<i>Thripinema nicklewoodi</i> (Tylenchida: Allantonematidae), a potential biological control agent of <i>Frankliniella occidentalis</i> (Thysanoptera: Thripidae)
Frankliniella occidentalis Pergande (Thysanoptera: Thripidae) is the most important insect pest for greenhouse flower crops. Thripinema nicklewoodi Siddiqi (Tylenchida: Allantonematidae) is an entomoparasitic nematode attacking and sterilizing the thrips.
Methods to propagate and study T. nicklewoodi were developed. I observed an excretion rate of 21.4 nematodes per day by parasitized female thrips. The sex ratio of the excreted nematodes was 6:1 (female:male). After exposing 50 healthy first instars to four parasitized female thrips in a rolled bean leaf, I obtained a 75.3% mean parasitization rate in the adult stage of the thrips. In contrast to previous reports, male thrips were found to be parasitized as readily as females. Parasitism reduced the longevity of both adult female and male thrips by 26% and 61%, respectively. T. nicklewoodi when presented with various thrips life stages achieved the highest attack rate in first and second instars and prepupa. Free-living nematodes were found to escape from hosts through the anus and penetrate new host thrips through the intersegmental membranes of the thorax and abdomen.
While nematode parasitization affected tospovirus propagation, it did not reduce transmission of impatiens necrotic spot virus even though parasitism reduced feeding activity of adult female thrips by 81% on leaves, 38% on pollen, and 22% on honey. However, despite lowered total feeding, probing by parasitized thrips (in honey) was not reduced, and this may explain why lowered feeding does not result in lowered virus transmission.
In a study of the population dynamics on caged impatiens in greenhouse, nematode transmission persisted for seven host generations and populations of normal (i.e., not parasitized) female thrips declined by 39–79% in the nematode treatment compared to the control. However, no significant reductions were found in numbers of larval thrips between nematode treatments and the control. A higher proportion of male thrips occurred in populations with nematodes in which adult female thrips declined significantly compared to the control population.
T. nicklewoodi released seven times on caged impatiens in a greenhouse did not provide preventative control of thrips, though the population growth of second instar, adult female, and male thrips was suppressed by 44, 68, and 49%, respectively.