On the basis of the most recent information we received under the Sunshine Act, the Twin Peaks population shows no sign of becoming self-sustaining. Even though some breeding occurs on on Twin Peaks, it’s not enough. The population would likely die out without injections of new butterflies from the larger population in San Bruno.
In the graph below, the dark bars show the butterflies spotted each year before the new batch from San Bruno are moved in. Those would be the ones known to be born on Twin Peaks. The light bars show the number of butterflies transferred from San Bruno.
[Edited to Add: This graph has an error for 2015; the native-born number should be 19. The others were spotted only after transfers from San Bruno had started and should have been excluded from our count.]
WHAT ARE MISSION BLUE BUTTERFLIES?
The Mission Blue butterfly (Aricia icarioides missionensis) is a subspecies of the quite widespread Boisduval’s Blue (Aricia icarioides). The species is not endangered, but the subspecies is found only from San Bruno to Marin, at a very few sites. The largest population is on San Bruno Mountain.
Lupine is the nursery plant of the Mission Blue. It’s the only plant on which it’s known to lay its eggs and which the caterpillars eat. (Specifically, it’s three varieties of lupine.)
Mission blue eggs hatch into caterpillars which eat the lupine, shedding their skins as they grow. The larger caterpillars are tended by native ant species, who protect them from predators while benefiting from “honeydew” – sugary caterpillar pee.
When they’ve grown to their full size, they form their pupae near the base of the plants, or even on the soil beneath, and remain there for months (in diapause). They hatch into butterflies in spring, sip nectar from a range of flowers (including the “invasive” non-native Italian thistle: Carduus pycnocephalus), mate, and lay eggs on lupines.
These butterflies have only one generation a year and an 8-10 week flight season, becoming visible in April and May. The males live an average of 7 days, and females for 8 days. The males usually hatch before the females do, so they are ready to mate when the females appear.
THE TWIN PEAKS MISSION BLUE BUTTERFLY PROJECT
Twin Peaks at one time had a population of Mission Blue Butterflies, but a fungus killed most of the lupine in the area. By 2008 they hadn’t been seen in years – 1997 was the last year any substantial number was spotted. The project was initiated in 2009 with the transfer of 22 female butterflies to Twin Peaks. (In fact, the project actually started earlier, with three kinds of lupine being planted on Twin Peaks.)
The project team – San Francisco Recreation and Parks Department’s Natural Areas Program (NAP) and outside consultants Creekside Center for Earth Observation – catch butterflies on San Bruno Mountain and bring them to Twin Peaks. A USFWS permit governs how many butterflies they can move, and from where. Most years, they can’t reach this limit.
- In 2009, the project staff moved 22 female butterflies to Twin Peaks and caged them over lupine plants until they laid their eggs. They hoped the butterflies would go forth and multiply. Only a small number made it.
- In 2010, observers counted 17 adult butterflies, and 14 larvae. This was not a self-sustaining population.
- In 2011, they spotted only 7 adults of which two were females, and 3 larvae. So they got US Fish and Wildlife Service permits to take more butterflies from San Bruno Mountain – 40 females and 20 males – which they released in May 2011.
- In 2012, they observed 7 butterflies (one female) and 6 larvae. Then they transported 11 female and 5 male butterflies from San Bruno Mountain. (They had permission to transport 60, but could not get them.)
- In 2013, they saw a total of 27 native-born butterflies, of which 6 were female – and 5 larvae. Then they caught 38 female and 20 male butterflies on San Bruno mountain and released them on Twin Peaks. Follow-up surveys observed a lot of eggs – 1120 – on Twin Peaks – much more than in previous years, when the highest number observed was 273.
- In 2014, they saw 23 native-born butterflies on Twin Peaks (5 female). This was despite spending more time looking than in the previous year (9 visits instead of 5) and going across the whole season. Since they didn’t move any butterflies from San Bruno in 2014, and so were not spending time capturing butterflies, project staff could spend more time observing them on Twin Peaks. The egg bonanza had not paid off; there’s a very high attrition rate between egg and adult butterfly from predators and parasites.
