A. nigricornis is an uncommon wasp  that can be found throughout the Netherlands [2, 4].
The species is active from March to end of September .
The females are the only Ancistrocerus wasps that overwinter and are active early in the year starting in March before the other species are active in May . This makes them easy to identify during that period. the males die after mating in the autumn . the females have mated last summer and are already ready to lay eggs .
As of June the new generation emerges.
A. nigricornis catches larvae of Leafroller moths (Tortricidae) [4,6].
They nest in all kinds of bore holes, natural or artificial. In my bee hotels they are interested in nesting holes with a diameter of ±5 mm which is in compliance with the literature that states holes with a diameter of 4 – 5 mm .
The females are regular guests on the bee hotels, see also here.
3. BROOD PARASITES
The following nest parasites of A. nigricornis present in the Netherlands are mentioned in the literature:
– Chrysis ignita 
– Amobia signata 
The following species have been observed in the garden:
Chrysididae (Cuckoo wasps)
– Ruby-tailed wasp (Chrysis ignita) 
Length male 7,5 – 10,5 mm, length female 10 – 13 mm.
1. Legs yellow with black 
2. Propodeum black 
3. Sternite 2 edge makes angle of ±90 degrees seen from the side, and flatly impressed behind it 
4. Tergite 1 normally with distinct marking 
5. Antenna scape underside yellow over entire length 
References1 Nederlands Soortenregister
3 SCHMID-EGGER, C. H. Bestimmungsschlüssel für die deutschen Arten der solitären Faltenwespen (Hymenoptera: Eumeninae). Deutscher Jugendbund für Naturbeobachtung, 1994, 54: 90.
4 Martynova, Kate & Fateryga, Alexander. (2015). Chrysidid wasps (Hymenoptera, Chrysididae)—Parasites of eumenine wasps (Hymenoptera, Vespidae: Eumeninae) in Crimea. Entomological Review. 95. 472-485. 10.1134/S0013873815040090.
5 Breugel, P. van 2014. Gasten van bijenhotels. – EIS Kenniscentrum Insecten en andere ongewervelden & Naturalis Biodiversity Center, Leiden.
6 Peeters, T.M.J., C. van Achterberg, W.R.B. Heitmans, W.F. Klein, V. Lefeber, A.J. van Loon, A.A. Mabelis, H. Nieuwen-huijsen, M. Reemer, J. de Rond, J. Smit, H.H.W. Velthuis, 2004. De wespen en mieren van Nederland (Hymenoptera: Aculeata). – Nederlandse Fauna 6. Nationaal Natuurhistorisch Museum Naturalis, Leiden, knnv Uitgeverij, Utrecht & European Invertebrate Survey – Nederland, Leiden.
7 POVOLNY, D. The flesh-flies of Central Europe (Insecta, Diptera, Sarcophagidae). Spixiana supplement, 1997, 24: 1-260.