Thursday, February 9, 2012

Revenge of the mealybugs!

A few years ago, I picked up a plant – not sure where exactly – that had mealybugs on it. They have been the bane of my plant growing ever since. Every year, they re-surface on one poor plant or another, appearing without much warning in their hundreds. By this stage, they will already be on plants nearby, and if I’m not quick enough with applying pesticide, they will be on every plant in my greenhouse.


Here is this year’s source plant (Sarracenia flava var. cuprea “Victorian Clone”). Embarrassing though such a festy plant is, I’m hoping my misfortune will help others out. Pests are, after all, just part of life.


Here’s a closeup of the culprits. Yuk! You can see every stage here, from the fluffy eggs to the tiny pink crawlers and second instar nymphs, to the spiny, silverfish-looking later instars and adults.


No, not a sundew-Sarracenia hybrid. The dewy looking things are the fruiting bodies of a siity mould growing on the excrement of the mealybugs. Like all True or Sucking Bugs (Insecta: Hemiptera), mealybugs have a very convoluted digestive system that allows them to process large amounts of sap – basically sugar water – in order to access the minute quantities of nitrogen and other compounds that are of use. The white mealy stuff is actually waxes that they extrude from tiny pores on their exoskeleton. I’m not sure what species (or even genus) these mealybugs are\. To find out, I would need to digest the fats out of them using sodium hydroxide and then perform a complex set of staining dips to colour their exoskeleton enough to see the layout and shape of their skin pores! From havcing done this, I can promise it is tedious and not fun at all! Butterflies and moths are far easier to work with, even with all their hairs and scales...

For those of us who don’t live where Sarracenia are endemic, mealybugs are perhaps the worst pest we have to deal with. There are several reasons why this is the case. For starters, mealybugs are relatively cryptic insects that avoid exposed sites until competition becomes intense. They like concealed places and exploit these first – the roots are usually where an infestation originates from, or under the pitcher scales (which are excellent egg laying sites). These mealybugs are the most damaging, as they directly drain resources from the rhizome, where it hurts the plant’s health the most.


This is what a severe infestation in the rhizome and pitcher scales looks like. The white stuff on the dried scales are predominantly egg masses, not mealybugs themselves.


From the rhizome and scales, they spread up the pitcher ala (left) and anywhere that two pitchers are touching (right). Unlike scale insects, mealybugs can move and are relatively active – I have seen them move along an entire pitcher at a surprising speed for such a flabby looking creature.


Once on the leaves, the underside of the peristome are used as egg laying sites, resulting in sudden population explosions that can spread to other pitchers. The tiny pink oblong shaped things in this photo are crawlers – so small they can drift on air currents moving at around 70 cm/second (less than a few miles per hour). These spread on the breeze to any nearby plants and are what allow mealybugs to really infest everything if left unchecked.

So what to do about them? From personal observations, mealybugs are always worse in greenhouses, mainly because their natural enemies can’t access them as easily as outside. Environmental stresses also reduce their numbers in the open – despite their waxy coatings, mealybugs (like all insects) are vulnerable to drying out and also to drowining. The only really practical option is to use a combination of pesticides, one followed by the other. I tend to use a flushing-contact type, aka one with a pyrethroid active, first, followed by a systemic.  The strategy is simple – flood the soil with a pyrthroid, which will drive most active mealybugs present in the roots and rhizome up onto the pitchers. By flood, I mean spray the pyrethroid directly onto the growth points and rhizome until the surrounding soil surface foams white for at least 10 seconds. Then wait a few minutes until the mealybugs make their escape up the pitchers, and spray them a second time with the pyrethroid to finish them off.

I then wait a week or two, and use a systemic like Confidor. Systemics are good, because they are dissolved within the plant’s sap and then ingested by the insects. The reason that you use the systemic second is that it will kill off any immatures hatching from eggs not killed by the pyrethroid. It will also finish of any adults you missed.

Regardless of what chemicals you use, ALWAYS FOLLOW THE LABEL DIRECTIONS! They are there to protect you – trust me on that one!

Even with this treatment, mealybugs can be already somewhat resistant to chemical pesticides, or hidden away in some obscure part of the plant or pot that misses treatment. I have had them pop up for three years in a row, but seem to be beating them back – the first year they were widespread, last year only half the plants had them, and this year it is limited to about half a dozen plants surrounding the S. flava var. cuprea above (at least that I know about). This year is repotting year, so I am now planning to wash the rhizomes in a pesticide dip prior to repotting, in the hope that it will mop up the last few insects.

With a bit of luck, it will finally be the end of them!