> layman's language? 

As adult female insects and mites have two sets of chromosomes, each gene has two versions (called alleles).  Sometimes having one in the resistant form is enough to show the full effect - that's dominance.  Sometimes you need to have both in the resistant form before you see the effect - that's recessive.

Somewhat confusingly for geneticists sticking to these terms, the apparent dominance and recessiveness can change according to how much of the chemical the organism is exposed to.

The last bit is just saying that a gene can have multiple effects.  Some effects may appear dominant, others may appear recessive.

Does that work for you?

I'd just have said that the trait is co-dominant, but that the level of challenge influences the response.

best wishes

Gavin

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