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Dead bumblebees under linden trees are a typical sight in mid and late summer when the trees are in full bloom. In this article you will learn what causes the bumble bee die-off.
In a nutshell
- Linden often the only source of food in midsummer
- are visited by numerous bumblebee colonies
- The food supply of the linden trees decreases sharply
- Bumblebees die due to the ensuing lack of food
- Bumblebee pastures help out the insects
Bumblebee deaths: causes
What causes the bumblebee die-off in July that lasts throughout mid and late summer? If you're also wondering why there are so many dead bumblebees under linden trees, you need to look at the food supply. Bumblebees (Bombus) cannot find enough late-blooming food sources in cities during mid and late summer and starve to death. For this reason, whole peoples gather around two species of linden trees specifically:
- Silver linden (Tilia tomentosa)
- Crimean Lime (Tilia Euchlora)
Source Crimean linden (left): Tilia x Euchlora 0001, edited by Plantopedia, CC BY-SA 3.0; Source silver linden (r.): Bruce Marlin, Tilia tomentosa, edited by Plantopedia, CC BY 3.0H. cell
Due to their drought and pollutant resistance, the two species are preferably planted in urban areas along roads. They flower after the large-leaved lime (Tilia platyphyllos) and small-leaved lime (Tilia cordata), making them an important source of food for bees and bumblebees from mid-July to mid-August.
The disadvantage: numerous colonies of bumblebees visit the trees, which consumes the nectar and pollen much faster. Since the insects often travel long distances to the trees and have to compete for the available food, they die directly in the tree or on the way back to the nest. Since the colonies hardly stock up, many workers even die on the way to the trees. This is why the bumblebee die-off is so extensive. Other causes are:
- “Empty” linden trees are preferred to new honey plants
- other food sources too far away
- There is a lack of food outside of cities
Dusky bumblebee (Bombus terrestris)
Notice: The common beetle (Bombus terrestris) is particularly badly affected by this problem. It is particularly difficult to adapt to new forage plants, which leads to the loss of numerous colonies.
help bumblebees
If you live in the city and want to help pollinator insects, you need to provide them with more food sources. In order for this to succeed, rely on bee and bumblebee friendly plants that only bloom later in the summer. It doesn't matter whether you have a garden, courtyard or balcony. Even a balcony box with richly flowering species is sufficient to provide a large number of bumblebees with food. The following plants are best suited for this purpose:
- Artichoke (Cynaria scolymus)
- Bearded Beard (Caryopteris)
- Common heather (Calluna vulgaris)
- Bee friend (Phacelia tanacetifolia)
- Blue Viper Bugloss (Echium vulgare)
- True lavender (Lavandula angustifolia)
- Common ivy (Hedera helix)
- Clover (Trifolium)
- Cornflower (Cyanus segetum)
- Globe thistles (Echinops)
- Mallows (Malva)
- Oregano (Origanum vulgare)
- Rudbeckia (Rudbeckia)
- Hollyhocks (Alcea)
- Summer lilac (Buddleja davidii)
- Coneflowers (Echinacea)
- Dead Nettles (Lamium)
- Water daffodil (Eupatorium cannabinum)
Bumblebee on Bee Friend (Phacelia tanacetifolia)
Emergency solution: sugar water
They can provide a small boost to exhausted bumblebees if you find them on your property. Just mix up some sugar water and feed the bugs. They recharge their energy reserves through the meal. All you need is half a teaspoon of sugar, which you dissolve in water at room temperature. The start-up aid is administered as follows:
- Take bumblebee to a safe place
- transport carefully on paper or similar
- also possible by hand
- Pour sugar water into teaspoons
- place directly in front of bumblebee
- on average 1/3 sugar water is drunk
Notice: Sugar water is only an emergency solution if you have already made your garden bee-friendly. If the bumblebee finds only heavily frequented linden trees after the sugar meal, it uses up too much energy again and dies.
frequently asked Questions
Why do bumblebees look for food in cities?Bumblebees are becoming more common in cities as rural areas become increasingly depleted in food availability and replaced by monocultures. Late-flowering plants in particular are becoming increasingly rare, which makes moving into urban areas attractive. Since many cities rely on Crimean and Silver lime trees, they often provide the only available energy source for the insects.
Do bees also suffer from the low food supply?Yes. Depending on the food supply, bees of all kinds also fly to the linden trees and die. However, since bees are not so focused on certain foraging plants, they quickly find other plants that serve as a food source. For this reason there are significantly fewer dead bees than bumblebees under the linden trees.
Are there other threats to pollinators?Predators are a common problem for the weakened insects, as they can no longer escape them due to the lack of energy reserves. Wasps, hornets and birds, which have an easy time with the bumblebees, are particularly problematic in late summer.
What is not responsible for bumble bee deaths?Previously, researchers suspected that the cause of the massive die-off of pollinators could be mannose. Mannose is a sugar that is toxic to bumblebees and bees. However, linden nectar does not contain mannose, which refutes this theory.