Rain Beetle

Adults of P. melanarius grow to be 12–18mm long. They appear black with straight striations on their elytra. They "are distinguished from other Pterostichus species by their laterobasal carina and denticulate posterior angles of the pronotum." Adults have wing dimorphism, characterized by brachypterous beetles and macropterous beetles. Brachypterous beetles have shortened hind wings and are unable to fly. Macropterous beetles have mature hind wings and are able to fly. Eggs are translucent white immediately after oviposition, but darken before hatching. The larvae have a hard dark reddish-brown head. The appearance of pupae is not well documented.

P. melanarius is found in a wide geographical range across Europe, the United States, and Canada. It can be found in "natural, anthropogenic, and agricultural biotopes." Beetles in similar geographical ranges tend to have similar development and life cycle stages, which differs from beetles in other geographical ranges. P. melanarius can traverse 2.5-5m daily, which can increase during the summer in search of mates. In addition to walking, flight capabilities resulting from wing dimorphism contribute to the beetle's ability to traverse wide ranges.

In Europe, the beetle can be found in Scandinavia, Italy, Greece, Ireland, Russia, and Serbia. In North America, the beetle is found on both the west and east coasts of the continent, but has been documented steadily moving into the center of the US. It is important to study this varying distribution to build an understanding how P. melanarius might affect its new habitats.

Mating of P. melanarius occurs during early fall months closely followed by oviposition. To maximize probability of larvae surviving and having access to resources with needing to travel, female beetles select shaded and wet environments to lay their eggs. Females typically lay about 130 eggs at a time. All larval stages happen in the soil. Colder temperatures are necessary for the larvae to develop into adults and warmer temperatures are necessary for the transition from final larval to pupae. Based on this developmental timeline, fully-developed adults emerge during late May to June.

Since P. melanarius inhabit arable fields, they are vulnerable to agricultural practices such as tilling fields and the use of insecticides. Tilling can be harmful to the life cycle and development of the beetles as it can occur during the fall and spring, which coincide with the oviposition and final development stages of the beetles. Tillage in the spring can disturb larvae and pupae in the soil, resulting in a decrease in the number of fully developed adults able to breed that emerge later in the fall. Some studies suggest that tillage has no effect on the beetles, but the contradictory results may be due to study methodology differences. The use of insecticides on fields that are inhabited by P. melanarius has negative consequences for the beetle because the insecticide is harmful to the beetles and its prey. Direct exposure to insecticides reduces the beetle population and their activity levels. The beetles can also be indirectly exposed to insecticides through consumption of prey affected by insecticides. According to one study, the consumption of affect prey results in total mortality for a beetle, a risk which was only slightly reduced when consuming the affected prey several days after it was exposed to insecticide. Further, insecticides can cause mortality in the beetle's prey, reducing food availability and potentially resulting in starvation of the beetles. …