Field infestation
Both ripeness stage and cultivation type had a significant effect on the
number of eggs per berry (Fig. 1a; ripeness stage*cultivation type: p
< 0.0001, F3, 1951 = 69.72, GLIMMIX). In both
years, cultivated ripe and purple berries carried more eggs than wild
berries at the same ripeness stage, while infestation in red, blush and
green berries were more similar. However, cultivated berries on average
were 3-4-fold the weight of wild ones, and after accounting for weight,
wild berries contained more eggs per gram than cultivated berries at all
ripeness stages (Fig. 1b; ripeness stage*cultivation type: p <
0.0001, F3,1786 = 9.08).
There was a three-way interaction effect of ripeness stage, week, and
cultivation type on infestation per gram of fruit over time (ripeness
stage*week*cultivation type: p = 0.01, F7,1759 = 2.52,
GLIMMIX). Cultivated berries appeared to maintain a consistent
infestation pattern throughout the season, with ripe fruit containing
the most eggs and the blush stage (least ripe stage collected) having
the fewest eggs (Fig. 2a). In contrast, the correlation between
infestation and ripeness stage in wild fruit is much less clear (Fig.
2b), even though ripeness overall was a significant factor (ripeness
stage: p < 0.0001, F4,1759 = 91.81). We
sampled two habitat types (woods and roadside) for wild-growing berries,
and unexpectedly the average infestation between the two types were
similar (Supp. Table S2).
Focusing only on the 2018 wild fruit samples collected at different
elevations, effects from elevation and ripeness stage were each
significant (elevation: p < 0.0001, F4,851 =
6.21; ripeness stage: p < 0.0001, F4,851 =
40.48, GLIMMIX), but their interaction was not, suggesting that all
elevations were infested to a similar degree (elevation*ripeness stage:
p = 0.35, F12,851 = 1.11). There is evidence for
differential timing of infestation, as the three-way interaction among
all variables was significant (Fig. 3; elevation*ripeness stage*week: p
= 0.002, F13,851 = 2.49). Lower elevations had available
fruit for collection two weeks before higher elevations. In general, the
only significant differences between ripeness stages within each
elevation was between the blush and ripe stage, otherwise mean
infestations were not significantly different.
To examine differences in the pattern of infestation among sample types,
we calculated the percentage of berries that contained at least one egg
per sample group. Ripe and purple wild berries and ripe cultivated
berries had above a 95% mean infestation across all timepoints (Table
1). Cultivated and wild fruits of the same ripeness stage (cultivation
type: p = 0.0625, F1, 55 = 3.61) were not significantly
different from each other, although green wild and blush fruits of both
types were significantly less infested overall than those at the ripe
stage (cultivation type*ripeness stage: p < 0.0001,
F3, 55 = 8.62). A weekly breakdown shows near 100%
infestation in wild ripe and purple fruits throughout the sampling
period, but more variable infestation in fruit at earlier ripeness
stages (Supp. Fig. S1). Sampling other wild-growing fruits found near
wild blackberry canes revealed a range of infestation patterns (Table
S3). Plant species phylogenetically close to known D. suzukiihost plants were more likely to be infested than those more distant
phylogenetically.