Dear college

There are many articles about penetration of pesticides in moth larvae cuticle. I think this paper can be useful for you:

thank for your attention

I am looking for protocols that showed pesticides penetration by cutting, fixing and staining of insect thorax and whether the type of compositions involved in thorax cutting, fixing and staining is effective in showing pesticides penetration to larvae cuticule ?

Asalaam-u-aleikham Samaneh

There is no simple way to demonstrate the presence of pesticides by staining of insect tissues. The best way to demonstrate their presence in any tissue at any particular time point after exposure is to use radio-labelled insecticide material and follow that up by autoradiography. This method has a minimal effect on the pharmacodynamics of the insecticide in the insect but at the same time allows you to detect a) the presence in each tissue, and b) the concentration effect either at the target site or in any depot accumulation.

Although penetration of insecticide through the cuticle is considered to be a hard fact and is the "orthodox" viewpoint disseminated by a few of the old insecticide toxicology "experts", there is an alternative viewpoint based on a series of more or less elegant experiments that suggest insecticides enter the insect through the tracheal system (See: the works of Philip Gerolt at

https://doi.org/10.1002/ps.2780010511;

https://doi.org/10.1002/ps.2780030108;

https://doi.org/10.1002/ps.2780060304;

https://doi.org/10.1002/ps.2780060603). The only reason his work was "dismissed" by people like Matsumura was that the absolute concentration of insecticide in the tracheae was considerably lower than in other tissues. However, if the tracheae deliver insecticide directly to the target nerves the quantity of insecticide required would be low and this approach avoids the complex explanations required to address the multiple physical quesiotns arising in explaining how a hydrophobic insecticide could penetrate and disseminate through the hydrophilic components of the cuticle (chitin) and also diffuse so quickly through the watery haeomocel fluid.

I hope this helps

Ian

Dear Mahmoudvand,

I think you can topically apply your insecticide through the larvae pronotum, which show a homogenous response (you can test it by radioautographic test). Keep in the mind that the rate of penetration at low concentrations tended to exceed that predicted by theory, whereas at high concentrations the rate distinctly fell away from theoretical linearity.

As well as, another factor which affects the rate of penetration is the polarity of the compound itself (this is another important point in your experiment). It has been generally considered that the rate of penetration is directly related to the liposolubility of the compound since the outer layers of the cuticle are highly apolar. 

Also, please attention to physicochemical properties of your insecticide such as molecular size and shape, solubility at the site of absorption, the degree of ionization, relative lipid solubility of ionized and unionized forms, and partition coefficient. They are critical parts of penetration tests.

I think below papers is useful for you:

- Mode of entry of contact insecticides, Gerolt, J. Insect Physiol. 

- Dynamic aspects of insect-insecticides interactions, Welling, Ann. Rev. Entomol.

Kind regards, Homayoon. 

Dear colleague,

Please find attached a publicaion dealing with pennetration and insceticidal activiity in Tenbrio molitor pupae. It can help.

Good luck,

Pr. N. Soltani

Dear colleague,

Thanks for your kind

I would start by addressing two main issues.

1) chemistry (Mohammad's response)

2) Research question.

If the research question involves basic toxicity testing or addressing resistance issues where the only factor of importance is metabolic or target site insensitivity then you are best off with a topical assay wherein you place a droplet of known concentration onto the insect (make sure the insect does not clean off the droplet). On the other hand, if you are interested in cuticular penetration, partitioning between sprayed surface and insect, residual activity, and other issues then you need to do a residual assay. In this sort of assay you coat a surface and then confine the insect to that surface. The surface may be a petri dish (plastic or glass), or a leaf from the host plant. The choice makes a difference. In this case uniformity of the coating is often desired, though the coating in the field is never uniform at a spatial scale relevant to the target insect.

You should also consider that the pesticide will degrade in the field. Degradation products may be more (or less) toxic than the parent compound. In any case, the application will slowly decline in efficacy.

Pesticide activity is dependent on the dose per weight of insect over time. Thus your evaluation time is critical. A pesticide that has no effect at 24 hours, might be 100% lethal at 72 hours. Droplet assays can quantify the dose, residual assay cannot. Insect weight is difficult to measure and is seldom taken into account. Thus, most assays are simplified to either look at dose, or time, and never consider individual insect body mass.