Unter jenen, welche die Erbsenpflanze besuchen, könnte die Käferspecies Bruchus pisi dem Versuche gefährlich werden, falls sie in grösserer Menge erscheint.
Among those which visit pea-plants, the beetle-species Bruchus pisi might become dangerous to the experiment, if it appears in larger amounts.
beetle-species = Käferspecies See p. 6, s. 15.
Bruchus pisi This is the pea weevil. The currently accepted zoological name is Bruchus pisorum. Mendel’s first publication was a short report on this species, and the damage it had caused around Brünn (Verhandlungen des zoologisch-botanischen Vereins in Wien 4 [1854], p. 27–28). Hugo Iltis took this as an indication that Mendel had started to experiment with peas in 1854 already (see Gregor Johann Mendel: Leben, Werk und Wirkung, Berlin: Springer, 1924, p. 24).
might = könnte The use of the subjunctive is revealing. Mendel is not reporting an actual incident, but warning his readers, thus apparently expecting that some of them would want to replicate his experiments.