**4. Discussion**

Trap cropping is a technique used in both ecological and agronomic fields and it is based on the use of plant species, particularly attractants and species susceptible towards certain pests, insects, or nematodes [23]. According to the characteristics of the plant used and the time or space of deploying, different modalities of trap cropping (perimeter, sequential, multiple, and push-pull) are reported for the managemen<sup>t</sup> of different pests [13]. Although trap cropping has usually been employed to control insect pests, few studies were previously performed on nematodes, such as cys<sup>t</sup> nematodes [24–26], and root-knot nematodes [27–29].

In the present research, the trap crop technique was evaluated to reduce the population density of *Meloidogyne graminicola* in upland rice field experiments where continuous flooding cannot be applied as a managemen<sup>t</sup> practice to control this pest.

To select the most successful trap cropping method, the host range, biology, development, and multiplication, spread and survival strategies of the pest are pivotal information for the correct managemen<sup>t</sup> [30].

Among the numerous host plants reported, *Oryza sativa* has been recorded to be the most attractive and susceptible one to *M. graminicola* [6,31]. For this reason, it was selected in this study as the trap crop plant. Concerning the time prior to the destruction, a sufficient period is required for the host plants to attract free-living second-stage juveniles (J2) and permit the root colonization to occur before nematode reproduction. In this work, the choice to destruct the rice plant at the second leaf stage (about 16–17 days from sowing to the trap crop destruction) was based on bibliographic information [32,33], *M. graminicola* cycle observations in the field, and analysis in the laboratory during the 2019 mandatory monitoring of this pest in the Lombardy region (Sacchi S, pers. obs.). In fact, Dabur et al. [32] observed that J2 of *M. graminicola* in the soil can enter the roots of host plants from the 5th day of sprouted rice seed sowing, increasing their number in the roots up to the 12th day of sowing. Moreover, from our observations, at the second/beginning third leaf stage, only J2 were found inside the roots, while at the end third/beginning fourth leaf stage, mostly J3, J4, and males were present. The female presence was observed from the fourth leaf unfolded stage.

At the end of the experiment, a reduction of the total number of *M. graminicola* was recorded only in the treated plots. Uncultivated and control managements gave similar results and are perhaps related to the several weeds present in the uncultivated plots. Some of them, such as *Echinocloa* spp. and *Cyperus* spp. are known as host plants of *M. graminicola* [6], and therefore the nematode can survive and reproduce in these alternative hosts. This result confirms and encourages efficient weed managemen<sup>t</sup> as an important tool to maintain a low nematode population in infested fields [29].

Concerning the reduction of the rice RKN number in the soil, the results also highlighted positive consequences directly on the plant health *status*, and plant population density per unit area, due to the lower stress. Indeed, the rice plants grown after the three trap crop cycles showed a significantly lower infestation index in treated plots than both control and uncultivated ones, notwithstanding the low root-gall index in all the plots due to the second leaf stage of the plants. Also, the rice plants grown in the treated plots were taller by about 12% than both plants in the control and uncultivated plots at this stage of plant development. Moreover, in the treated plots the plant population density increased by 25% and 34% compared to the control and uncultivated ones, respectively.
