Hymenostilbe nutans

Hymenostilbe nutans

Saturday, April 26, 2014

Aleyrodidae as a source of nutrients for fungi. Are they really?

It is well known that aleyrodids are hosts to a wide variety of insect fungi. Some of the earliest attempts at biological control were in the early 1900s when Aschersonia and others were tested as control agents for citrus whitefly. Work at the Florida University Agricultural Experimental Station over a period of 15-20 years looked at the possible use of these 'friendly fungi'. Sadly by the 1920s attention had switched to the use of chemical means which were quicker and more reliable and cheaper. Interest in Aschersonia went into decline until the mid 80s when the Dutch company Koppert started looking at the use of Aschersonia against glasshouse whitefly pests.

I have studied Aschersonia-Hypocrella for over 20 years and one matter has always intrigued me. A typical whitefly larva is less than 3 mm and is so very dorso-ventrally flattened that it barely reaches 0.1 mm thick. And yet.... After it has been killed by Aschersonia the resulting fruitbody can be 4-5 mm diameter and 1-2 mm high. That is a large volume of fungus to emerge from such a small host.

Figure One - Aleyrodid host infected with Aschersonia luteabadia
It is almost certain that the fungus, once it has killed its host, continues to get nutrients from the plant on which the host was feeding. Fig. 1 shows an aleyrodid larva with the stroma of Aschersonia luteabadia developing over it. The traditionally accepted development is that the abdomen of an insect/spider host is overcome first and only later the thorax. But in Fig. 1 the abdominal segments are still visible while the yellow stroma has grown over the thorax/head region.

After death the stylet of the aleyrodid must still be stuck in the phloem of the host plant. Phloem sap will continue to pump up that stylet thanks to phloem pressure. Phloem sap is a rich source of nutrients - sugars, hormones, minerals etc. All useful for a still-developing Aschersonia-Hypocrella.

Figure Two - Aschersonia confluens with sticky droplet
In March 2005 Rung and I were doing a survey in the south of Thailand and came across Aschersonia confluens in big numbers. It was early in the morning and what struck me was the numbers of specimens that had little droplets on the developing stromas. Fig. 2 is slightly out of focus for the stroma but shows a glistening drop of liquid. It was viscous and when I tasted it it was sweet. My conclusion was that this was a droplet of phloem sap that had been pumped up the stylet of the now-dead aleyrodid.

Gary Samuels and myself drew attention to this phenomenon in 1998 when we were discussing the large stroma  Hypocrella species on bamboo scale insects. At that time we assumed the fungus kept the host alive while it continued to grow over it. Much liked Septobasidium. I think, now, it is more likely the host is killed and it is only the stylet that continues to provide nutrients for the fungus host.