We now have a name, Polytoxon robustum, for the big yellow slugs that have become conspicuous during the wet season in many gardens in the north-eastern suburbs of Harare. The slugs are large with an extended length of up to 165 mm and are pale yellow to mustard in colour, sometimes mottled with brown.
The slugs were first noticed in Greystone Park during the late 1990s. By 2015 they had been spotted in many other north-eastern suburbs including Borrowdale Brooke, Chisipite, Greendale, Groombridge, and Northwood but not, so far, from elsewhere in Harare. Polytoxon robustum has never been recorded in Zimbabwe before. The nearest previous record is from central Tanzania, more than 1000 km to the north. Polytoxon flourishes in a variety of places in Tanzania, Kenya, eastern DRC, Rwanda and South Sudan at altitudes ranging from 3200 metres down to near sea-level. We know neither the source, nor the means, by which it arrived in Harare. Maybe it came as a stowaway from East Africa in the form of eggs or young slugs hidden in soil or on plants?
There is no apparent reason why Polytoxon robustum should not continue to spread. Elsewhere, the slug lives in habitats that range from densely treed montane and bamboo forests, through coffee and tea plantations, to savanna grassland where there are just a few scattered trees and bushes. Although it is doing particularly well in Harare’s green and watered gardens, Polytoxon robustum can also be abundant in parts of East Africa where the annual rainfall is only 500–700 mm. Two-thirds of Zimbabwe receives at least 600 mm of rain, so low rainfall alone is unlikely to prevent further expansion of its geographical range, although East Africa does have the advantage, for a slug, of rainfall that is spread over two wet seasons each year.
Where they occur in Harare, the slugs are common, but unlike many alien invasive slugs elsewhere in the world, they do not seem to create a nuisance by eating garden plants. They often climb plastered walls on which they leave conspicuous trails of droppings.
Dr Ben Rowson from the National Museum of Wales, who identified the slugs, observes that no one has ever recorded the reproductive behaviour of this particular species. All slugs are hermaphrodite. Before they mate, some slugs will shoot ‘love darts’ into their partner’s bodies, each trying to subdue the other hormonally. Our species has at least 22 of these love darts and the name Polytoxon, which means ‘many arrows’, refers to them. The slugs are harmless to people, but if you touch one it produces copious, sticky mucus that is difficult to scrub from your fingers.
Polytoxon robustum has crossed both the Enterprise and the Borrowdale Roads and the slugs have popped up in some north-eastern suburbs, but not in others, which makes you wonder how they got there. Now that they are so abundant and widespread in Harare, it is not going to be possible to eradicate these slugs, even if we wanted to. We have no idea what effects their arrival has had on other invertebrates.
The arrival of this slug is a reminder that we should not transport soil and plants around the place – and especially not into National Parks – without proper precautions because of the alien invasive plants and animals that could take the opportunity to hitch a ride. Mana Pools National Park and the Urungwe Safari area are so far relatively free of alien invasive plants and animals, but it has been proposed that tree seedlings grown in Harare should be planted there. One way to keep the Parks and Wild Life Estate pest-free is to not to import soil or plants.
See www.researchgate.net/profile/Ben_Rowson (click on view all) and you can download a free copy of ‘First reports of two terrestrial slugs from southern Africa’
Image: The big yellow slugs that have become conspicuous during the wet season in many gardens in the Northern suburbs of Harare.