Cacyreus marshalli (Geranium Bronzei) which belongs to the family Lycaenidae, is a butterfly native to South Africa that recently arrived in Italy in the late 1980s (1). It is called "the plague of geraniums and pelargoniums" because of its ability to cause these and other flowers to die by laying eggs in the stems. In England it is called Geranium bronze. The color is in fact dark brown similar to bronze.
Introduced to Europe probably with a few batches of geraniums, it spread rapidly first to the Balearic Islands, then later to Spain, Italy and France.
The female lays round, white-colored eggs in the stems and twigs, and the larvae, light green in color, resembling small caterpillars, which
eat the containing
and make the flowers die.
The male is slightly longer reaching about 15 mm with a wingspan of 30 mm, while the female measures 12 mm long and has a wingspan of 24 mm.
It is equipped with a long brown proboscis about 3 mm long with which it sucks and pierces.
It is most visible during full sunlight hours, when it is very active, while in the shade it tends to remain motionless in the leaves or on the ground. During the night it has no activity.
Distribution. Originally from southern Africa, this small butterfly has spread to Europe since the 1990s, becoming a well-known pest of geraniums.
Morphology.
Size: Its wingspan typically ranges between 1.5 to 2.3 cm.
Weight: The Geranium Bronze is lightweight, weighing just a few milligrams.
Colors: It sports brown wings with a slight lighter border. The underwings display small pale spots.
Life Cycle. The butterfly undergoes full metamorphosis. Geraniums and pelargoniums are the main host plants for the caterpillars, which bore tunnels into the plant stems, causing significant damage.
Dietary Habits. Caterpillars feed on the inner tissues of geraniums, while adult butterflies seek nectar from small flowers.
Conservation Status. Due to its rapid expansion and the abundant presence of geraniums as host plants, the Geranium Bronze has become quite common in many parts of Europe.
References_____________________________________________________________________
(1) Rocchia E, Luppi M, Paradiso F, Ghidotti S, Martelli F, Cerrato C, Viterbi R, Bonelli S. Distribution Drivers of the Alien Butterfly Geranium Bronze (Cacyreus marshalli) in an Alpine Protected Area and Indications for an Effective Management. Biology (Basel). 2022 Apr 7;11(4):563. doi: 10.3390/biology11040563. PMID: 35453762; PMCID: PMC9027867.
Abstract. Cacyreus marshalli is the only alien butterfly in Europe. It has recently spread in the Gran Paradiso National Park (GPNP), where it could potentially compete with native geranium-consuming butterflies. Our study aimed to (1) assess the main drivers of its distribution, (2) evaluate the potential species distribution in GPNP and (3) predict different scenarios to understand the impact of climate warming and the effect of possible mitigations. Considering different sampling designs (opportunistic and standardised) and different statistical approaches (MaxEnt and N-mixture models), we built up models predicting habitat suitability and egg abundance for the alien species, testing covariates as bioclimatic variables, food plant (Pelargonium spp.) distribution and land cover. A standardised approach resulted in more informative data collection due to the survey design adopted. Opportunistic data could be potentially informative but a major investment in citizen science projects would be needed. Both approaches showed that C. marshalli is associated with its host plant distribution and therefore confined in urban areas. Its expansion is controlled by cold temperatures which, even if the host plant is abundant, constrain the number of eggs. Rising temperatures could lead to an increase in the number of eggs laid, but the halving of Pelargonium spp. populations would mostly mitigate the trend, with a slight countertrend at high elevations.
Herrero S, Borja M, Ferré J. Extent of variation of the Bacillus thuringiensis toxin reservoir: the case of the geranium bronze, Cacyreus marshalli butler (Lepidoptera: Lycaenidae). Appl Environ Microbiol. 2002 Aug;68(8):4090-4. doi: 10.1128/AEM.68.8.4090-4094.2002.
Abstract. Despite the fact that around 200 cry genes from Bacillus thuringiensis have already been cloned, only a few Cry proteins are toxic towards a given pest. A crucial step in the mode of action of Cry proteins is binding to specific sites in the midgut of susceptible insects. Binding studies in insects that have developed cross-resistance discourage the combined use of Cry proteins sharing the same binding site. If resistance management strategies are to be implemented, the arsenal of Cry proteins suitable to control a given pest may be not so vast as it might seem at first. The present study evaluates the potential of B. thuringiensis for the control of a new pest, the geranium bronze (Cacyreus marshalli Butler), a butterfly that is threatening the popularity of geraniums in Spain. Eleven of the most common Cry proteins from the three lepidopteran-active Cry families (Cry1, Cry2, and Cry9) were tested against the geranium bronze for their toxicity and binding site relationships. Using 125I-labeled Cry1A proteins we found that, of the seven most active Cry proteins, six competed for binding to the same site. For the long-term control of the geranium bronze with B. thuringiensis-based insecticides it would be advisable to combine any of the Cry proteins sharing the binding site (preferably Cry1Ab, since it is the most toxic) with those not competing for the same site. Cry1Ba would be the best choice of these proteins, since it is significantly more toxic than the others not binding to the common site.
Cacyreus marshalli 
