Feature

A massive plague of moths grips SA's Eastern Cape

Prof Martin Villet

Scientists, puzzled over a massive outbreak of "fascinating" moths  (no two are alike) reveal that there are two main culprits involved in loss of citrus crops and the mass defoliation witnessed on some game farms in the region

It’s happening again: the Eastern Cape is being invaded by a plague of insects. Walk past almost any hedge in many towns and you’ll notice these large, dark moths erupting like miniature bats as you approach and bumbling off to new hiding places. There are probably at the very least a dozen perched up near the ceiling of your garage and they are roosting in most of the verandas and porches around your town or city. In short, they are everywhere, it seems. Residents, crop and game farmers across the Eastern Cape heading towards Natal have viewed these visitors with justifiable suspicion.

The moth’s official name is Lienard’s Achaea, a name derived uninventively from its scientific name, Achaea lienardi, although farmers have various other names for it. The species was originally described in 1833, from specimens caught in Madagascar. It belongs to the family Noctuidae, which includes a large number of the moths that attack crops, such as Armyworm, Tomato Semilooper, and Cotton Bollworm, as well as the Sundowner Moth that is attracted to beer.

However, Lienard’s Achaea is not a threat to crop farmers or drinkers. In fact, if there was going to be an agricultural problem, we would know about it already because it is the caterpillars that do the damage, and for there to be so many moths now, there must have been equal numbers of caterpillars in December and January. However, it is a case of mistaken identity it seems as some crop farmers have experienced loss of crops but the culprit is a different moth.

Fruit piercing moth

Although Lienard's Achaea is known to some farmers as the ‘Fruit-Piercing Moth’, the true culprit responsible for the loss of citrus crops in the region is another moth named Serrodes partita, which uses the barbed tip of its tongue or proboscis to drill into fruits such as peaches, grapes and sweet citrus. (This leads to bacterial rot and the fruit falls from the trees, and Lienard’s Achaea gets the blame because it feeds at the rotting punctures.) However, Lienard’s Achaea is known to other farmers as ‘Wattle Looper’ because the caterpillars attack black wattle trees. While this is not strictly a crop, it is still an economically significant plant. Their larvae have been recorded feeding on potato plants and castor oil bushes.

Moths defoliating entire hillsides

These activities normally go unnoticed, but in outbreaks like the one we are having this year, the damage can be severe. Entire hillsides in some of the local game reserves have been turned grey as over 90% of the leaves were eaten. What do these moths feed on, then? Well, the list is long and includes many valley bushveld plants. This has helped to spread the load on the plants, although they have been hard-hit in some areas like Kwandwe.

The list of hosts includes Soetdoring (Acacia karroo), Small-leafed Ghwarrie (Euclea undulata), Jacket plum (Pappea capensis), Karoo boerbean (Schotia afra), Katdoring (Scutia myrtina), White milkwood (Sideroxylon inerme), and about a dozen others. Spekboom (Portulacaria afra) and Bee-sting Bush (Azima tetracantha) are new additions to the list this season.

Lienard’s Achaea is well-know to lepidopterists. The species is distributed from the Eastern Cape through KwaZulu-Natal and the bushveld to Zimbabwe to Sudan and West Africa. There are rumours of it in Panama and Costa Rica, but these are probably misidentifications. There was a severe outbreak in Grahamstown around 1860, and two others before that. If plagues have been noticed at least since settlers arrived in 1830, they are not due to global climate change. By 1953 more than 20 outbreaks had been reported. There was a spectacular eruption around East London in early 1993 and a small one around Grahamstown in 2004. The current outbreak runs from near Port Elizabeth in the west to Durban in the east, and from the Beach to the Amatole Mountains. It is especially phenomenal in East London. It is seen in Grahamstown every year in March or April, although not usually in such large numbers.

Weather triggered eruptions

What triggers eruptions? Long-term records show that vital ingredients are drought in spring and early summer followed by late summer rains. This delays the flushing of new leaves until later in the season. At this point, night-time air temperatures are higher and more conducive to the activities of the adult moths, which lay their eggs then. The caterpillars take full advantage of the young leaves and complete development in four or five weeks. It is possible that the early drought and late availability of prey also depresses the populations of predatory insects that would otherwise eat the caterpillars.

The adult moths are eaten by a variety of birds, including red-winged starlings, olive thrushes, wagtails, hadedas, karoo prinias, rock kestrels and yellow- billed kites. Even monkeys, cats and dogs eat them. The caterpillars are attacked by other insects, including predatory stinkbugs, parasitic flies and mud wasps, and these will help to regulate the numbers back to more normal levels. Starvation will also bring down the numbers. A second generation of adult moths has started to appear, but in far fewer numbers and some of them are dwarfed, indicating that they were starved as caterpillars.

Darwin's individually patterned moths

Lienard’s Achaea has been a scientific curiosity for many years. Charles Darwin, who was born 200 years ago this year, was interested to receive news from Grahamstown that the adult was a fruit-piercing pest. Its reputed ability to puncture the toughened skins of fruits was his first evidence that moths could be the pollinators of certain orchids that stored their nectar in nectaries that had to be punctured.

The extreme variation in the wing patterns has also fascinated biologists, and been the focus of much theorising. The moths are unusual because no two are marked exactly the same. If they will stay quiet as you approach them, roosting on walls, you will discover that those dark, triangular wings are marked with patches and lines that vary in size and colour from specimen to specimen. Every one is an individual, even thought they are ‘just’ insects. So, appreciate them while you can, because they probably won’t be as plentiful next year.

More information:

 Professor Martin Villet is an entomologist based in the Department of Zoology and Entomology at Rhodes University in Grahamstown, South Africa, and a regular contributor to Science in Africa.

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