Tri-trophic interactions and the minimal effect of larval microsite and plant attributes on parasitism of Sphenella fascigera (Diptera: Tephritidae)

Figures & data

Figure 1 Pteromalus sp. Photograph courtesy of F-R Schnitzler.

Figure 2 Map showing the locations of the natural Senecio lautus populations sampled in this study (black points). White circles mark urban areas.

Table 1 Summary statistics of Sphenella fascigera and Pteromalus sp. incidence by site.

Site	No. of 2-m quadrats	No. of plants	No. of flower heads	Total no. of flies^1	No. of pupae collected^2	Reared flies^3	Reared wasps^3	Fly density per plant^4	Level of parasitism
Breaker Bay	4	7	50	14	6	4	0	2.00	0
Eastbourne	6	15	670	7	11	0	0	0.47	0
Makara Beach	2	2	15	1	0	0	0	0.50	0
Moa Point	3	9	183	6	3	0	0	0.67	0
Owhiro Bay 1	29	181	2336	660	486	91	32	3.65	0.260
Owhiro Bay 2	9	95	949	526	232	49	20	5.54	0.289
Owhiro Bay 3	4	15	57	39	7	1	1	2.60	0.5
Owhiro Beach	7	21	318	165	63	23	3	7.86	0.115
Petone	1	7	162	2	2	1	0	0.29	0
Total	68	356	4740	1420	823	178	58	3.99	0.246

¹ Total number of flies includes all larvae and pupae.

² Collected pupae are all the closed pupae found.

³ Reared from field collected S. fascigera pupae.

⁴ Fly density per plant is the total number of flies divided by the number of plants.

Table 2 Parasitism depending on the location of the pupa in the stem or flower head of the plant.

Location	Parasitised	Not parasitised	n	% of parasitism
Flower head	29	61	90	0.32
Stem	29	116	145	0.20
Total	58	177	235	0.25

Table 3 Results of a multi-variable (stepwise deletion) minimum adequate model and single-variable linear models for parasitism of Sphenella fascigera by Pteromalus sp.

	Minimum adequate model		Variables tested one by one
Variable	β coefficient	P-value	β coefficient	P-value	Significance
1. Location on plant
Flower head-vs-stem	dropped		−0.634	0.066	–
2. Other plant-level factors
Total number of flies	−0.182	0.473	−0.220	0.403	NS
Other insects	NA	0.447	NA	0.306	NS
3. Plant attributes
Maturity	0.517	0.079	0.483	0.085	–
Vigour	peaked at vig. = 3	0.324	NA	0.244	NS
Total flowers	dropped		−0.078	0.696	NS
Size	dropped		−0.393	0.116	NS
4. Neighbourhood plant densities
No. of Senecio lautus within 50 cm	dropped		0.565	0.051	–
No. of Senecio lautus within 2 m	dropped		0.299	0.319	NS
No. of Senecio lautus within 6 m	0.456	0.230	0.728	0.056	–
No. of other Senecio plants within 2 m	dropped		0.009	0.995	NS

NA, single value β coefficients are not applicable for multi-level factors; NS, not significant; –, marginal significance when 0.05 < P < 0.1.