FWIW--here are some ideas from ChatGPT: Idea "C" was interesting; I hadn't considered that one
I want you to take a deep look into silba adipata, the black fig fly, to postulate why it spread rapidly through much of Southern California in 2022, spread to the bay area to some extent the following year, but then has failed to expand into the Central Valley and foothills despite a large native fig tree population in the Central Valley and surrounding foothills.
Here’s a “deep look” hypothesis set that tries to explain the
fast SoCal spread (2022) →
Bay Area appearances (2023) →
apparent Central Valley/foothills lag, using what we actually know about
Silba adipata biology and the CA invasion timeline.
What’s firmly known (and matters for spread)
- Host specificity: black fig fly (BFF) is essentially a fig-only pest (Ficus carica), including caprifigs. CDFA Blogs+2UC IPM+2
- Attack window: females lay eggs via the ostiole into unripe (often unfertilized) figs; larvae feed inside and commonly cause premature fruit drop; larvae then exit and pupate in soil; they overwinter as pupae. UC IPM+2UC Agriculture and Natural Resources+2
- High intrinsic growth potential: it’s multivoltine (often cited ~4–6 generations/year in its native range), which makes population build and “explosive” local spread plausible once established. OUP Academic
- Early CA distribution (the baseline for your observation): after discovery in 2021, early reports were concentrated in coastal/southern counties—and researchers explicitly noted that temperature thresholds were unknown, limiting confident range predictions into inland CA. OUP Academic+1
Also important: your premise (“failed to expand into the Central Valley”) was
true for a while, but UC ANR’s updated pest alert now says BFF
has reached parts of the Central Valley (e.g., Merced is listed) as of their October 2025 update.
UC Agriculture and Natural Resources
That doesn’t invalidate your question—it reframes it as:
why the big lag before meaningful inland establishment?
Why SoCal could blow up fast in 2022
1) Human-assisted “jump” dispersal likely dominated early expansion
The original pest alerts explicitly warn that
moving figs and/or potted fig plants can cause “artificial spread.”
UC IPM+1
SoCal has:
- Dense backyard fig plantings (fresh-fruit sharing culture, swaps, farmers markets)
- Massive intra-region driving and logistics corridors
- Long fig season in many microclimates
Postulate: 2022 looked like rapid spread because it was a blend of:
- Local diffusion (short-distance movement between nearby figs)
- Repeated long jumps from people moving infested fruit/plant material
2) Coastal SoCal microclimate is “forgiving” for a soil-pupating, multivoltine fly
BFF
overwinters as pupae in soil.
UC IPM+2UC Agriculture and Natural Resources+2
Coastal Southern CA generally means:
- Fewer hard freezes (higher overwinter survival)
- Moderated summer highs (less lethal heat stress)
- Longer shoulder seasons for early/late generations
Net effect: higher year-to-year population carryover + more generations realized.
Why it could show up in the Bay Area in 2023, but not roar inland (yet)
3) Bay Area is “climate-similar” to coastal SoCal in the ways that matter
If early spread leaned on
human movement plus
coastal-moderate survival, then the Bay Area is a natural next stop: milder extremes, lots of backyard figs, strong travel links.
So the Bay Area presence in 2023 fits a model of:
- Transport introduction + coastal moderation = establishment
The Central Valley + foothills lag: the most plausible mechanisms (ranked)
A)
BFF’s critical life stage for persistence is the
pupa in soil (and larvae exiting dropped fruit into soil).
UC IPM+2UC Agriculture and Natural Resources+2
Central Valley summers add two stressors that coastal zones don’t:
- Very high soil surface temperatures
- Very low humidity / desiccating soils
Postulate: even if adults arrive (hitchhike),
soil stages fail more often, so you get “sparks” without sustained fire—especially in unirrigated or bare, sun-baked soils under roadside/wild figs.
Testable prediction: inland finds should correlate strongly with
irrigated, shaded, mulched backyards or orchards (cooler/moister soil), and be rarer in fully exposed dry sites.
B)
Because they
overwinter as pupae, a few colder nights (or repeated near-freezing soil temps) could reduce survival enough to slow establishment.
UC Agriculture and Natural Resources+1
Foothills (like Newcastle/Auburn bands) can get sharper cold swings than the valley floor
and far sharper than coastal zones.
Testable prediction: inland persistence should be better in
urban heat islands and low-frost pockets; worse in open foothill sites with radiational cooling.
C)
BFF prefers
unripe (often unfertilized) figs.
UC IPM+2UC Agriculture and Natural Resources+2
Central Valley heat can compress fig development:
- Fruit may pass through susceptible stages faster
- Drop patterns differ (less time for larvae to complete development before fruit desiccates)
Postulate: coastal zones provide a longer window of “perfect” fruit stages per generation, increasing realized reproduction.
D)
You’re right that the Central Valley/foothills have many figs—but distribution matters:
- In coastal metros, figs are often dense and continuous (yards, parks, streets)
- In inland areas, figs can be more patchy, separated by larger agricultural blocks without figs
If early natural dispersal is limited (and it often is for small flies), patchiness slows spread unless humans keep reseeding new pockets.
E)
CDFA notes uncertainty about how widely established the fly may be and discusses attraction to common fruit-fly traps (e.g., McPhail + torula yeast), but also implies surveillance may not perfectly reveal distribution.
CDFA Blogs+1
So the Central Valley could have had
small, intermittent populations before they were confidently reported.
Putting it together as one coherent story
A single model that fits your timeline is:
- 2021–2022: multiple SoCal introductions + high human-mediated movement + mild coastal conditions → rapid county-to-county spread. UC IPM+1
- 2023: Bay Area gets seeded through travel/plant/fruit movement; coastal moderation supports establishment. UC Agriculture and Natural Resources
- 2022–2024-ish lag inland: repeated introductions happen, but summer soil heat/desiccation + winter cold events + phenology compression + patchiness suppress the probability that any one introduction becomes a self-sustaining population.
- Eventually (by 2025 update): enough introductions land in the right inland microhabitats (irrigated/shaded/urban heat islands), and the fly begins to show up in Central Valley counties. UC Agriculture and Natural Resources
