Header: AI-generated editorial illustration, not a field photograph.
Some of the bumps on a scale-infested plant are insects. That takes a moment to appreciate, because many of them look about as animated as the pot.
Armored scale insects feed on the plant beneath separate protective covers. Those covers can look like small, flat lids attached to leaves or stems. Underneath sits the softer insect—and that is the part Rhyzobius lophanthae has come to eat.
Rhyzobius is a predatory lady beetle, although it does not wear the familiar red-and-black uniform. The adult has dark wing covers, a reddish-brown front, and fine hairs best appreciated through a lens. At roughly 1/12 inch long, it fits between ruler marks spaced 1/8 inch apart. Its larvae, the young feeding stage before adulthood, eat scale too. [UC IPM]
Following the trail to a colony
Getting a beetle onto the plant gives it a place to start. It still has to find the scale. For an insect this small, a few leaves and branches provide a considerable search area.
Plant odors can help. In experiments with scale-infested cycads, Rhyzobius responded to leaf odors and preferred leaves from mature plants over those from seedlings. That preference was clearer in filtered laboratory air than in the forest, where other smells competed for attention. [Marler & Marler] The study gives us evidence of a searching cue, rather than a promise that the beetles will find every colony on every houseplant.
You can shorten the search by releasing beside living scale. On a large plant, check several branches and distribute the adults among affected areas. Finding one colony does not mean the beetle has completed an inspection of the premises.
Getting beneath the cover
Finding the scale leaves another problem: reaching it. The armored scale’s shield lies over its body, close to the plant surface. A beetle that can feed on an exposed scale still has to deal with that cover.
Small Rhyzobius larvae can feed beneath shields, reaching eggs and crawlers—the tiny, mobile young scales that have not yet settled under covers of their own. Adults and larger beetle larvae also feed openly on scale. Both stages chew their prey, but their different sizes let them use different parts of a colony. [UC IPM]
The prey itself can be difficult to eat too. Honda and Luck found that adult beetles often could not penetrate mature, reproductive California red scales, and took longer to eat third-stage California red scale than oleander scale. [Honda & Luck] “Armored scale” includes several kinds of meal, some more accessible than others.
That is why the pest’s identity matters before you choose a beneficial. If you have identified only “a brown bump,” start with There’s Someone Under That Bump. More beetles cannot settle an identification question.
Raising the next generation beside dinner
When a female finds suitable scale, she can leave the next generation close to its food. Researchers rearing Rhyzobius on dictyospermum scale found beetle eggs beneath the scale shields. [Branco et al.] The cover that protects the pest can also shelter the predator’s young.
Those eggs hatch into elongated larvae, which feed and grow before becoming pupae. A pupa stays attached while the adult beetle develops inside. The adult then emerges and begins the cycle again.
Knowing those stages helps you recognize what follows a release. The dark larvae do not look like miniature rounded beetles, and a pupa will not walk away when you inspect it. Leave them undisturbed. A test that requires every useful insect to move would reject most pupae on principle.
Giving that generation time to develop
A new generation takes time. In Branco and colleagues’ laboratory study, egg-to-adult development averaged 24.82 days, or about three and a half weeks, with suitable prey freely available at about 77°F and 75% relative humidity. [Branco et al.] That is the time to produce an adult beetle under those conditions, not a deadline for clearing scale from your plant.
The adults you release can feed before any offspring appear. Finding larvae later suggests reproduction; finding successive stages shows development continuing. Neither observation, by itself, tells you how much scale remains. Check the prey as well as the predators: inspect beneath a small sample of covers and watch for new young scales settling nearby.
When the food supply dwindles
As the scale population declines, the beetles have less to eat. In laboratory starvation tests, adults without prey survived much less time than fed adults. [Branco et al.] That does not tell us exactly how long they will remain on a houseplant, but it does explain why a successful release cannot promise permanent protection.
Keep checking new growth and incoming plants after the original problem subsides. Use the release guide for placement and the monitoring guide to follow changes in the living scale population. Old shields may remain attached after their occupants die, so a plant can still look untidy while the infestation improves.
The beetles handle the eating. Removing the old packaging belongs to a separate department.
Sources and further reading
- UC IPM. Scale Predatory Rhyzobius Lady Beetles. University extension identification reference; covers two Rhyzobius species.
- Honda & Luck (1995). Scale Morphology Effects on Feeding Behavior and Biological Control Potential of Rhyzobius lophanthae. Peer-reviewed research.
- Branco et al. (2017). Life-history traits of the predator Rhyzobius lophanthae reared on the scale Chrysomphalus dictyospermi. Peer-reviewed research.
- Marler & Marler (2018). Rhyzobius lophanthae Behavior is Influenced by Cycad Plant Age, Providing Odor Samples in a Y-tube Olfactometer. Peer-reviewed odor-choice experiments; not a houseplant release trial.