- In 2015, they observed 22 native-born butterflies – 17 males and 5 females. [Edited to Correct: Actually, the number observed *before* transfers was 19 – 16 mles and 3 females.] Then they brought in 22 more Mission Blues from San Bruno (13 females, 9 males).
- In 2016, they didn’t specifically go looking for Mission Blue butterflies – they mainly surveyed the lupine plants, which is where the butterflies lay their eggs, and the caterpillars live and eat. They saw only 7 butterflies, all males, before bringing in 44 more from San Bruno.
The project team considers it a continued success, based on limited objectives that include observing evidence that caterpillars are feeding on lupines; that male and female free-flying butterflies are seen both in and outside the release areas; spotting eggs. The other objectives are entirely about habitat management – more lupine, more nectar plants (preferably native ones), managing grass and other unwanted plants around the lupine.
(You can read the whole 2015 report here as a PDF: TwinPeaksProgressReportmbb2015 )
[Edited to Add: This graph has an error for 2015; the native-born number should be 19 – 16 males and 3 females. The others were spotted only after transfers from San Bruno had started and should have been excluded from our count.]
MAINTAINING A MISSION BLUE POPULATION ON TWIN PEAKS?
The only way to have Mission Blue butterflies on Twin Peaks, is to intervene continually in two ways:
Regularly importing Mission Blue butterflies from elsewhere
Though the Mission Blue butterfly does breed on Twin Peaks, it is only moderately successful. A bad year, or just plain attrition, could quickly drop numbers below a sustainable level. (If there are too few butterflies, many males will die before finding mates.)
So the only way to have Mission Blue butterflies at Twin Peaks is to keep bringing them from San Bruno (or maybe other areas if strong numbers pop up). In addition to San Bruno, an effort is being made to introduce them into the Golden Gate National Recreation Area. Populations have also shown up spontaneously in other areas.
Gardening for lupine
Twin Peaks will need continual work to maintain the Mission Blue garden. Without ongoing effort, the area will naturally change to other vegetation.
- In a “natural” setting, patches of lupine shift around because they thrive on disturbance. (So, therefore, do the Mission Blue butterflies.) Without such disturbance, lupine will need to be replanted from time to time. They will also need to plant the nectar sources for the butterfly.
- Gardening is also needed to block the natural succession of grassland into scrub land. The main problem here is coyote brush, a native plant that would normally invade the grassland and overtake the lupine. NAP uses volunteers for this. It also uses pesticides to control other plants, including Garlon 4 Ultra for oxalis.
- Also, the grass around the lupine patches needs to be trimmed back so the butterflies can spot it more easily.
The program continues: gardening for lupine on Twin Peaks, and transferring in butterflies from other locations. The most recent report (Feb 2016) says they’re stopping butterfly counts as a way of surveying the population on Twin Peaks, but instead just going by whether the butterfly is present or absent at specific locations. The main population estimate will be based on whether the lupine is being eaten.
This may not be very accurate, since other insects – including the closely-related Acmon Blue butterfly that also occurs on Twin Peaks – also eat lupine and it’s difficult to tell what’s causing the leaf-damage. Also, most caterpillars will not survive to become butterflies, which means they will not reproduce. The attrition rate exceeds 80% and may be quite variable depending on predators like rodents and parasites – especially certain wasps.
But perhaps accuracy is not very important; unless the number is very large, they will continue to transfer in butterflies from San Bruno mountain. Their USFWS permit is valid through 2020.
On the whole, though, the project seems harmless. Aside from the continuing use of pesticides, and some diversion of resources, there seems no reason not to continue. Some concerns were raised as to whether San Bruno mountain’s Mission Blue population would be affected, but Creekside thinks these transfers are entirely sustainable.