CONTROLLING INSECTS WITH BACILLUS THURINGIENSIS

In agriculture there are many ways to balance the biological balance between mobile living beings and plants. This is based on the use of organisms that, in a totally ecological way, are capable of reducing the current high density of insects and pathogens in our plants. So today we introduce you to Bacillus thuringiensis.

Bacteria play a fundamental role in life, and of course, included in plants. There are a lot of plant diseases caused by bacteria and not fungi, as we generally think. In this case, Bacillus thuringiensis  has a protective role, since it is capable of parasitizing and eliminating some potential pests, such as that of Lepidoptera, specifically, caterpillars .

WHAT IS BACILLUS THURINGIENSIS ?

It has no common name, so we have to get used to calling it that, even if it has a somewhat intricate name. It is a Gram positive bacteria (due to the cell envelope, which distinguishes them in staining from Gram negative ones) that we can find in the great microbiological load that our soil has (and if we take care of it, even more so).

As we cannot find  enough Bacillus thuringiensis  to have total control of all the caterpillars and moths that plague our crops, there are phytosanitary products on the market that contain, in a concentrated form, this bacterium. Once we add water to the broth that we will apply to our plants, the bacteria are activated and do their job.

HOW DOES IT WORK AND WHY IS IT INSECTICIDE?

When  Bacillus thuringiensis  sporulates, it forms crystals of  protein δ-endotoxins , with insectidal action. They are commonly known as protein crystals  and have insecticidal action against:

• Lepidopteran larvae (Lepidoptera).
• Flies and mosquitoes (Diptera).
• Beetles (beetles).
• Bed bugs (hemiptera).
• Nematodes (worms).

However, we must bear in mind that its effectiveness has dropped a lot (we will see it in the next point). Currently, and depending on the strains within  Bacillus thuringiensis  that is used, it has control over defoliating and frugivorous caterpillars.

B. thuringiensis var. aizawai

This strain is capable of controlling the following species:

• Gusanos gris ( Agrotis ipsilon ).
• Plusia (Autographa gamma).
• Tomato meter ( Chrysodeixis chalcites ).
• Cotton spiny caterpillar ( Earias insulana ).
• Bean moth ( Etiella zinckenella ), cluster moth ( Eupoecilia ambiguella, Lobesia botrana), olive tree moth ( Prays oleae )  and leek moth ( Acrolepiosis assectella ).
• Tomato caterpillar ( Helicoverpa armigera ).
• Cabbage donut ( Plutella xylostella ) and black donut ( Spodoptera littoralis ).
• Gardama ( Spodoptera exigua ).

B. thuringiensis var. israelensis

This strain is capable of controlling the following species of the following genera:

• Aedes
• Anopheles
• Culex
• Backstage
• Orthocladius
• Tipula
• Black flies of the genus  Simulium.

B. thuringiensis var. kurstaki

This strain is capable of controlling the following species:

• Caterpillar of the almond tree ( Aglaope infausta ).
• Gray worm ( Autographa gamma ).
• Minador del tomate ( Chrysodeixis chalcites ).
• Cusano de la soja ( Clytie including ).
• Corn borer ( Ostrinia nubilalis).
• Cabbage white butterfly ( Pieris brassicae ).
• Citrus moth ( Prays citri ), olive moth ( Prays oleae ), cluster moth ( Lobesia botrana ), peach moth ( Anarsia lineatella ) and cabbage moth ( Plutella xylostella ).
• Piral de la vid ( Sparganothis pilleriana ).

SOME PRODUCTS ON THE MARKET …

BACILLUS THURINGIENSIS Kurstaki 32%. WP. (32 Millones U.I./g)

It is a wettable powder at different concentrations depending on the crop and pest to be treated, which contains the kurstaki strain and is especially effective against defoliating larvae.

The treatment usually has a dose of between 250 and 500 grams / ha, considering a broth for said surface of between 800 and 1,000 L

BACILLUS THURINGIENSIS Kurstaki 18%. WG

It would be made up of the same previous product, but with a lower concentration of Bacillus,  (18 Million IU / g), so the dosage will be lower and it will be designed for smaller crops such as horticultural crops. It is effective and of residue 0 at doses of 30-50 grams / hl, and it is especially indicated for gray worms, green tomato caterpillar, mamestra, pieris, moths, donuts, etc.

NOT ALL THE FOREST IS …  BACILLUS THURINGIENSIS

As we have mentioned, it seems that we have a totally effective and ecological weapon to control and eliminate leaf-chewing caterpillars. However, it is not entirely true. Although Bacillus thuringiensis  is known to  have great potential against Lepidopteran larvae, the biggest problem we face is the generation of resistance.

If exposed to toxin that this bacterium produces year after year and crop after crop, selective pressure  causes pests to build resistance if treatment is rendered ineffective. This is what has been happening in today’s agriculture, where the concentrations of active materials have had to increase every time. In this case, when the barrier of said Bacillus thuringiensis toxin is overcome  ,  there is no going back.

GENETIC ENGINEERING THROUGH THIS BACTERIUM

Of course, in the world of biotechnology, where bacteria are included,  Bacillus thuringiensis  has been and is (well, not so much now ..) a potential for plant development. Imagine that protein crystals toxic to caterpillars are extracted from said bacterium and, somehow, inoculated into a plantation through its seed. The larvae of Lepidoptera and other genera, at the moment in which they bite the leaf or the stem, will become intoxicated and will die. Well, or at least they won’t want to “sink their teeth” into it again.

There were many advantages , such as the great inoculation capacity of said protein in the culture, so that the insecticidal action was multiplied. As the toxin is inside the plant, only those larvae that feed on the stems and have the capacity to become intoxicated will die.

However, the great disadvantage is that it works with genetics ( transgenic products ), which not everyone likes, and the ability to overcome resistance from these organisms. A genetically modified seed that has taken a great effort in time and money to obtain, can last a few years (it cannot be predicted exactly) until the selective barrier is overcome.

 Well, we will see how effective Bacillus thuringiensis is   without the appearance of resistance … What do you think?