Some may think I am a little crazy to harvest a puncturevine (PV) plant and count all of the seeds that it produces; however, that is what I did last year to make a point about puncturevine weevils.
Growers may recall back in the 1960s and 1970s when puncture-vine weevils (Microlarinus lareynii) were available for release in fields where PVs were a problematic weed. These parasites were native to India and Europe, and were introduced into the U.S. in 1961. They did a very good job of becoming established and parasitizing PV seeds. The question keeps arising, though: “Why aren’t PV weevils available to release or purchase?”
Actually, there still are commercial companies that offer PV weevils for sale; however, close examination of existing PV plants in the late summer will reveal that the weevil populations are doing quite well. There is another PV weevil (Microlarinus lypriformis) that feeds on the bottom side of the PV stems, but it is the seed-feeding weevil that does the most to suppress the PV populations. The key word is suppress. This weevil on its own cannot eradicate PVs.
Female PV seed weevils chew through the side of green PV seed capsules and deposit eggs. The larvae develop inside of the seed and pupate. The adult emerges in about three weeks in central California. There are multiple generations per year based on local temperatures. So why don’t these weevils eliminate PVs? The answer is all in the numbers, and this gets back to me counting seeds.
I have read in the literature that a typical PV plant can have up to 5,000 seeds. The particular plant I counted had a 4-foot diameter and had 28,896 seeds. Of that, 45% (13,003) were parasitized by PV seed weevils. That means that of the original 28,896 seeds, 15,893 were still viable. If each seed produced a plant that yielded the same number of seeds as the mother plant, that would mean this plant could yield more than 459 million seeds. We all know that 100% of the seeds won’t grow into the same 4-foot diameter plant. However, even if only a few do, it illustrates how rapidly PV can proliferate, even when 13,000 seeds on the original mother plant were parasitized by the weevil.
The reality is that PV seed weevils are widely present and doing a good job of feeding on PV seeds. Examination of mature plants in late-summer will reveal their activity. It’s just that, as with most parasites, they only suppress the target to levels that require additional management. Cultivation or herbicides are necessary to complement the PV weevils in their effort to control PV.