Size Does Matter
Broom flower containing seed beetles of various sizes.
The broom seed beetle (Bruchidius villosus) was released in New Zealand in 1988 to help manage infestations of Scotch broom (Cytisus scoparius). The larvae of this beetle live inside the tough little seeds and eat out the contents, destroying seeds and reducing germination rates. Even though the beetle has been estimated to destroy about 75% of seeds here now, models suggested that this was unlikely to be sufficient to reduce broom populations alone. However, Quentin Paynter presented a talk at ISBCW reassessing these predictions, looking at the quality of seeds produced by the broom plant instead of their quantity.
In its home range (Europe) broom relies on disturbance to regenerate or it disappears, but not so in New Zealand where seedlings happily grow up below existing stands, ensuring populations are perpetuated. There is evidence to suggest that big broom seeds are more successful at producing seedlings that can survive and grow in the shade of existing stands. Studies have shown that, on average, broom seeds in the native range are smaller than broom seeds in the introduced range. Quentin and his colleagues have collected and weighed broom seeds from 14 locations around New Zealand. “The seeds were highly variable in size but still on average around 40% bigger than their European counterparts, which helps explain why broom is so invasive here,” said Quentin.
Quentin has discounted the possibility that the increase in seed size is due to founder effects or genetic drift. He has suggested though that evolution may be responsible for the change, with seed size gradually becoming bigger in the exotic range because of an absence of seed beetles. This is because seed size has implications for the seed beetles.
The larger the seed, the larger the beetle that emerges, and larger beetles are more successful at surviving winter and producing offspring. Quentin measured the size of the broom seed beetles emerging from different sized broom seeds, confirming a strong positive correlation between seed size and beetle size. “I also found that on average the largest females laid over three times more eggs than the smallest and that large beetles were better at surviving the winter than small ones,” confirmed Quentin.
Thinking ahead it is possible to imagine a scenario where the New Zealand seed beetles may over time create a selection pressure that favours broom plants that produce small seeds, as these will reproduce more successfully than the big-seeded plants favoured by the beetles. This could result in less competitive broom like that seen in the native range. However, the broom seed beetle would not do as well under that scenario so biocontrol could break down, potentially allowing broom to bounce back again. However, if this was to happen, another seed-feeder that does not appear to rely on large seeds is available that could be introduced to New Zealand. Larvae of the broom seed weevil (Exapion fuscirostre) feed externally on multiple seeds and are therefore not affected by seed size, and would be fi ne regardless of seed size. Also, with the way the broom gall mite (Aceria gentistae) is performing, many broom plants may in future not survive to produce much if any seed. We will continue to monitor broom seed size in New Zealand every 5–10 years to see if changes are occurring and so we know if any other actions will be needed to stay on track with our goal to successfully biologically control broom.
This research was funded by the Ministry of Business,Innovation and Employment as part of Landcare Research’sBeating Weeds Programme.