Wasps, honeydew and forest birds

Walk through a South Island beech forest on a still summer morning and the trunks can look as if they've been painted black. Look closer and you'll see thousands of fine white threads sticking out of the bark, each tipped with a tiny, glistening drop. That drop is honeydew, and for native forest birds it is one of the richest foods in the bush.

From late summer, the same trunks are often humming with introduced wasps. This note explains how honeydew works, why wasps are such a problem in these forests, and what is being done about it.

What honeydew is

Honeydew is made by native scale insects that live inside the bark of beech trees. The insects tap into the tree's sap, which is high in sugar, and pass the excess out through a long waxy thread. The droplet at the end of each thread is mostly sugar and water. A black sooty mould grows on the sugary residue, which is what gives honeydew beech its dark, velvety trunks.

It's a remarkable resource. Honeydew is available for much of the year, and it feeds native birds, bats, lizards and insects. Studies in beech forest have found that korimako (bellbird) and tūī were more common where honeydew was more plentiful. Kākā (forest parrot) also drink it: NZ Birds Online lists honeydew alongside nectar, sap, seeds, fruit and wood-boring grubs in the kākā diet. Tauhou (silvereye) take it too.

Most honeydew beech forest is in the northern South Island, and the best-known study sites are around Nelson Lakes.

Enter the wasps

New Zealand has two introduced social wasps that cause most of the trouble: the German wasp, which arrived in the 1940s, and the common wasp, which arrived in the late 1970s. According to the Department of Conservation, New Zealand has some of the highest densities of these wasps anywhere in the world.

In honeydew beech forests the numbers are startling. DOC estimates that, seasonally, some of these forests have around 12 nests, or about 10,000 worker wasps, per hectare. The combined weight of those wasps is greater than that of all the native birds, stoats and rodents in the same forest put together.

Wasps are after sugar for energy, and honeydew is an easy supply. Research in beech forest found that birds made little difference to the amount of honeydew on the trunks, but wasp feeding in summer and autumn sharply cut the number, size and sugar content of the drops. At the peak of the wasp season, the energy available in honeydew fell by more than 90 percent.

Why it matters for birds

That leaves forest birds short of food at exactly the time many of them are raising chicks or building condition for winter.

• Kākā have long been described by researchers as competing directly with wasps for honeydew. Kākā are also threatened by stoats and possums at their tree-hole nests, so a food shortage adds pressure.
• Tūī and korimako rely on nectar and honeydew for energy. When wasps take the honeydew, these birds have less to fall back on outside the flowering season.
• Insect-eating birds such as tītitipounamu (rifleman), riroriro (grey warbler) and miromiro (tomtit) lose out too, because wasps also hunt huge numbers of native insects and spiders. Wasps have even been seen killing newly hatched birds in the nest.

Long-term bird counts in the Nelson Lakes area have recorded declines in several of these species, and wasps are one of the pressures researchers point to, alongside rats, stoats and possums.

How wasps are controlled

Wasps can't be removed from whole forests, but they can be knocked down where it matters most. The main tool is Vespex, a protein bait developed with DOC. It contains a small amount of the insecticide fipronil and is set out in bait stations. Worker wasps carry the bait back to the nest to feed their larvae, and the nest dies.

A few points about how it works:

• It only works in the warmer months, when wasps are hunting protein for their young.
• It isn't attractive to bees, which are after sugar rather than meat.
• DOC doesn't supply it. Community groups and landowners can buy it after passing an online training test, and it can be used on private land or, with the right permissions, on public conservation land.

The results can be big. In one long-running Nelson Lakes project, wasp control cut wasp numbers by around 90 percent, and monitoring found more honeydew left for birds.

Birding in honeydew forest

If you want to see the honeydew story for yourself:

1. Pick the right forest. Beech forests of the northern South Island are the classic places. Look for blackened trunks and the fine white threads.
2. Go early in the season. Spring and early summer, before wasp numbers peak, is when you are most likely to see korimako, tūī and kākā feeding on the trunks.
3. Watch the trunks, not just the canopy. Birds will work their way up and down a tree, sipping from the threads.
4. Be wasp-aware from January to April. Wasp numbers peak in late summer and autumn. Wear closed shoes, don't sit on logs or banks without checking first, and avoid sweet drinks in open cups. Anyone with a known sting allergy should carry their medication and tell their group.

Wasp nests near your home

The same wasps nest around houses as well as in the bush. Nests turn up in wall cavities, roof spaces, under decks, in garden banks and in old rodent burrows. A nest near a doorway, clothesline or play area can send hundreds of wasps out if it is bumped, mown over or sprayed badly. If you find one, don't poke, block or try to burn it, and keep children and pets well away. Nests inside walls or roofs, or high and hard to reach, are a job for a professional with the right gear rather than a ladder and a can of fly spray; in Auckland, Waspari explains what is involved with a nest in the roof or eaves. Late summer and autumn, when nests are largest, is when people most often get stung.

How you can help

• Support local wasp control. Many conservation groups run bait stations in summer and welcome volunteers.
• Record what you see. Notes of birds feeding on honeydew, or of high wasp numbers, are useful to local groups and to eBird.
• Don't feed sugar water carelessly. Open sugar feeders in late summer can draw in wasps as well as tūī. Keep feeders clean and take them in if wasps take over.

Honeydew beech is one of the quiet wonders of New Zealand's forests. Keeping wasps down gives the birds that depend on it a better chance.