Density-independent reproductive success of the hemiparasitic plant Striga hermonthica, despite positive and negative density-dependent phases

The hemiparasite Striga hermonthica is a major constraint to smallholder farmer livelihoods and food security in sub-Saharan Africa. A better understanding of its life-cycle can help developing more effective management strategies. Here, we studied density dependence in S. hermonthica on Sorghum bicolor. We exposed two genotypes of S. bicolor that differed in the level of tolerance and resistance to S. hermonthica to a range of seed densities of the parasite. We evaluated multiple host and parasite performance parameters through periodic, destructive harvests and related these to the initial seed density using model selection. Initially, the probability for attachment was positively density-dependent, suggesting facilitation of new infections. However, at host maturity, S. hermonthica infection probability showed strong negative density dependence, indicating severe competition, in particular in the early developmental stages. Although parasite shoot dry weight showed a strong negative density dependence at host maturity, flower production per parasite exhibited positive density dependence again, suggesting compensation. The two host genotypes had similar responses to increased parasite densities, indicating differences between the genotypes in tolerance but not resistance. Consequently, despite density dependence in life-cycle components, the per capita reproductive output of S. hermonthica, R-0 (flowersseed(-1)) was density-independent. Apparently, management of the hemiparasite can neither benefit from a negatively density-dependent bottleneck, nor from a positively density-dependent Allee effect. The most promising suggestion to obtain S. hermonthica population decline (R-0<1) and long-term control is to increase host shading by growing a vigorous, competitive crop.