d- LIMONENE
Limonene is a plant origin pesticide (which means that it comes from plants!). It is present as a major component of citrus fruits' rind and skin. In fact, we also consume it. In other words, it is very safe for us! But, fortunately, the same can''t be said for insects. Fruit flies and mealy bugs (the first 2 of the 4 insects we are tackling- click here if you have forgotten) are killed by limonene.
On fruit flies, limonene acts a nervous system stimulator, which means that its nervous system is over stimulated. What happens is that limonene will cause motor nerves of these flies to get hyper-excited. This causes convulsions, paralysis and eventually death. While why limonene does this isn't exactly known; it is probably due to its action as an acetylcholinesterase inhibitor. When any impulse is passing through a synapse, acetylcholine (a neurotransmitter) is needed and once the impulse is passed, acetylcholinesterase (an enzyme) breaks it down so that the neuron isn't over-stimulated. Limonene, by acting as a acetylcholinesterase inhibitor, doesn't allow this acetylcholine to be broken down and so the impulse keeps passing. The poor fly, after some time just dies of exhaustion.
But mature fruit flies are braver. They are affected to the extent of being tired, but don't die of exhaustion since they are able to recover. Here, a pesticide synergist in the form of PBO helps us. It ensures that limonene isn't decomposed (and thereby detoxified) in the fly's body. This is done by its action as an inhibitor; inhibiting an enzyme called cytochrome P450. This enzyme would have otherwise decomposed limonene, but PBO ensures that it stays put in the insect body for a longer time period and even kills mature fruit flies.
On mealy bugs, the same very limonene acts in a very different manner. Here, it acts as a dehydrator. Remember the waxy protective layer these bugs had (if not, click here)? This layer gets dissolved under the action of limonene. This is going to kill the insect; but in the following manner:
Thus, there is no scope for mealy bugs to survive this triple form of attack by limonene.
Now, I am sure you are convinced that fruit flies and mealy bugs have been tackled and brought down to ecological threshold limit (in simpler words, to a point just absolutely essential for the food web to remain balanced or to a point necessary for the environment); lets get to saving mango from the other two insects also.
TRIFLUMURON
It is pretty obvious now, that MSW, MPW and RBMC are to be killed by triflumuron (since that's left!). Triflumuron, however works in a very different manner to that of limonene. Triflumuron comes in a category of insecticides called Insect Growth Regulators (IGR). These work by tampering insect growth instead of acting on some other body system of the insects (like the nervous or digestive system). IGRs are of 2 types-Juvenile hormones and Chitin Synthesis Inhibitors(CSI).
Triflumuron is a CSI. This means that it acts by inhibiting the synthesis of chitin. Why does chitin matter so much? That's because, chitin is to insects what skin is to us (but in a more integral manner). Chitin is an essential component of the exoskeleton of insects. They need this skeleton as a very important protective layer. As they proceed in their life cycle by ecdysis (molting, if that sounds better); these insects shed new 'skin' and grow new skin. This requires chitin since that is what the exoskeleton(or the skin) is principally composed of. Now, since triflumuron is going to inhibit the growth of chitin in the insect body; it can't grow, molt and proceed to the next stage in its life cycle. This kills these insects.
Now, (only if you are really inquisitive!) how does triflumuron make this happen? It isn't a miracle. To inhibit the growth of chitin; you need something which is going to break down bonds within chitin. Or, you need something which is going to inhibit the growth of chitin in the first place. Triflumuron does one of these things- it either acts as a chitinase mimic or as a chitin synthase inhibitor. This means that it either prevents chitin formation altogether (like a synthase inhibitor) or that it breaks down chitin after its formation (chitinase does that). This uncertainty stems from the fact that chitin synthesis isn't a very clearly understood process. Scientists greatly differ in their explanations of this process; which makes it all the more confusing to understand mode of actions of various pesticides. That is why, the fact that whether triflumuron acts as a chitinase mimic or a chitin synthase inhibitor isn't yet known. So till the world finds out how insects make their 'skin', we are good to go as long as you understand that triflumuron's going to kill weevils and RBMCs because of its action on chitin.
Due to the specific nature of the chitin and the corresponding enzymes (its something similar to homeopathic medicine),unnecessary insects aren't damaged by triflumuron. Plus, since humans don't have any chitin, we are also safe. That's why triflumuron comes in a category of pesticides known as 'reduced risk pesticides'. So, while weevils and RBMCs siblings from the Lepidopteran and Coleopteran family aren't spared, other insects are affected to a lesser extent.
