tropics and provides fruits that yield neem oil and neem seeds, among other things, which have been shown to be effective in pest control. In particular, biting insects such as the rape pollen beetle can be combated with agents containing neem. The following preparations are approved for use in home and allotment gardens:

• Organic pest-free neem from Bayer Garten (approved until December 31, 2023)
• Lizetan Neem from Bayer Garten (approved until 12/31/2023)
• Compo Bio insect-free (approved until December 31, 2023)
• Naturen organic pest-free neem (approved until 31.12.2023)

Since these preparations are semi-systemic, they should only be considered as a last resort. In addition to the natural active ingredient of the neem tree, the products also contain azadirachtin. Although this chemical compound is also based on neem oil, it is produced synthetically.

preventive measures

The species-appropriate cultivation of endangered ornamental and useful plants plays an important role in prevention. Avoid locations where there is a risk of waterlogging. A location with soil compaction weakens growth and opens the door to the destructive activity of pollen beetles. Where organic fertilizers are used exclusively, vital soil life reigns supreme and harmful over-fertilization does not occur. If you have access to unpolluted liquid manure, this not only acts as an excellent starter fertilizer in early spring, but also drives away rape pollen beetles that are approaching. If a garden is permeated by the scent of essential oils, the pests obviously give the beds a wide berth. At least in laboratory tests, the repellent effect of mint oil or lavender oil turned out. While scientists strive to find the right application technique and product formulation To work out, hobby gardeners plant lavender, lemon balm, camphor and similar Plant.

Conclusion

In order to stop one of the most important springtime pests, the timing of the fight is just as relevant as the fight itself. If temperatures exceed 15 degrees, ornamental plants and crops from the cruciferous family should be checked daily for infestation by the pollen beetle. Since the use of chemical insecticides in private gardens is taboo, use the biological control methods presented here to combat the destructive beetles. It's a good thing that well-known remedies, such as rock flour, beneficial insects and the classic vegetable oil-soap mixture, have proven themselves in the fight against the beetles and their larvae.

Learn more about plant pests