Funny how one thing leads to another …
I posted a comment on the Garden Dum website (based in Australia) to the effect that the yellow and black ladybirds (Illeis galbula) we find on cucurbit leaves and that are thought to control powdery mildew, actually spread it, don’t they?
The article author, Jennifer Stackhouse, politely queried where I might have got such an odd notion from, so I linked to this Royal New Zealand Institute of Horticulture article about powdery mildew by Jane Wrigglesworth that says: “But if you think these beetles will help control powdery mildew, think again. They actually carry spores of the disease under their wings from plant to plant.”
I also quoted from the 2005 book, Backyard Battlefield (Random House), by entomologist Ruud Kleinpaste who says, “Sadly, observing these elegant creatures in my veggie garden I think I have gathered enough evidence to accuse them of spreading the fungus from leaf to leaf and plant to plant.”
So far, so good. Jennifer double-checked my information against that available in Australia and came back with information from Denis Crawford, an insect photographer and garden writer.
“I have heard this story before,” Denis says, “but it is not supported by any scientific literature I can find.” He quotes an excerpt from a paper (one of the few, he says) which examines the biology and behaviour of the fungus-eating ladybird Illeis galbula:
“Feeding behaviour is remarkably uniform, both larvae and adults graze fungal spores and hyphae from surfaces of leaves. When Oidium sp. is dense, they feed on a front and visibly clear large areas of the leaf’s white fungal covering; if infestation is light, both larvae and adults search leaf surfaces at random and if nothing is found, adults fly off.”
Denis suggests it may be possible that the beetles spread the fungus as would any other insect which walks over the spores and moves to another leaf, as would water drops, wind, etc. “The ladybirds almost certainly do more good than harm.”
So I contacted Murray Dawson, RNZIH webmaster, to share this information and wonder if he knew any more … and Murray was kind enough to let me know that Denis who, it turns out, is writing a book on garden insects had also been in touch seeking clarification.
I wonder if it’s one of those things that has been said often enough that people now accept that the ladybirds spread the fungus, and repeat it as a truth to other gardeners. If any readers have any ideas or knowledge, I’d be pleased to hear them. Just click on the “reply” tag underneath this post.
PS (May 5): In the latest edition of the weekly Get Growing email newsletter I find Lynda Hallinan also repeating the story that the ladybirds are pests.