On fruit flies, limonene acts a nervous system stimulator, which means that its nervous system is over stimulated. What happens is that limonene will cause motor nerves of these flies to get hyper-excited. This causes convulsions, paralysis and eventually death. While why limonene does this isn't exactly known; it is probably due to its action as an acetylcholinesterase inhibitor. When any impulse is passing through a synapse, acetylcholine (a neurotransmitter) is needed and once the impulse is passed, acetylcholinesterase (an enzyme) breaks it down so that the neuron isn't over-stimulated. Limonene, by acting as a acetylcholinesterase inhibitor, doesn't allow this acetylcholine to be broken down and so the impulse keeps passing. The poor fly, after some time just dies of exhaustion.
But mature fruit flies are braver. They are affected to the extent of being tired, but don't die of exhaustion since they are able to recover. Here, a pesticide synergist in the form of PBO helps us. It ensures that limonene isn't decomposed (and thereby detoxified) in the fly's body. This is done by its action as an inhibitor; inhibiting an enzyme called cytochrome P450. This enzyme would have otherwise decomposed limonene, but PBO ensures that it stays put in the insect body for a longer time period and even kills mature fruit flies.
On mealy bugs, the same very limonene acts in a very different manner. Here, it acts as a dehydrator. Remember the waxy protective layer these bugs had (if not, click here)? This layer gets dissolved under the action of limonene. This is going to kill the insect; but in the following manner:
- A essential protective layer is lost.
- Their respiratory spiracles (holes through which they breath) are clogged by limonene and so, they can't breath. This results in suffocation and by implication death.
- Limonene after dissolving the layer, also sucks body fluids from the poor bug- dehydrating and thereby killing it.
Thus, there is no scope for mealy bugs to survive this triple form of attack by limonene.
Now, I am sure you are convinced that fruit flies and mealy bugs have been tackled and brought down to ecological threshold limit (in simpler words, to a point just absolutely essential for the food web to remain balanced or to a point necessary for the environment); lets get to saving mango from the other two insects also.
TRIFLUMURON
It is pretty obvious now, that MSW, MPW and RBMC are to be killed by triflumuron (since that's left!). Triflumuron, however works in a very different manner to that of limonene. Triflumuron comes in a category of insecticides called Insect Growth Regulators (IGR). These work by tampering insect growth instead of acting on some other body system of the insects (like the nervous or digestive system). IGRs are of 2 types-Juvenile hormones and Chitin Synthesis Inhibitors(CSI).
Triflumuron is a CSI. This means that it acts by inhibiting the synthesis of chitin. Why does chitin matter so much? That's because, chitin is to insects what skin is to us (but in a more integral manner). Chitin is an essential component of the exoskeleton of insects. They need this skeleton as a very important protective layer. As they proceed in their life cycle by ecdysis (molting, if that sounds better); these insects shed new 'skin' and grow new skin. This requires chitin since that is what the exoskeleton(or the skin) is principally composed of. Now, since triflumuron is going to inhibit the growth of chitin in the insect body; it can't grow, molt and proceed to the next stage in its life cycle. This kills these insects.
Now, (only if you are really inquisitive!) how does triflumuron make this happen? It isn't a miracle. To inhibit the growth of chitin; you need something which is going to break down bonds within chitin. Or, you need something which is going to inhibit the growth of chitin in the first place. Triflumuron does one of these things- it either acts as a chitinase mimic or as a chitin synthase inhibitor. This means that it either prevents chitin formation altogether (like a synthase inhibitor) or that it breaks down chitin after its formation (chitinase does that). This uncertainty stems from the fact that chitin synthesis isn't a very clearly understood process. Scientists greatly differ in their explanations of this process; which makes it all the more confusing to understand mode of actions of various pesticides. That is why, the fact that whether triflumuron acts as a chitinase mimic or a chitin synthase inhibitor isn't yet known. So till the world finds out how insects make their 'skin', we are good to go as long as you understand that triflumuron's going to kill weevils and RBMCs because of its action on chitin.
Due to the specific nature of the chitin and the corresponding enzymes (its something similar to homeopathic medicine),unnecessary insects aren't damaged by triflumuron. Plus, since humans don't have any chitin, we are also safe. That's why triflumuron comes in a category of pesticides known as 'reduced risk pesticides'. So, while weevils and RBMCs siblings from the Lepidopteran and Coleopteran family aren't spared, other insects are affected to a lesser extent.