In the last decade of this study, there was a decrease in the number of invasive insects that entered japan. One likely reason for this decrease is that people have

1) produced instecides that are strong enough to kill every insect that is present in crops from one country to another

2) improved inspections of crops that are transported from one area of the world to another area of the world

3) genetically altered all insects so they don't feed on crops that humans use

4) stopped the transportation of all food crops from other countries , requiring each area to use only locally grown crops