Bed bugs are small, parasitic insects that feed on human blood. They are usually reddish-brown in color, oval shaped, and wingless. Newly hatched bed bugs, called nymphs, are very tiny when they first begin feeding.
When do bed bugs start biting?
Bed bugs need blood in order to grow and develop. Nymphs will take their first blood meal shortly after hatching from the egg. They must feed on blood before molting to the next stage.
Newly hatched bed bugs, known as first instar nymphs, are only about the size of a poppy seed. They are translucent whitish in color, making them very difficult to see with the naked eye.
First instar nymphs will take approximately 3-10 minutes to engorge with blood. They will consume roughly 0.15-0.3 μl of blood in their first feeding.
Second instar nymphs
After molting to the second instar, bed bug nymphs are slightly bigger, about the size of a pinhead. They are opaque whitish in color. Second instar nymphs feed for approximately 15-20 minutes, consuming around 0.6-1.1 μl of blood.
Third instar nymphs
In the third instar stage, bed bug nymphs are about 1.5 mm long, around the size of a sesame seed. They are tan in color and begin to show some of the reddish hue of adult bed bugs. Third instar nymphs feed for roughly 30-40 minutes and will ingest 1.1-1.6 μl of blood.
Fourth instar nymphs
Fourth instar bed bugs are noticeably larger, about 2.5 mm in length. They are more elongated in shape compared to earlier nymphal stages. Fourth instars feed for approximately 45-60 minutes and will consume 1.6-2.4 μl of blood.
Fifth instar nymphs
In the fifth instar stage, bed bugs reach approximately 3 mm in length. They now resemble miniaturized adult bed bugs in both size and shape. Fifth instars feed for 60-90 minutes and ingest 2.4-7.0 μl of blood.
Adult bed bugs
By the time bed bugs have reached the adult stage, they are roughly 4-5 mm long—about the size of an apple seed. Adults are broad, flat, and oval-shaped. They are reddish-brown in color, although they can appear more brownish after feeding.
Adult bed bugs feed for 5-10 minutes and will consume anywhere from 7.0-15.0 μl of blood in one sitting. Females must take larger blood meals in order to develop eggs.
Size comparison
Here is a size comparison of bed bugs at different life stages:
| Life Stage | Size |
|---|---|
| 1st instar nymph | 1 mm |
| 2nd instar nymph | 1.5 mm |
| 3rd instar nymph | 2 mm |
| 4th instar nymph | 2.5 mm |
| 5th instar nymph | 3 mm |
| Adult | 4-5 mm |
Do bed bugs bite at every stage?
Yes, bed bugs require blood meals at each stage of development in order to grow, molt, and reach the next instar. They begin biting and feeding immediately after hatching from the egg as first instar nymphs.
All bed bug instars—including adults—will bite humans and feed on blood. Bites from earlier nymphal stages may be less noticeable due to the smaller amount of saliva injected.
Conclusion
In summary, bed bugs start biting at a very small size, as tiny as 1 mm in length as first instar nymphs. With each subsequent molt, they increase slightly in size. By the adult stage, bed bugs reach 4-5 mm in length. But they will bite and feed at every life stage in order to survive and develop.