Mulch improves biological control of thrips in Alstroemeria

The poor establishment of predatory -mites and -bugs in ornamentals such as rose, chrysanthemum and potted-plants is a bottle-neck for successful biological control of thrips and white-fly in these crops. Due to limited damage tolerance in ornamentals, prey densities are typically low, resulting in poor availability of food for predators. Wageningen UR Greenhouse Horticulture has developed a mulch that improves the establishment of predatory mites. This new method was recently tested in a commercial Alstroemeria nursery, with funding of the PT (Dutch Horticultural board). Mulch application resulted in a strong increase of predatory mites on the soil and on the crop and a better control of thrips.

The mulch created favourable conditions for predators. It includes nutrients for alternative prey – astigmatic mites, that thrive in the mulch and form an excellent food source for predators. The stimulating effect of mulch application on soil-predatory mites was discovered several years ago by Wageningen UR Greenhouse Horticulture. Later studies have shown that also Amblyseius swirskii, a predator that is normally used to control thrips and white-fly on the crop, was able to move between the crop and the soil and feed on prey in the mulch. The alternative prey supported the predators in periods of prey-scarcity on the crop, resulting in higher predator densities and improved control of thrips in potted chrysanthemum.

The recent experiments in Alstroemeria show that the mulch can also support predators in higher crops and that besides A. swirskii, also other ‘leaf’ predatory mites descend to the mulch to feed: 8 weeks after introduction, the predatory mite Neoseiulus cucumeris have attained a factor 30 higher densities in plots treated with mulch. With mulch application, predatory mites establishment was poor and thrips density increased gradually. In mulch treated plots, thrips infection was followed by an increase in predator density on the crop and effective thrips control. The effectivity of this innovative method is now being tested on roses.