Several bugs and spiders have red or reddish markings on their backs. Here is a list of five that are the most common, or most likely what you have seen.
Boxelder Bug
(Boisea trivittata)
While more black than red, these bugs have noticeable red lines and markings on their back.
Jadera Bugs
(Jadera haematoloma)
Commonly referred to as the “red-shouldered bug” due to its distinctive red pronotum, the part of the insect directly behind the head. They are about 1/2 inch long. Nymphs (juveniles) are brighter red. They are primarily found in the southern United States.
Spotted Lanternfly
The Spotted Lanternfly (Lycorma delicatula) is an invasive planthopper that was first detected in the US in 2014 (in PA). Nymphs (juveniles) are black with white spots and develop red patches as they grow. Adult Spotted Lanternflies are approximately 1 inch long and have greyish wings with black spots. When they open their wings, you can see the hind wings which are red with black spots at the front and white and black bars at the rear.
Red Velvet Mite
(Balaustium mites)
Balaustium mites, especially in their adult form, can be quite striking. They are often bright red and covered with fine, velvety hairs. Depending on the species and the life stage, they can range in size, but many are tiny and may only be a few millimeters long.
Red Spider Mite
(Tetranychus urticae)
These tiny mites are pests that can damage plants. They are red or reddish-brown. Certain species of Balaustium mites are often colloquially referred to as “concrete mites” or “sidewalk mites” due to their propensity to appear on concrete surfaces, particularly in warm, sunny conditions.
Remember, color alone isn’t a definitive identification factor, and other characteristics should be considered when identifying bugs. If you ever need to handle or remove a bug or spider, it’s essential to do so safely, especially if you’re unsure of its species or potential harmful effects.